WEBVTT 00:00:00.090 --> 00:00:01.670 The following content is provided 00:00:01.670 --> 00:00:03.820 under a Creative Commons license. 00:00:03.820 --> 00:00:06.550 Your support will help MIT OpenCourseWare continue 00:00:06.550 --> 00:00:10.160 to offer high quality educational resources for free. 00:00:10.160 --> 00:00:12.700 To make a donation or to view additional materials 00:00:12.700 --> 00:00:16.620 from hundreds of MIT courses, visit MIT OpenCourseWare 00:00:16.620 --> 00:00:17.275 at ocw.mit.edu. 00:00:26.130 --> 00:00:28.740 PROFESSOR: And I thought that what we'd do today 00:00:28.740 --> 00:00:32.900 is first go over this syllabus for audition, which 00:00:32.900 --> 00:00:35.040 is the second part of the course. 00:00:35.040 --> 00:00:37.670 Just so you get an idea of what's in store for you. 00:00:38.800 --> 00:00:42.610 And then today's lecture will have a big part 00:00:42.610 --> 00:00:47.780 on sounds, which have physical properties that 00:00:47.780 --> 00:00:50.630 are very different than the light stimulus you guys have 00:00:50.630 --> 00:00:53.610 been talking about so far in the course. 00:00:53.610 --> 00:00:56.900 And we're going to illustrate the different types of sounds, 00:00:56.900 --> 00:00:59.195 very simple sounds like pure tones. 00:01:00.490 --> 00:01:03.450 And very complex sounds like human speech, 00:01:03.450 --> 00:01:06.770 which have many, many components in them. 00:01:07.980 --> 00:01:10.970 And then we'll get into the auditory system, 00:01:10.970 --> 00:01:13.295 first starting with the auditory periphery. 00:01:15.300 --> 00:01:18.090 And we'll talk about the three basic divisions 00:01:18.090 --> 00:01:22.080 of the auditory periphery, which are the outer, middle, 00:01:22.080 --> 00:01:23.850 and inner ear. 00:01:23.850 --> 00:01:25.440 And today we're really only going 00:01:25.440 --> 00:01:28.470 to have a chance to focus on the functions 00:01:28.470 --> 00:01:30.219 of the outer and the middle ears. 00:01:30.219 --> 00:01:32.635 And so we'll talk about the functions of those structures. 00:01:35.520 --> 00:01:43.050 So as far as the syllabus goes, each of the lectures 00:01:43.050 --> 00:01:43.840 has a title. 00:01:43.840 --> 00:01:47.620 So today's title, October 28th, is sound, external, middle, 00:01:47.620 --> 00:01:48.345 and inner ears. 00:01:49.530 --> 00:01:55.270 And each of the lectures has a reading or more 00:01:55.270 --> 00:01:56.720 accompanying it. 00:01:56.720 --> 00:02:00.680 And so most of the readings are from the textbook. 00:02:00.680 --> 00:02:02.960 So there's a textbook, Schnupp, Nelken, 00:02:02.960 --> 00:02:08.820 and King, which is a very good, up-to-date textbook written 00:02:08.820 --> 00:02:10.710 by two psychophysicists. 00:02:12.530 --> 00:02:14.880 And one physiologist, Israel Nelken. 00:02:16.410 --> 00:02:20.570 And it's written at just the right level for this class. 00:02:20.570 --> 00:02:23.060 That is an advanced undergraduate textbook. 00:02:23.060 --> 00:02:26.970 So it's pretty easy to read, or should be very easy to read. 00:02:26.970 --> 00:02:28.284 And it's written very well. 00:02:28.284 --> 00:02:29.450 These guys are good writers. 00:02:30.590 --> 00:02:34.740 They have many examples of auditory demonstrations 00:02:34.740 --> 00:02:38.180 that you can listen to just by clicking 00:02:38.180 --> 00:02:39.670 in the margin of the text. 00:02:39.670 --> 00:02:41.030 The demonstration we'll come up. 00:02:41.030 --> 00:02:44.020 And as you can see, after today's lectures, 00:02:44.020 --> 00:02:45.440 I like to give demonstrations. 00:02:45.440 --> 00:02:47.250 Because I like to listen to what we're 00:02:47.250 --> 00:02:48.970 talking about in terms of how does 00:02:48.970 --> 00:02:52.355 it really sound to you as a listener. 00:02:53.630 --> 00:02:57.370 So I'd encourage you to get that textbook. 00:02:57.370 --> 00:03:00.960 Now you could buy a hard copy, or if I'm not mistaken, 00:03:00.960 --> 00:03:03.410 Michelle you can get a free copy online. 00:03:03.410 --> 00:03:04.598 Is that right? 00:03:04.598 --> 00:03:06.056 AUDIENCE: There's an online version 00:03:06.056 --> 00:03:08.757 as well that you can read on [INAUDIBLE]. 00:03:08.757 --> 00:03:09.340 PROFESSOR: OK. 00:03:09.340 --> 00:03:09.839 Great. 00:03:09.839 --> 00:03:14.545 So if you have any trouble figuring that out, let me know. 00:03:14.545 --> 00:03:16.420 But I think you should easily be able to find 00:03:16.420 --> 00:03:17.211 the online version. 00:03:18.390 --> 00:03:20.320 And it should have the demonstrations 00:03:20.320 --> 00:03:22.845 that you can listen to with earbuds or headphones. 00:03:26.300 --> 00:03:30.900 There is for today these passages from the textbook. 00:03:30.900 --> 00:03:34.760 And then for today and for many of the lectures, 00:03:34.760 --> 00:03:39.490 there is another reading, which is a research paper. 00:03:39.490 --> 00:03:42.440 This one is by Hofman, Van Riswick, and Opstal. 00:03:43.530 --> 00:03:46.410 And it's titled, Relearning Sound Localization 00:03:46.410 --> 00:03:48.300 with New Ears. 00:03:48.300 --> 00:03:52.380 And this we'll talk about in class right 00:03:52.380 --> 00:03:53.980 at the end of today's lecture when 00:03:53.980 --> 00:03:56.250 we talk about the function of the outer ear, 00:03:56.250 --> 00:03:57.410 the pinna, so-called. 00:03:58.720 --> 00:04:01.050 And they did a very interesting experiment 00:04:01.050 --> 00:04:04.700 that addresses what is the function of your external ear. 00:04:04.700 --> 00:04:09.310 So people always ask me, what am I responsible 00:04:09.310 --> 00:04:10.470 for in these readings. 00:04:10.470 --> 00:04:13.240 Well this is a very specific paper. 00:04:13.240 --> 00:04:15.750 It has a lot of interesting research methods. 00:04:15.750 --> 00:04:18.696 The subjects were human volunteers. 00:04:20.170 --> 00:04:22.700 And there are a lot of details in there 00:04:22.700 --> 00:04:24.610 that are not that important. 00:04:24.610 --> 00:04:27.380 What I'm really focused on having you learn 00:04:27.380 --> 00:04:29.900 is the take-home message. 00:04:29.900 --> 00:04:33.320 And the take-home message from this paper 00:04:33.320 --> 00:04:35.575 is, what is the function of the outer ear. 00:04:36.760 --> 00:04:40.280 And what is this twist in the title, how can 00:04:40.280 --> 00:04:44.900 you relearn sound localization with different outer ear. 00:04:44.900 --> 00:04:46.560 We'll talk about it in class. 00:04:46.560 --> 00:04:50.270 But I want you to get the take-home message 00:04:50.270 --> 00:04:52.950 from these research studies. 00:04:52.950 --> 00:04:56.700 Because there sort of what we do as professionals 00:04:56.700 --> 00:04:57.990 in the auditory system. 00:04:57.990 --> 00:05:00.310 Our day-to-day living is doing research. 00:05:00.310 --> 00:05:03.930 In some cases on human subjects, in some cases 00:05:03.930 --> 00:05:04.933 on individual molecules. 00:05:06.050 --> 00:05:08.530 But how can we learn about hearing 00:05:08.530 --> 00:05:10.420 from doing these research studies? 00:05:10.420 --> 00:05:16.270 And I have picked good papers, good research studies. 00:05:16.270 --> 00:05:17.920 Because they really tell us something. 00:05:17.920 --> 00:05:19.480 There's plenty of stuff out there 00:05:19.480 --> 00:05:22.350 that gives sort of equivocal results. 00:05:22.350 --> 00:05:23.940 But this is a really good paper. 00:05:23.940 --> 00:05:27.530 And you have a take-home point from it 00:05:27.530 --> 00:05:32.040 about how you use your outer ears to localize sounds. 00:05:32.040 --> 00:05:34.850 So that's an example of a research paper 00:05:34.850 --> 00:05:37.220 that goes along with this lecture. 00:05:37.220 --> 00:05:43.040 So just coursing through the syllabus-- on Wednesday, 00:05:43.040 --> 00:05:44.775 we'll have a lecture on hair cells. 00:05:47.900 --> 00:05:51.820 Next week we'll talk about the auditory nerve, which 00:05:51.820 --> 00:05:54.960 is the nerve that sends hearing information from your ear 00:05:54.960 --> 00:05:55.745 into your brain. 00:05:56.830 --> 00:06:00.320 And we'll talk about frequency resolution-- 00:06:00.320 --> 00:06:02.540 how we can tell one frequency from another. 00:06:03.925 --> 00:06:05.300 At the end of next week, we'll be 00:06:05.300 --> 00:06:09.600 talking about the brain, the cochlear nucleus, and all 00:06:09.600 --> 00:06:12.595 the interesting unit and cell types in the cochlear nucleus. 00:06:14.310 --> 00:06:16.210 The following week, we're going to be talking 00:06:16.210 --> 00:06:18.970 about hearing loss, how there can 00:06:18.970 --> 00:06:20.310 be problems with your hearing. 00:06:20.310 --> 00:06:22.450 Many of them are treated at the hospital where 00:06:22.450 --> 00:06:25.850 I do my research, which is Massachusetts Eye and Ear 00:06:25.850 --> 00:06:27.380 Infirmary across the river. 00:06:28.510 --> 00:06:33.820 And there, when the surgeons in my department 00:06:33.820 --> 00:06:35.550 encounter a deaf person, they give them 00:06:35.550 --> 00:06:38.470 the option to get a cochlear implant. 00:06:38.470 --> 00:06:40.690 So a cochlear implant is a device 00:06:40.690 --> 00:06:42.680 that can be put in your inner ear. 00:06:42.680 --> 00:06:45.420 And it can restore your sense of hearing. 00:06:45.420 --> 00:06:48.260 And we'll have a demonstration by that cochlear implant 00:06:48.260 --> 00:06:51.120 user who comes to class on that date 00:06:51.120 --> 00:06:55.180 and gives a demonstration of her cochlear implant, which 00:06:55.180 --> 00:06:57.505 has restored hearing to her, although not perfectly. 00:06:59.530 --> 00:07:01.420 So then later on in the semester, 00:07:01.420 --> 00:07:06.800 we'll talk about various other topics 00:07:06.800 --> 00:07:08.460 on up through the auditory cortex. 00:07:10.020 --> 00:07:15.130 And finally, we're going to have a tour of the Hearing Research 00:07:15.130 --> 00:07:17.930 Laboratory at the Massachusetts Eye and Ear Infirmary, 00:07:17.930 --> 00:07:21.270 where we'll meet over there and we'll 00:07:21.270 --> 00:07:23.590 encounter various research projects 00:07:23.590 --> 00:07:26.047 that are currently going on. 00:07:26.047 --> 00:07:27.130 And we'll talk about them. 00:07:28.690 --> 00:07:30.780 There is a written assignment. 00:07:30.780 --> 00:07:33.390 I guess you guys had an assignment for vision 00:07:33.390 --> 00:07:35.460 in the class-- a written paper? 00:07:35.460 --> 00:07:37.970 So we have an analog here on the auditory system. 00:07:39.030 --> 00:07:42.540 And this is the assignment you can read it later 00:07:42.540 --> 00:07:43.370 at your leisure. 00:07:43.370 --> 00:07:46.180 It won't make much sense right now, 00:07:46.180 --> 00:07:48.710 because we haven't talked about neural circuits 00:07:48.710 --> 00:07:50.880 for localization of sounds yet. 00:07:50.880 --> 00:07:52.120 You can look on the syllabus. 00:07:52.120 --> 00:07:55.840 It's about halfway through the second part of the class. 00:07:56.860 --> 00:07:58.950 And there's a lot of details here. 00:07:58.950 --> 00:08:04.130 And it asks you what's updated since an original model was 00:08:04.130 --> 00:08:08.650 postulated by a researcher called [? Jeffrus ?]. 00:08:08.650 --> 00:08:11.600 So that's a paper-- I don't think 00:08:11.600 --> 00:08:13.940 I said how long it should be. 00:08:13.940 --> 00:08:16.250 How long was the paper for vision? 00:08:16.250 --> 00:08:16.960 Was there a link? 00:08:18.535 --> 00:08:19.410 AUDIENCE: [INAUDIBLE] 00:08:19.410 --> 00:08:20.770 PROFESSOR: Four to six pages? 00:08:20.770 --> 00:08:23.165 OK, four pages sounds good. 00:08:23.165 --> 00:08:25.820 If you really want to write six, you probably could. 00:08:29.070 --> 00:08:31.480 But we'll talk about this when we 00:08:31.480 --> 00:08:34.020 talk about sound localization in the class. 00:08:34.020 --> 00:08:37.480 And I think the due date here is written. 00:08:37.480 --> 00:08:39.210 It's the date of the lab tour. 00:08:42.100 --> 00:08:44.340 And then, we have a final exam in the class. 00:08:44.340 --> 00:08:47.010 And I think as Doctor Schiller talked about 00:08:47.010 --> 00:08:49.440 at the very first day, the final exam 00:08:49.440 --> 00:08:52.890 will be waited toward the auditory system, 00:08:52.890 --> 00:08:56.181 which we haven't had a test on by the time this exam rolls 00:08:56.181 --> 00:08:56.680 around. 00:08:56.680 --> 00:09:00.340 So I think it's going to be 2/3 audition 00:09:00.340 --> 00:09:02.960 on the final exam, and 1/3 vision. 00:09:02.960 --> 00:09:05.070 And there are several review sessions 00:09:05.070 --> 00:09:08.960 for both senses planned at the end of the semester. 00:09:11.210 --> 00:09:13.822 So any questions about the organization 00:09:13.822 --> 00:09:15.340 of what we're going to do? 00:09:18.130 --> 00:09:21.100 OK, so I'll start today's lecture. 00:09:30.780 --> 00:09:34.910 And I think the PowerPoint files-- for today's lecture 00:09:34.910 --> 00:09:38.280 and all the rest of the lectures for the rest of the semester 00:09:38.280 --> 00:09:40.260 are available in the course website. 00:09:40.260 --> 00:09:45.380 So you can look at them now or as the lecture comes up. 00:09:46.390 --> 00:09:48.820 So first, we're going to talk about 00:09:48.820 --> 00:09:51.820 the physical characteristics of sound-- 00:09:51.820 --> 00:09:54.450 just very, very different than the characteristics 00:09:54.450 --> 00:09:55.755 of the light stimulus. 00:09:56.880 --> 00:10:02.550 And maybe light stimuli are so obvious that Peter Schiller 00:10:02.550 --> 00:10:05.450 probably didn't spend much time in his lecture about it. 00:10:05.450 --> 00:10:08.450 But I'm going to spend 10 or 15 minutes here 00:10:08.450 --> 00:10:10.290 on the physical characteristics of sound. 00:10:10.290 --> 00:10:13.720 Because it's very different than the light stimulus. 00:10:13.720 --> 00:10:19.880 So sound is a mechanical, radiated energy, 00:10:19.880 --> 00:10:24.660 transmitted by longitudinal vibrations of a m 00:10:24.660 --> 00:10:27.485 so you have to have a medium to transmit sound. 00:10:28.520 --> 00:10:30.490 You can have light go through outer space 00:10:30.490 --> 00:10:31.415 in a complete vacuum. 00:10:32.630 --> 00:10:36.379 But in outer space, you can't have sound 00:10:36.379 --> 00:10:37.795 because you have to have a medium. 00:10:39.180 --> 00:10:42.510 The medium can be various types of things. 00:10:42.510 --> 00:10:44.690 We're going to talk mostly about sound in air. 00:10:45.840 --> 00:10:49.830 But you could have sound in water-- whales make songs, 00:10:49.830 --> 00:10:54.120 and they sing to each other, and one whale listens to another. 00:10:54.120 --> 00:10:56.640 And in between the two is a medium of water. 00:10:56.640 --> 00:11:01.670 You can have sound in a solid-- if you live in an apartment 00:11:01.670 --> 00:11:06.330 room, and you hear your neighbors' music, you 00:11:06.330 --> 00:11:07.500 especially here the base. 00:11:07.500 --> 00:11:11.090 Because the low frequency sound transmits pretty well 00:11:11.090 --> 00:11:14.380 through solids, like the solid of the wall 00:11:14.380 --> 00:11:15.635 in between the two apartments. 00:11:17.600 --> 00:11:21.240 Sound can go in many, many different types of media. 00:11:21.240 --> 00:11:26.220 In air, like we're going to use mostly for this course, 00:11:26.220 --> 00:11:28.430 you can think of sound is being produced 00:11:28.430 --> 00:11:32.630 by a sound source like the piston of your loudspeaker. 00:11:33.960 --> 00:11:37.240 And the piston goes back and forth. . 00:11:37.240 --> 00:11:40.840 It's driven back and forth by an electric voltage 00:11:40.840 --> 00:11:44.720 and when it goes this way, it presses on the air molecules 00:11:44.720 --> 00:11:46.440 in front of it. 00:11:46.440 --> 00:11:49.640 And it presses them so they're closer together 00:11:49.640 --> 00:11:51.795 and makes them a little bit higher in pressure. 00:11:53.150 --> 00:11:58.330 And that's what's meant by this compression or condensation. 00:11:58.330 --> 00:12:01.140 And these dots close together means a little bit 00:12:01.140 --> 00:12:02.335 of an area of high pressure. 00:12:03.800 --> 00:12:06.730 Then as the piston moves the other direction, 00:12:06.730 --> 00:12:08.590 it rarefies the air. 00:12:08.590 --> 00:12:11.020 It drags some of the air with it. 00:12:11.020 --> 00:12:13.320 And so that little space right in front of the piston 00:12:13.320 --> 00:12:15.140 has a lower pressure. 00:12:15.140 --> 00:12:19.210 Because there are fewer molecules per volume 00:12:19.210 --> 00:12:19.765 than before. 00:12:21.060 --> 00:12:28.420 So this energy, then, is transmitted through the medium 00:12:28.420 --> 00:12:32.700 to whatever-- a microphone, which detects sound, 00:12:32.700 --> 00:12:35.290 or a listener, which can listen to the sound. 00:12:37.290 --> 00:12:41.870 If you have a microphone or some kind of detector that 00:12:41.870 --> 00:12:45.390 can plot that pressure at any one point-- 00:12:45.390 --> 00:12:47.312 let's say the microphone is right at the edge 00:12:47.312 --> 00:12:47.895 of this paper. 00:12:48.910 --> 00:12:53.980 And you graph the pressure as a function of time on this graph. 00:12:53.980 --> 00:12:56.385 So here's pressure and here's time. 00:12:57.590 --> 00:13:02.040 As those radiated energy wave fronts pass you, 00:13:02.040 --> 00:13:03.860 the pressure will go up. 00:13:03.860 --> 00:13:05.070 And then it will go down. 00:13:05.070 --> 00:13:07.150 And then it will go up and go down. 00:13:07.150 --> 00:13:10.045 And it will repeat over and over as long as that piston moves. 00:13:12.030 --> 00:13:15.500 So this horizontal line is simply 00:13:15.500 --> 00:13:20.140 the barometric or static pressure of the air. 00:13:20.140 --> 00:13:23.210 And sure, the barometric pressure changes a little bit. 00:13:23.210 --> 00:13:27.060 If there's a hurricane coming, it gets way low. 00:13:27.060 --> 00:13:30.100 If there's a high pressure like we have right now-- 00:13:30.100 --> 00:13:32.260 sunny climate, the barometric pressure goes up. 00:13:33.300 --> 00:13:35.186 But those are very slow fluctuations. 00:13:36.350 --> 00:13:41.340 And the sound wave form is a very, very fast waveform 00:13:41.340 --> 00:13:43.960 that goes many times per second. 00:13:44.990 --> 00:13:48.580 In fact, what we call as sound frequency 00:13:48.580 --> 00:13:53.945 is the number of oscillations of that pressure wave per second. 00:13:55.530 --> 00:13:58.650 And they are very fast. 00:13:58.650 --> 00:14:03.480 As you can see down here on this so-called audiogram 00:14:03.480 --> 00:14:06.820 or frequency curve for human hearing, 00:14:06.820 --> 00:14:10.290 the frequencies, which are on the x-axis here, 00:14:10.290 --> 00:14:19.490 go from 10 Hertz-- Hertz means cycles per second-- 00:14:19.490 --> 00:14:26.250 so one Hertz is one cycle per second. 00:14:28.540 --> 00:14:30.270 And that is a frequency that's so 00:14:30.270 --> 00:14:31.610 low it didn't get on this graph. 00:14:32.930 --> 00:14:34.750 Because humans aren't sensitive to 00:14:34.750 --> 00:14:37.970 frequencies that slow or that low. 00:14:37.970 --> 00:14:40.550 Usually the lower limit for human hearing 00:14:40.550 --> 00:14:44.640 is considered to be about 10 cycles per second, or 10 Hertz. 00:14:44.640 --> 00:14:51.290 And it extends all the way up to 20,000 cycles per second. 00:14:51.290 --> 00:14:52.910 And in the middle of the human range, 00:14:52.910 --> 00:14:54.440 we'll be talking about hearing a lot 00:14:54.440 --> 00:14:57.640 at a middle frequency of about 1,000 Hertz. 00:14:57.640 --> 00:15:01.890 So that's a nice, round, middle frequency for you 00:15:01.890 --> 00:15:03.900 to remember for human hearing. 00:15:05.810 --> 00:15:08.620 So we're talking about pressure oscillations 00:15:08.620 --> 00:15:11.900 in terms of thousands of times per second, 00:15:11.900 --> 00:15:13.436 or hundreds of times per second. 00:15:13.436 --> 00:15:14.310 So they're very fast. 00:15:16.360 --> 00:15:19.020 There will be examples during our course 00:15:19.020 --> 00:15:22.210 where the auditory system-- the auditory neurons 00:15:22.210 --> 00:15:24.660 keep track of those cycles, even though they're 00:15:24.660 --> 00:15:28.160 going back and forth thousands of times per second. 00:15:29.710 --> 00:15:31.710 So we'll come back to that in future lectures. 00:15:34.530 --> 00:15:38.000 Now this is the audiogram for human hearing 00:15:38.000 --> 00:15:39.280 in the solid curve here. 00:15:39.280 --> 00:15:42.290 This is supposed to say human, if you could read it. 00:15:42.290 --> 00:15:48.350 And on the y-axis is how strong the stimulus 00:15:48.350 --> 00:15:55.250 is, how loud it is, or in terms of physical characteristics, 00:15:55.250 --> 00:15:58.390 what the sound pressure is. 00:15:58.390 --> 00:16:01.950 And this scale goes from minus 20 to 140. 00:16:01.950 --> 00:16:06.150 And the units are dB SPL and that 00:16:06.150 --> 00:16:10.605 stands for decibels sound pressure level. 00:16:12.220 --> 00:16:14.600 And whenever you hear level in a formula, 00:16:14.600 --> 00:16:18.910 you should perk up your ears and say oh, that 00:16:18.910 --> 00:16:22.170 means there's a log-- a logarithm-- in the formula. 00:16:22.170 --> 00:16:25.790 And sure enough, the formula for a sound pressure level 00:16:25.790 --> 00:16:31.124 is 20 times the log of whatever sound pressure 00:16:31.124 --> 00:16:32.540 you're talking about, whatever you 00:16:32.540 --> 00:16:37.350 were listening to or measured by your microphone divided 00:16:37.350 --> 00:16:38.980 by some reference pressure. 00:16:40.520 --> 00:16:41.860 That's the formula. 00:16:41.860 --> 00:16:43.520 And the reference pressure is given 00:16:43.520 --> 00:16:47.670 as 20 micronewtons per square meter. 00:16:47.670 --> 00:16:50.040 OK, so let's figure that out. 00:16:50.040 --> 00:16:53.660 What is Newton a unit of? 00:16:53.660 --> 00:16:54.160 Anybody? 00:16:54.160 --> 00:16:54.826 AUDIENCE: Force. 00:16:54.826 --> 00:16:58.950 PROFESSOR: Right, force-- and meter squared is area. 00:16:58.950 --> 00:17:03.645 So we're talking about force per area, and that's pressure. 00:17:05.609 --> 00:17:09.060 So Newton obviously was like Hertz, one of the people 00:17:09.060 --> 00:17:10.380 who was interested in physics. 00:17:12.079 --> 00:17:17.010 And a Newton is a unit of force per square meter is pressure. 00:17:17.010 --> 00:17:21.380 Now in more modern terms, the unit micronewton 00:17:21.380 --> 00:17:23.369 per square meter has been renamed 00:17:23.369 --> 00:17:33.340 to be a pascal, abbreviated Pa. 00:17:35.630 --> 00:17:36.530 So it's the same. 00:17:36.530 --> 00:17:41.630 One Pascal is one Newton per square meter. 00:17:41.630 --> 00:17:44.100 In this case, we're talking about micro-- Newtons 00:17:44.100 --> 00:17:45.875 are micro Pascals. 00:17:51.540 --> 00:17:57.520 So why is that number chosen as the reference 00:17:57.520 --> 00:18:00.070 for this very important sound pressure level scale? 00:18:00.070 --> 00:18:04.975 Well, it's actually chosen with the hearing system in mind. 00:18:06.910 --> 00:18:11.930 What they did in the 1930s, when this was being developed, 00:18:11.930 --> 00:18:14.500 is they rounded up a bunch of people at a county fair, 00:18:14.500 --> 00:18:16.600 gave them headphones, and said we're 00:18:16.600 --> 00:18:20.330 going to try a nice mid-frequency. 00:18:20.330 --> 00:18:23.200 Let's try 1,000 Hertz. 00:18:23.200 --> 00:18:25.110 They gave them a tone at 1,000 Hertz. 00:18:25.110 --> 00:18:27.290 The listeners listened to it. 00:18:27.290 --> 00:18:29.430 Then they said I can hear that fine. 00:18:29.430 --> 00:18:31.930 Then they turned the level down a little bit. 00:18:31.930 --> 00:18:33.930 And the person said yeah, I can still hear that. 00:18:33.930 --> 00:18:36.980 Then they turned it down so much that the person didn't say, 00:18:36.980 --> 00:18:38.530 I hear something. 00:18:38.530 --> 00:18:40.060 There were silent. 00:18:40.060 --> 00:18:41.840 They turned it up a little-- says yeah, 00:18:41.840 --> 00:18:43.730 I hear-- They turned it down. 00:18:43.730 --> 00:18:47.320 They titrated the levels until it was right at threshold, 00:18:47.320 --> 00:18:48.555 just barely detectable. 00:18:50.280 --> 00:18:53.420 And they took an average of 30-some people. 00:18:53.420 --> 00:18:56.240 And they said that is going to be 00:18:56.240 --> 00:18:59.910 the basis of our sound pressure level scale. 00:18:59.910 --> 00:19:03.940 So it's actually a term that was derived biologically 00:19:03.940 --> 00:19:05.810 by testing people's hearing. 00:19:08.250 --> 00:19:11.870 So that's kind of a nice story. 00:19:11.870 --> 00:19:13.030 I wonder if it's true. 00:19:13.030 --> 00:19:14.780 Well, let's look at it. 00:19:14.780 --> 00:19:22.930 Where does the human hearing curve, that 1,000 Hertz, fall? 00:19:22.930 --> 00:19:28.850 Where should it fall if 20 micronewtons per square meter 00:19:28.850 --> 00:19:30.510 is the pressure you're talking about? 00:19:30.510 --> 00:19:32.440 It's the same as the reference pressure. 00:19:32.440 --> 00:19:34.050 What's 20 over 20? 00:19:34.050 --> 00:19:35.360 It's 1. 00:19:35.360 --> 00:19:36.705 What's the log of 1? 00:19:41.040 --> 00:19:41.730 Zero. 00:19:41.730 --> 00:19:42.600 Correct. 00:19:42.600 --> 00:19:47.432 20 times the log of 1 is 0-- sound pressure level 0. 00:19:47.432 --> 00:19:48.890 Well, look at our curve right here, 00:19:48.890 --> 00:19:52.540 that 1,000 Hertz-- it's pretty close to 0. 00:19:53.570 --> 00:19:55.920 Why might it not be exactly zero? 00:19:55.920 --> 00:19:58.014 Well the people that were used for this curve 00:19:58.014 --> 00:20:00.555 were a little bit different than the ones in the county fair. 00:20:02.300 --> 00:20:05.615 We'll study later on that some people have a hearing loss. 00:20:06.950 --> 00:20:10.354 Hearing can be affected by the room that you used. 00:20:10.354 --> 00:20:12.270 Maybe there was a lot of yelling and screaming 00:20:12.270 --> 00:20:13.225 at the county fair. 00:20:15.350 --> 00:20:17.545 We have better rooms to test hearing now. 00:20:18.730 --> 00:20:22.140 It turns out that the human hearing curve is actually 00:20:22.140 --> 00:20:26.720 a little more sensitive at 2,000, 3,000, and maybe 4,000. 00:20:26.720 --> 00:20:31.510 So when the pressures go below the reference pressure, 00:20:31.510 --> 00:20:34.030 the number becomes less than 1. 00:20:34.030 --> 00:20:36.620 And the logarithm becomes negative. 00:20:36.620 --> 00:20:40.290 It's perfectly fine to have a negative SPL. 00:20:40.290 --> 00:20:45.500 We have some points on the graph for that-- minus 2, minus 3 dB. 00:20:46.720 --> 00:20:51.630 This other dashed audiogram, or hearing sensitivity curve, 00:20:51.630 --> 00:20:53.890 is for a different species-- the cat. 00:20:53.890 --> 00:20:56.770 And the cat here's down to about minus 10 00:20:56.770 --> 00:21:00.490 dB SPL-- at least this group of cats did. 00:21:00.490 --> 00:21:04.210 The cats also hear higher in frequency than humans. 00:21:05.350 --> 00:21:07.650 Dogs and cats can hear about an octave 00:21:07.650 --> 00:21:10.730 higher-- that is a doubling of frequency 00:21:10.730 --> 00:21:13.200 higher than humans do, and maybe some of you 00:21:13.200 --> 00:21:15.990 have had dog whistles that you blow. 00:21:15.990 --> 00:21:17.210 And you don't hear anything. 00:21:17.210 --> 00:21:21.760 But the dog comes because it's a very high frequency 00:21:21.760 --> 00:21:24.280 beyond the upper limit of human hearing, 00:21:24.280 --> 00:21:28.275 but well within the hearing range of those species. 00:21:30.900 --> 00:21:33.760 So different species have different hearing ranges. 00:21:37.854 --> 00:21:38.775 AUDIENCE: Professor? 00:21:38.775 --> 00:21:39.400 PROFESSOR: Yes. 00:21:39.400 --> 00:21:42.352 AUDIENCE: Sorry-- just to clarify, 00:21:42.352 --> 00:21:46.288 is a micropascal then [INAUDIBLE]? 00:21:46.288 --> 00:21:47.280 PROFESSOR: No. 00:21:47.280 --> 00:21:54.070 These are units of pressure-- micronewtons per square meter-- 00:21:54.070 --> 00:21:56.395 and this is a unit of pressure. 00:21:58.310 --> 00:22:02.455 SPL is just in these units called decibels. 00:22:03.740 --> 00:22:05.769 And it it's not a pressure-- 00:22:05.769 --> 00:22:07.060 AUDIENCE: It's the log of that. 00:22:07.060 --> 00:22:07.600 PROFESSOR: That's right. 00:22:07.600 --> 00:22:08.675 It's the log of that. 00:22:10.767 --> 00:22:11.600 Any other questions? 00:22:16.000 --> 00:22:20.700 So these are sort of the lower limits of hearing. 00:22:20.700 --> 00:22:26.020 When you go into conversational levels, or the level of a lawn 00:22:26.020 --> 00:22:30.870 mower, or the level of a concert, 00:22:30.870 --> 00:22:33.390 the levels get higher-- still certainly 00:22:33.390 --> 00:22:35.850 within your audibility range. 00:22:35.850 --> 00:22:38.700 As you go to a higher and higher level, 00:22:38.700 --> 00:22:41.190 you risk damage to your hearing. 00:22:41.190 --> 00:22:45.690 And at that risk level, which it says high risk thresholds here. 00:22:45.690 --> 00:22:50.790 And right around 120 dB, sounds become painfully 00:22:50.790 --> 00:22:54.400 loud and damaging to your hearing. 00:22:54.400 --> 00:22:58.600 And that's what this shaded area refers 00:22:58.600 --> 00:23:00.960 to-- gunshots, jet aircraft engine. 00:23:00.960 --> 00:23:03.760 And we'll talk about that during our lecture of hearing loss. 00:23:09.580 --> 00:23:10.933 So I have some demonstrations. 00:23:14.020 --> 00:23:17.880 Because a lot of people have trouble with the decibel scale. 00:23:17.880 --> 00:23:19.350 So what is a decibel? 00:23:19.350 --> 00:23:23.840 And what does it sound like when you change the sound from 50 00:23:23.840 --> 00:23:26.820 dB to 60 dB? 00:23:26.820 --> 00:23:28.710 Well this demonstration has three parts. 00:23:29.880 --> 00:23:32.670 And let me read the text first. 00:23:34.060 --> 00:23:39.120 Broadband noise-- sometimes it's called white noise. 00:23:39.120 --> 00:23:41.840 Broadband noise and white noise are synonyms. 00:23:41.840 --> 00:23:45.337 And what is white light as a visual stimulus? 00:23:45.337 --> 00:23:46.420 AUDIENCE: All wavelengths. 00:23:46.420 --> 00:23:48.350 PROFESSOR: All wavelengths, right? 00:23:48.350 --> 00:23:51.862 And so broadband noise means it has all frequencies. 00:23:51.862 --> 00:23:55.140 It has 10 Hertz, 20 Hertz, 30 Hertz, 1,000 00:23:55.140 --> 00:23:57.565 Hertz, 2,000-- it has all frequencies. 00:23:58.620 --> 00:24:01.270 And it sounds like the "shh" sound. 00:24:03.220 --> 00:24:08.420 So you hear this "shh." it'll start out pretty loud. 00:24:08.420 --> 00:24:15.220 It'll be reduced in ten steps of six decibels for each step. 00:24:15.220 --> 00:24:18.211 And I think you'll be able to very clearly hear 00:24:18.211 --> 00:24:20.460 the difference between the first and the second steps. 00:24:21.680 --> 00:24:24.500 And demonstrations are repeated once. 00:24:24.500 --> 00:24:27.900 The second demonstration is same noise 00:24:27.900 --> 00:24:31.680 is reduced in 15 steps-- now of three decibels. 00:24:31.680 --> 00:24:35.290 So this is a little bit of a smaller scale, 00:24:35.290 --> 00:24:37.240 though you'll still be clearly audible. 00:24:38.580 --> 00:24:40.450 Third, broadband noise is reduced 00:24:40.450 --> 00:24:42.880 in 20 steps of now one dB. 00:24:42.880 --> 00:24:46.255 So let's listen to see if we can hear 1 dB steps. 00:24:49.568 --> 00:24:52.526 RECORDING: The decibel scale-- broadband 00:24:52.526 --> 00:24:57.949 noise is reduced in 10 sets of 6 decibels. 00:24:57.949 --> 00:25:00.907 [INAUDIBLE] repeated once. 00:25:00.907 --> 00:25:12.290 [TONE] [TONE] 00:25:12.290 --> 00:25:14.787 OK, was that clear-- the difference 00:25:14.787 --> 00:25:15.870 between one and the other? 00:25:15.870 --> 00:25:17.920 So that's what 6 dB sounds like? 00:25:17.920 --> 00:25:21.780 Now, you guys who are up here close to the speakers, 00:25:21.780 --> 00:25:26.240 you might be starting at 85 dB SPL on the first ones-- pretty 00:25:26.240 --> 00:25:27.540 loud. 00:25:27.540 --> 00:25:28.976 6 dB lower is 79. 00:25:30.390 --> 00:25:31.780 And then, so on and so forth. 00:25:31.780 --> 00:25:33.960 You guys at the back are further from the speaker. 00:25:33.960 --> 00:25:36.190 You're not starting at the same level. 00:25:36.190 --> 00:25:39.520 You might be starting at 60 dB. 00:25:39.520 --> 00:25:43.700 You're still going down 6 dB to 54 dB in the next step. 00:25:43.700 --> 00:25:45.200 Everything is linear in here. 00:25:45.200 --> 00:25:47.980 It doesn't matter where you start from, 00:25:47.980 --> 00:25:50.150 as long as you're going down 6 dB. 00:25:50.150 --> 00:25:53.830 So where you start doesn't really matter in these demos. 00:25:53.830 --> 00:25:55.790 RECORDING: Broadbad noise is reduced 00:25:55.790 --> 00:25:59.710 in 15 steps of 3 decibels. 00:25:59.710 --> 00:26:16.880 [TONE] [TONE] 00:26:16.880 --> 00:26:18.817 PROFESSOR: OK, still clear the increment 00:26:18.817 --> 00:26:19.900 between one and the other? 00:26:19.900 --> 00:26:22.480 OK, now here's the one dB steps. 00:26:22.480 --> 00:26:24.021 RECORDING: Broadband noise is reduced 00:26:24.021 --> 00:26:26.967 in 20 steps of one decibel. 00:26:28.440 --> 00:26:52.150 [TONE] [TONE] 00:26:52.150 --> 00:26:54.270 PROFESSOR: OK so how about for that? 00:26:54.270 --> 00:26:56.740 Would you be able to stake your life on the fact 00:26:56.740 --> 00:26:58.700 that you could tell one from another? 00:26:58.700 --> 00:27:00.110 No, I see a lot of heads shaking. 00:27:01.210 --> 00:27:05.390 Well if you sit there and do this over and over again, 00:27:05.390 --> 00:27:09.870 and really train yourself, apparently 1 dB 00:27:09.870 --> 00:27:11.950 is the just noticeable difference 00:27:11.950 --> 00:27:14.665 that most observers can here. 00:27:16.230 --> 00:27:27.465 So 1 dB is the just noticeable difference in SPL. 00:27:48.790 --> 00:27:50.860 So how do we do that? 00:27:50.860 --> 00:27:53.490 Well you have an auditory nerve. 00:27:53.490 --> 00:27:57.740 And at 60 dB, your auditory nerve fibers 00:27:57.740 --> 00:27:59.730 are sending this many spikes to the brain. 00:27:59.730 --> 00:28:05.340 At 61 dB, they're sending maybe a few more spikes-- something 00:28:05.340 --> 00:28:05.840 like that. 00:28:05.840 --> 00:28:08.040 It's not absolutely clear how you do that. 00:28:08.040 --> 00:28:12.060 There is more information coming in from the ear to the brain 00:28:12.060 --> 00:28:13.580 as a function on sound level. 00:28:13.580 --> 00:28:14.746 We'll talk a lot about that. 00:28:15.490 --> 00:28:20.915 Now, we also talked about sound frequency. 00:28:22.750 --> 00:28:25.640 JND for sound level is about 1 dB. 00:28:25.640 --> 00:28:27.110 What is it for sound frequency? 00:28:27.110 --> 00:28:29.960 We're going to have pretty much a whole lecture on that. 00:28:31.010 --> 00:28:35.620 But your ear is extremely good at telling one frequency 00:28:35.620 --> 00:28:37.000 from another. 00:28:37.000 --> 00:28:44.390 So if you start at 1,000 Hertz and change it to 1,002 00:28:44.390 --> 00:28:47.560 Hertz-- very, very small change-- 00:28:47.560 --> 00:28:49.522 you can tell the difference. 00:28:49.522 --> 00:28:52.435 Your ear is a fantastic frequency analyzer. 00:28:53.442 --> 00:28:54.900 We're going to have a whole lecture 00:28:54.900 --> 00:28:57.440 on exactly how your ear does that. 00:28:57.440 --> 00:29:01.490 But the JND for sound frequency is also a good demonstration. 00:29:01.490 --> 00:29:05.560 We'll play that when we talk about sound frequency coding. 00:29:07.551 --> 00:29:08.050 OK. 00:29:08.050 --> 00:29:09.715 Any questions about that so far? 00:29:14.980 --> 00:29:15.860 OK. 00:29:15.860 --> 00:29:20.075 Let's switch back to the physical characteristics 00:29:20.075 --> 00:29:20.575 of sound. 00:29:21.740 --> 00:29:24.140 And these are some very common auditory stimuli. 00:29:25.560 --> 00:29:28.140 We've heard a noise just now. 00:29:28.140 --> 00:29:33.450 And if you graph the sound pressure as a function of time, 00:29:33.450 --> 00:29:35.470 this is what the waveform looks like. 00:29:36.520 --> 00:29:37.730 How could you do that? 00:29:37.730 --> 00:29:39.670 If you take a microphone, stick it out 00:29:39.670 --> 00:29:43.970 in front of a noise source, and run that into an oscilloscope, 00:29:43.970 --> 00:29:48.650 the microphone converts the sound pressure into a voltage, 00:29:48.650 --> 00:29:51.900 the oscilloscope displays the voltage signal 00:29:51.900 --> 00:29:52.960 as a function of time. 00:29:52.960 --> 00:29:54.102 You can look at that. 00:29:55.580 --> 00:29:58.150 Auditory scientists like to look at things 00:29:58.150 --> 00:30:00.750 as a function of time, of course. 00:30:00.750 --> 00:30:02.770 They also like to look at things as a function 00:30:02.770 --> 00:30:04.275 of sound frequency. 00:30:07.240 --> 00:30:12.400 This is a graph for this same stimulus, a noise stimulus, 00:30:12.400 --> 00:30:14.320 now as a function of frequency. 00:30:14.320 --> 00:30:17.890 And we said before, the noise is broadband. 00:30:17.890 --> 00:30:18.790 It's white noise. 00:30:18.790 --> 00:30:20.250 It has all frequencies. 00:30:20.250 --> 00:30:22.640 And here is the graph to show you that. 00:30:22.640 --> 00:30:25.330 This might be the energy, and this 00:30:25.330 --> 00:30:27.310 is as a function of frequency. 00:30:27.310 --> 00:30:28.590 So it has all frequencies. 00:30:28.590 --> 00:30:31.400 It's trailing off a little at the very highest. 00:30:31.400 --> 00:30:34.450 That may be because the microphone couldn't wiggle 00:30:34.450 --> 00:30:37.490 back and forth at very, very high frequencies. 00:30:37.490 --> 00:30:40.690 But it's essentially a flat frequency curve. 00:30:40.690 --> 00:30:44.213 And sometimes this display is called the spectrum. 00:30:50.250 --> 00:30:54.724 So spectrum or spectra are graphs as a function 00:30:54.724 --> 00:30:55.265 of frequency. 00:30:56.750 --> 00:30:59.000 Sometimes people talk about this as a frequency 00:30:59.000 --> 00:31:01.940 domain and the time domain. 00:31:01.940 --> 00:31:04.960 If you've taken any electrical engineering courses here 00:31:04.960 --> 00:31:09.660 at MIT, people will talk about the time and frequency domains. 00:31:09.660 --> 00:31:12.920 And how can you go from one representation to another? 00:31:12.920 --> 00:31:14.720 Well, you can take your microphone signal 00:31:14.720 --> 00:31:16.710 instead of going to the oscilloscope, 00:31:16.710 --> 00:31:18.860 going to the spectrum analyzer, which 00:31:18.860 --> 00:31:21.660 is a machine that can give you this nice plot. 00:31:21.660 --> 00:31:23.390 But how about mathematically? 00:31:23.390 --> 00:31:24.500 How can you do that? 00:31:25.860 --> 00:31:27.460 The Fourier Transform, right. 00:31:41.750 --> 00:31:45.470 Of course, Fourier was a mathematician 00:31:45.470 --> 00:31:50.450 who studied various things, heat transfer and other things. 00:31:50.450 --> 00:31:52.030 He developed this transformation. 00:31:52.030 --> 00:31:54.240 If you have the mathematical description 00:31:54.240 --> 00:31:57.290 of a time-varying signal, you can 00:31:57.290 --> 00:32:00.790 plug it through his equation, the Fourier transform, 00:32:00.790 --> 00:32:04.520 and come out with the frequency representation or the frequency 00:32:04.520 --> 00:32:05.020 domain. 00:32:06.070 --> 00:32:08.740 Or, vice versa, if you have the frequency domain, 00:32:08.740 --> 00:32:10.729 you can inverse Fourier transform and go back 00:32:10.729 --> 00:32:11.520 to the time domain. 00:32:13.470 --> 00:32:16.140 We're not going to talk too much about transforms here. 00:32:16.140 --> 00:32:21.140 But it is interesting, because, as it turns out, 00:32:21.140 --> 00:32:25.910 your inner ear is a wonderful frequency analyzer. 00:32:25.910 --> 00:32:30.350 It can tell the difference between 1,000 and 1,002 Hertz. 00:32:30.350 --> 00:32:33.380 This is a very nice way in the ear 00:32:33.380 --> 00:32:35.170 of detecting the different frequencies. 00:32:35.170 --> 00:32:39.780 And so these time and frequency domain representations 00:32:39.780 --> 00:32:41.720 are very convenient for us to look at. 00:32:41.720 --> 00:32:43.010 So just keep that in mind. 00:32:44.780 --> 00:32:50.570 Here's a very common auditory stimulus, the pure tone 00:32:50.570 --> 00:32:51.330 or the sinusoid. 00:32:53.470 --> 00:32:56.910 This is a sinusoidal waveform in the time domain. 00:32:56.910 --> 00:33:00.866 In the frequency domain, it only has one frequency-- 00:33:00.866 --> 00:33:02.240 the frequency at which that thing 00:33:02.240 --> 00:33:05.050 is going back and forth in terms of Hertz. 00:33:06.510 --> 00:33:08.230 This is in a Hertz axis. 00:33:09.240 --> 00:33:12.050 So sometimes it's called a pure tone. 00:33:12.050 --> 00:33:13.060 Why is it so pure? 00:33:14.080 --> 00:33:16.230 Does it have high morals or what? 00:33:16.230 --> 00:33:19.255 No, it just has one sound frequency. 00:33:21.910 --> 00:33:24.900 These other stimuli, we're going to listen 00:33:24.900 --> 00:33:26.110 to this in just a minute. 00:33:26.110 --> 00:33:27.945 This is a so-called square wave. 00:33:31.160 --> 00:33:37.910 Imagine trying to add up a whole bunch of pure tones 00:33:37.910 --> 00:33:40.030 to result in a square wave. 00:33:40.030 --> 00:33:41.660 It seems impossible, right? 00:33:42.780 --> 00:33:45.770 Well, it's possible if you use an infinite number 00:33:45.770 --> 00:33:46.570 of frequencies. 00:33:46.570 --> 00:33:48.690 And this frequency representation 00:33:48.690 --> 00:33:52.550 for a square wave goes on basically forever. 00:33:52.550 --> 00:33:55.090 To get those corners of the square wave 00:33:55.090 --> 00:33:58.380 sharp like a true square wave, you 00:33:58.380 --> 00:34:01.560 need lots of individual frequencies, 00:34:01.560 --> 00:34:03.770 lots of pure tones, if you will. 00:34:06.360 --> 00:34:10.468 Tone bursts are some common auditory stimuli. 00:34:10.468 --> 00:34:12.259 We'll talk about those later in the course. 00:34:13.560 --> 00:34:16.710 Click is a very common auditory stimulus. 00:34:16.710 --> 00:34:18.860 It's a sound like this. 00:34:18.860 --> 00:34:23.250 Or last night, it was the sound of a fastball 00:34:23.250 --> 00:34:24.724 hitting a wooden baseball bat. 00:34:26.530 --> 00:34:30.210 It's a very sharp, impulsive sound, very nice sound 00:34:30.210 --> 00:34:32.400 if you're behind the team who's batting. 00:34:34.489 --> 00:34:37.300 So a click, that baseball hitting the bat, 00:34:37.300 --> 00:34:39.290 doesn't happen for very long. 00:34:39.290 --> 00:34:43.539 A click can be infinitesimally short. 00:34:45.210 --> 00:34:47.480 The time that the baseball is in contact with the bat 00:34:47.480 --> 00:34:48.409 is pretty short. 00:34:49.929 --> 00:34:52.780 And if it's very short in the time domain, 00:34:52.780 --> 00:34:54.394 then you have all frequencies. 00:34:55.870 --> 00:35:00.290 So it's another example of a broadband or broad spectrum 00:35:00.290 --> 00:35:01.590 sound. 00:35:01.590 --> 00:35:04.310 If the click is infinitesimally short, 00:35:04.310 --> 00:35:06.580 the spectrum is completely flat. 00:35:09.510 --> 00:35:11.240 Those are some common auditory stimuli. 00:35:14.720 --> 00:35:19.870 Let's go through some more complicated, 00:35:19.870 --> 00:35:22.170 and maybe more interesting, sounds. 00:35:22.170 --> 00:35:25.420 Well, all of us like to listen to music, right? 00:35:25.420 --> 00:35:28.900 So here are some examples of musical sounds. 00:35:28.900 --> 00:35:31.006 This is a piano keyboard. 00:35:32.790 --> 00:35:38.000 And here is the spectrum or frequency representation 00:35:38.000 --> 00:35:43.290 of what you get when you strike one key on the piano keyboard. 00:35:43.290 --> 00:35:44.470 So that's one note. 00:35:46.360 --> 00:35:48.890 Well, sure, it sounds like one thing, 00:35:48.890 --> 00:35:52.910 but you have a whole bunch of different frequencies 00:35:52.910 --> 00:35:54.160 that go along with it. 00:35:54.160 --> 00:35:55.640 And why is that true? 00:35:55.640 --> 00:35:56.390 Does anybody know? 00:35:56.390 --> 00:35:58.598 Why do you get a whole bunch of different frequencies 00:35:58.598 --> 00:36:01.550 when you strike a key on the piano keyboard? 00:36:04.389 --> 00:36:04.889 Yeah? 00:36:04.889 --> 00:36:07.857 AUDIENCE: Isn't it vibrating all along the length 00:36:07.857 --> 00:36:12.330 so there's different wavelengths? 00:36:12.330 --> 00:36:13.555 PROFESSOR: What's vibrating-- 00:36:13.555 --> 00:36:14.735 AUDIENCE: It's not-- 00:36:14.735 --> 00:36:15.200 PROFESSOR: In the piano? 00:36:15.200 --> 00:36:17.650 AUDIENCE: It's not-- it's like an infinitely small portion 00:36:17.650 --> 00:36:19.210 of the string. 00:36:20.470 --> 00:36:22.812 It's the longer string. 00:36:22.812 --> 00:36:28.660 It's parts that are shorter still vibrating. 00:36:28.660 --> 00:36:31.240 PROFESSOR: Yeah, you're getting there. 00:36:31.240 --> 00:36:35.470 In the piano, the string is fixed at one end, 00:36:35.470 --> 00:36:37.415 and it's a long string. 00:36:37.415 --> 00:36:39.030 It [? fits ?] [? in ?] the other. 00:36:39.030 --> 00:36:43.900 And your key that you press down makes a hammer go up, 00:36:43.900 --> 00:36:45.340 and there's a bunch of linkages. 00:36:45.340 --> 00:36:48.450 And eventually, the hammer hits that string somewhere. 00:36:49.980 --> 00:36:51.970 And the string, it's fixed here. 00:36:51.970 --> 00:36:53.620 It's not going to move. 00:36:53.620 --> 00:36:54.370 It's fixed here. 00:36:54.370 --> 00:36:55.400 It's not going to move. 00:36:55.400 --> 00:36:58.670 But in between those points, it can move. 00:36:58.670 --> 00:37:04.070 So it can vibrate like this, or it can go up and down. 00:37:06.410 --> 00:37:08.240 It can also vibrate like this. 00:37:14.430 --> 00:37:16.860 You can have what's called a node in the middle. 00:37:18.300 --> 00:37:24.310 In fact, if you put your finger right here and fix that middle, 00:37:24.310 --> 00:37:26.470 it wouldn't allow the string to vibrate 00:37:26.470 --> 00:37:28.570 in this uniform fashion. 00:37:28.570 --> 00:37:31.460 But it would allow this half to vibrate and that half 00:37:31.460 --> 00:37:32.210 to vibrate. 00:37:32.210 --> 00:37:35.795 This node is sort of a constraint for this string. 00:37:35.795 --> 00:37:37.675 It can also vibrate like this. 00:37:38.739 --> 00:37:40.030 I wish I had a different color. 00:37:41.820 --> 00:37:42.497 Over here? 00:37:42.497 --> 00:37:42.997 Great. 00:37:48.530 --> 00:37:49.030 OK. 00:37:49.030 --> 00:37:56.930 You can also have the string vibrate like this. 00:37:59.265 --> 00:37:59.765 OK. 00:38:02.110 --> 00:38:05.010 And it can vibrate in many, many different patterns. 00:38:05.010 --> 00:38:06.700 I've just drawn a few. 00:38:06.700 --> 00:38:10.580 What's interesting is that this length is twice 00:38:10.580 --> 00:38:15.990 as long as this length, which is twice as long as this length. 00:38:15.990 --> 00:38:20.150 And what would you expect the time of those vibrations to be? 00:38:20.150 --> 00:38:23.920 Well, the big long thing is going to vibrate pretty slowly. 00:38:26.592 --> 00:38:28.550 That's what's called the fundamental frequency. 00:38:30.480 --> 00:38:32.970 The thing that's vibrating in two parts, it's shorter 00:38:32.970 --> 00:38:34.175 and it can vibrate faster. 00:38:35.690 --> 00:38:39.000 In fact, it vibrates twice as fast. 00:38:39.000 --> 00:38:44.000 So the first harmonic is twice the frequency 00:38:44.000 --> 00:38:49.040 of the fundamental, and so on and so forth. 00:38:49.040 --> 00:38:51.820 You can get from the physical characteristics 00:38:51.820 --> 00:38:55.710 of the vibration of that string a whole bunch 00:38:55.710 --> 00:38:58.320 of different vibration patterns. 00:38:58.320 --> 00:39:03.120 And they're usually a harmonic series-- twice, three times, 00:39:03.120 --> 00:39:07.880 four times, five times, six times-- the fundamental, 00:39:07.880 --> 00:39:10.630 just because of the physical characteristics of vibration 00:39:10.630 --> 00:39:14.320 of the string, and the wind column 00:39:14.320 --> 00:39:16.260 in the case of an Alto saxophone. 00:39:18.930 --> 00:39:23.730 When you hear that one note hit by the hammer, 00:39:23.730 --> 00:39:27.110 all of these vibrations are happening at once. 00:39:27.110 --> 00:39:32.910 And so that one sound sounds like one thing. 00:39:32.910 --> 00:39:38.527 Musicians will say it sounds like a note-- A above C. 00:39:38.527 --> 00:39:40.860 But you have a whole bunch of different harmonics in it. 00:39:45.150 --> 00:39:46.000 What is pitch? 00:39:47.410 --> 00:39:51.490 Pitch is very interesting to people 00:39:51.490 --> 00:39:54.320 who study the auditory system, to musicians. 00:39:54.320 --> 00:39:59.520 Pitch is that attribute of the sensation, auditory sensation, 00:39:59.520 --> 00:40:02.580 in terms of which sounds can be ordered on a musical scale. 00:40:04.080 --> 00:40:07.300 Let's say I didn't let you see the keyboard, 00:40:07.300 --> 00:40:10.910 but I recorded the sounds, and I press some sounds down there, 00:40:10.910 --> 00:40:14.870 some in the middle, some way up here, some way at the high end, 00:40:14.870 --> 00:40:17.145 and I gave you 20 different recordings, 00:40:17.145 --> 00:40:21.040 and I said, well, make a ranking of them. 00:40:21.040 --> 00:40:22.300 Put these down low. 00:40:22.300 --> 00:40:23.680 Those are number one and two. 00:40:24.720 --> 00:40:26.470 Put these in the middle-- those are number 00:40:26.470 --> 00:40:28.810 10-- up to the high end. 00:40:28.810 --> 00:40:30.390 The highest one is 20. 00:40:30.390 --> 00:40:31.435 You could do that. 00:40:33.020 --> 00:40:34.740 The ones that were down low would 00:40:34.740 --> 00:40:36.350 be called those with low pitch. 00:40:39.250 --> 00:40:41.120 The pitch of a pure tone, of course, 00:40:41.120 --> 00:40:43.080 depends on the frequency. 00:40:43.080 --> 00:40:45.240 That's as if you were just giving one. 00:40:46.410 --> 00:40:48.870 If you move that around, up high end frequency, 00:40:48.870 --> 00:40:52.020 it sounds like a really shrilly, high-pitched sound. 00:40:52.020 --> 00:40:54.430 If you move it down low, it sounds like a real low sound. 00:40:56.700 --> 00:41:00.280 The pitch of a complicated sound-- that is, 00:41:00.280 --> 00:41:03.150 with many overtones and harmonics-- 00:41:03.150 --> 00:41:06.170 depends strongly on the fundamental frequency. 00:41:06.170 --> 00:41:09.150 But sometimes, the fundamental-- for example, in this guitar 00:41:09.150 --> 00:41:10.135 sound-- is pretty weak. 00:41:11.510 --> 00:41:14.220 And in some cases, you can take it out altogether. 00:41:14.220 --> 00:41:16.640 The pitch doesn't change that much, surprisingly. 00:41:18.540 --> 00:41:21.835 So somehow, the ear knows by this pattern of spectrum 00:41:21.835 --> 00:41:23.960 that there should be a fundamental [INAUDIBLE] that 00:41:23.960 --> 00:41:24.834 can stick it back in. 00:41:27.040 --> 00:41:29.140 So that's what pitch is. 00:41:29.140 --> 00:41:32.210 Another sensation that musicians often talk about 00:41:32.210 --> 00:41:34.050 is the timbre of a sound. 00:41:34.050 --> 00:41:38.295 And the timbre is the quality or the identification of a sound. 00:41:39.620 --> 00:41:42.060 It relates to the highest harmonics here 00:41:42.060 --> 00:41:44.340 and the pattern of this harmonics. 00:41:44.340 --> 00:41:47.610 For the piano, it's starting big and sloping down. 00:41:47.610 --> 00:41:51.050 For a guitar, it's starting small, sloping up, 00:41:51.050 --> 00:41:53.130 and then sloping down. 00:41:53.130 --> 00:41:58.210 The timbre is what allows you to identify that sound that you 00:41:58.210 --> 00:41:59.270 heard as a piano. 00:41:59.270 --> 00:42:02.062 We can all hear a piano and say, that's a piano. 00:42:02.062 --> 00:42:04.145 We can all hear a guitar and say, that's a guitar, 00:42:04.145 --> 00:42:06.960 or that's an electric guitar, because its pattern 00:42:06.960 --> 00:42:11.220 of harmonics, its fundamental harmonics, differs. 00:42:11.220 --> 00:42:15.210 That's how we identify sounds is by their timbre 00:42:15.210 --> 00:42:16.960 or their spectrum, if you will. 00:42:21.090 --> 00:42:23.410 Those are pretty complicated sounds. 00:42:26.220 --> 00:42:27.150 What do I have next? 00:42:27.150 --> 00:42:28.108 I have a demonstration. 00:42:30.380 --> 00:42:33.590 This one is called Canceled Harmonics. 00:42:33.590 --> 00:42:36.340 And it's a very nice demonstration 00:42:36.340 --> 00:42:39.370 to illustrate the idea that I said, 00:42:39.370 --> 00:42:42.350 when you have all these harmonics go on together, 00:42:42.350 --> 00:42:46.449 it sounds like one thing, one note, one sound. 00:42:46.449 --> 00:42:47.990 But if you take some of the harmonics 00:42:47.990 --> 00:42:53.220 out and put them back in, you're aware of that 00:42:53.220 --> 00:42:55.680 taking out and putting back in. 00:42:55.680 --> 00:42:59.530 So what they're going to do is a complex tone 00:42:59.530 --> 00:43:03.510 is presented, followed by several cancellations 00:43:03.510 --> 00:43:07.210 and restorations of a particular harmonic. 00:43:07.210 --> 00:43:09.477 And let me show you what complex tones 00:43:09.477 --> 00:43:10.560 they're going to give you. 00:43:12.990 --> 00:43:14.925 It's simply this square wave. 00:43:16.986 --> 00:43:18.860 This is what you're going to be listening to. 00:43:18.860 --> 00:43:20.401 It sounds like [MAKES BUZZING NOISE]. 00:43:20.401 --> 00:43:22.060 It's not very musical at all. 00:43:23.770 --> 00:43:25.830 And it has a fundamental and a whole bunch 00:43:25.830 --> 00:43:27.490 of harmonics, an infinite number. 00:43:29.680 --> 00:43:32.850 When that complex goes on at once, you're going to say, 00:43:32.850 --> 00:43:35.730 that sounds like a nasty sound. 00:43:35.730 --> 00:43:37.165 It sounds like a buzz almost. 00:43:38.450 --> 00:43:44.150 Then they're going to take this one harmonic and pull it out, 00:43:44.150 --> 00:43:45.960 and then they're going to put it back in. 00:43:47.380 --> 00:43:48.999 As they do that, you're going to say, 00:43:48.999 --> 00:43:50.290 well, that sounded differently. 00:43:51.460 --> 00:43:53.510 When it was out and when it was back in, 00:43:53.510 --> 00:43:55.390 I could hear that thing going in and out. 00:43:55.390 --> 00:43:58.600 And then they're going to do that for the second, third, 00:43:58.600 --> 00:44:01.340 and fourth on up to, I think about 10 or so. 00:44:02.540 --> 00:44:04.470 Even though this whole constellation 00:44:04.470 --> 00:44:09.920 sounds like one sound, when they pulse these things in and out, 00:44:09.920 --> 00:44:11.740 you can tell. 00:44:11.740 --> 00:44:13.410 Let's listen to the demonstration, 00:44:13.410 --> 00:44:15.800 and let's see how many times they're going to do it. 00:44:15.800 --> 00:44:19.255 This is done for harmonics one through ten. 00:44:26.855 --> 00:44:28.322 Canceled Harmonics. 00:44:28.322 --> 00:44:31.010 A complex tone is presented, followed 00:44:31.010 --> 00:44:34.160 by several cancellations and restorations 00:44:34.160 --> 00:44:36.136 of a particular harmonic. 00:44:36.136 --> 00:44:39.594 This is done for harmonics one through 10. 00:46:05.502 --> 00:46:06.030 OK. 00:46:06.030 --> 00:46:09.330 Could everybody hear when this complex went on all at 00:46:09.330 --> 00:46:11.780 once it sounded like one sound? 00:46:11.780 --> 00:46:15.600 Then when individual components were taken out and pulsed back 00:46:15.600 --> 00:46:17.490 in, you could identify them. 00:46:17.490 --> 00:46:20.640 Your ear is very good at distinguishing 00:46:20.640 --> 00:46:22.825 the various frequencies in a complex spectrum. 00:46:24.190 --> 00:46:27.360 All that message is sent to the brain as individual channels, 00:46:27.360 --> 00:46:29.920 and the brain somehow perceives that when everything 00:46:29.920 --> 00:46:33.910 is going on at the same time, that's one sound. 00:46:33.910 --> 00:46:35.610 It's really not of interest to the brain 00:46:35.610 --> 00:46:38.270 that the string is vibrating a whole bunch 00:46:38.270 --> 00:46:40.090 of different frequencies. 00:46:40.090 --> 00:46:41.985 It's that there's one string vibrating. 00:46:43.530 --> 00:46:46.550 But if you took out one of these modes-- in other words, 00:46:46.550 --> 00:46:50.420 if I put my finger here and the fundamental goes away, 00:46:50.420 --> 00:46:53.350 you ear is very good at detecting that. 00:46:53.350 --> 00:46:55.170 And it sends a message to the brain 00:46:55.170 --> 00:46:56.970 that the fundamental is no longer there. 00:46:56.970 --> 00:46:59.178 And the brain says, something different has happened. 00:47:00.670 --> 00:47:03.700 So the ear is very good at recognizing 00:47:03.700 --> 00:47:05.260 those different characteristics. 00:47:05.260 --> 00:47:07.720 The brain is good at putting them back together and saying, 00:47:07.720 --> 00:47:10.194 they started at one time, so it's one object. 00:47:13.290 --> 00:47:14.680 Questions about that so far? 00:47:18.790 --> 00:47:24.920 Now, the last type of complex sound that I want to cover 00:47:24.920 --> 00:47:26.040 is speech sounds. 00:47:26.040 --> 00:47:29.530 And I want to save most of this for the end of the semester 00:47:29.530 --> 00:47:34.700 when we talk about the parts of the auditory system that 00:47:34.700 --> 00:47:38.220 are active in distinguishing different speech sounds. 00:47:38.220 --> 00:47:41.140 But let me just-- because we're talking about sounds 00:47:41.140 --> 00:47:44.520 and complex sounds, talk about speech sounds. 00:47:44.520 --> 00:47:46.730 This is a diagram of your vocal cavity. 00:47:48.100 --> 00:47:50.530 Way down at the bottom here, you get air 00:47:50.530 --> 00:47:53.410 from your lungs that goes through your trachea. 00:47:53.410 --> 00:47:57.430 And in the trachea, there's these vocal cords, if you will, 00:47:57.430 --> 00:48:00.190 that are scientifically called the glottis. 00:48:02.060 --> 00:48:05.890 The opening in between is the glottis. 00:48:05.890 --> 00:48:09.670 So air can come out, or if you use muscles associated 00:48:09.670 --> 00:48:11.910 with your vocal cords, you can close that off. 00:48:14.650 --> 00:48:21.010 As the air comes out from here, it moves those vocal cords 00:48:21.010 --> 00:48:22.230 back and forth. 00:48:22.230 --> 00:48:24.540 And they hit each other, and they open up, 00:48:24.540 --> 00:48:26.060 and they hit each other and open up. 00:48:27.310 --> 00:48:30.210 And as they do that, they interrupt the airflow 00:48:30.210 --> 00:48:31.642 and they allow it to pass through. 00:48:31.642 --> 00:48:34.100 And they interrupt it, and they allowed it to pass through. 00:48:35.340 --> 00:48:38.720 And if you were to put a microphone 00:48:38.720 --> 00:48:43.340 way down your trachea right above those vocal cords, 00:48:43.340 --> 00:48:45.980 you would see this time waveform. 00:48:45.980 --> 00:48:50.340 The pressure would go up right as the air pressure is coming 00:48:50.340 --> 00:48:52.650 from the lungs when the vocal cords were open. 00:48:53.720 --> 00:48:57.620 When the vocal cords are shut, there's no pressure there, 00:48:57.620 --> 00:49:00.580 or it's just atmospheric pressure. 00:49:00.580 --> 00:49:05.210 So this opening and closing of the air through the glottis 00:49:05.210 --> 00:49:08.450 forms this very complicated waveform. 00:49:08.450 --> 00:49:11.344 If you look at the spectrum of it, 00:49:11.344 --> 00:49:13.260 it has a whole bunch of different frequencies. 00:49:14.530 --> 00:49:16.890 The lowest of the frequencies is the frequency 00:49:16.890 --> 00:49:19.050 that these things are opening and closing. 00:49:19.050 --> 00:49:21.180 But there's a whole bunch of harmonics. 00:49:21.180 --> 00:49:22.675 It's a very complicated spectrum. 00:49:25.200 --> 00:49:27.620 The upper part of your vocal tract 00:49:27.620 --> 00:49:28.820 is what's called the filter. 00:49:30.730 --> 00:49:35.180 And it serves to emphasize some of those harmonics 00:49:35.180 --> 00:49:36.715 and de-emphasize others. 00:49:37.800 --> 00:49:41.070 And the filter function is indicated here 00:49:41.070 --> 00:49:42.765 having three peaks. 00:49:43.910 --> 00:49:46.130 Those peaks are called formant peaks. 00:49:58.620 --> 00:50:03.090 They have to do with the shape and dimensions, lengths 00:50:03.090 --> 00:50:06.310 and widths of your upper vocal tract. 00:50:06.310 --> 00:50:10.980 What's kind of neat is by manipulating, let's say, 00:50:10.980 --> 00:50:15.320 where your palate is, and where your lips are, 00:50:15.320 --> 00:50:19.160 and where your tongue is, you can change that filter function 00:50:19.160 --> 00:50:21.350 by using the muscles that move things around 00:50:21.350 --> 00:50:24.000 in your upper vocal tract. 00:50:24.000 --> 00:50:27.600 And after you've filtered this complex spectrum, 00:50:27.600 --> 00:50:30.780 you come out with a function where 00:50:30.780 --> 00:50:34.560 some of these spectral peaks are emphasized 00:50:34.560 --> 00:50:36.077 and some are not emphasized. 00:50:36.077 --> 00:50:37.660 And here's the function that you would 00:50:37.660 --> 00:50:40.990 get right outside in the air outside the front 00:50:40.990 --> 00:50:42.400 of your mouth. 00:50:42.400 --> 00:50:44.045 This is the time wave form here. 00:50:46.800 --> 00:50:51.140 Here are some examples of manipulation 00:50:51.140 --> 00:50:53.480 of your upper vocal tract. 00:50:53.480 --> 00:50:55.720 For instance, here the lower part of the mouth 00:50:55.720 --> 00:51:01.310 is moved way up high, and it produces an acoustic spectrum 00:51:01.310 --> 00:51:02.600 where you have a big f1. 00:51:04.120 --> 00:51:06.630 And f2 and f3 are small, and they 00:51:06.630 --> 00:51:08.080 are way up high in frequency. 00:51:10.600 --> 00:51:15.560 Contrast this with when the bottom of your mouth 00:51:15.560 --> 00:51:17.370 is lowered and moved backward. 00:51:18.560 --> 00:51:21.160 Here, F1 is even lower. 00:51:21.160 --> 00:51:23.500 F2 is quite low. 00:51:23.500 --> 00:51:25.330 And F3 is moderately low. 00:51:25.330 --> 00:51:27.100 And these are, of course, the way 00:51:27.100 --> 00:51:29.860 you pronounce different vowels. 00:51:31.470 --> 00:51:33.300 We can all say these two vowels. 00:51:33.300 --> 00:51:36.640 This is the vowel "i" as in "hit." 00:51:36.640 --> 00:51:39.640 Everybody say that-- hit, hit. 00:51:39.640 --> 00:51:44.790 You can kind of feel that the lower part of your mouth 00:51:44.790 --> 00:51:46.400 is moved upward. 00:51:46.400 --> 00:51:50.544 Whereas if you do something like this-- "a" in call. 00:51:50.544 --> 00:51:52.120 Call-- everybody say that. 00:51:52.120 --> 00:51:53.120 Call. 00:51:53.120 --> 00:51:54.930 You can feel the lower part of your mouth 00:51:54.930 --> 00:51:59.690 dropping down as indicated here in making a big cavity, 00:51:59.690 --> 00:52:02.110 whereas here the cavity is very small. 00:52:02.110 --> 00:52:04.090 It changes the acoustic spectrum. 00:52:04.090 --> 00:52:05.510 Our ears pick it up. 00:52:06.730 --> 00:52:09.960 And our ears are very good frequency analyzers. 00:52:09.960 --> 00:52:13.730 And they say the spectrum here sounds like hit, because you've 00:52:13.730 --> 00:52:17.830 learned to associate that spectrum with that vowel. 00:52:17.830 --> 00:52:19.220 This is a different spectrum. 00:52:19.220 --> 00:52:20.970 Our ears pick it up and they say, 00:52:20.970 --> 00:52:25.580 that's the vowel "a" as in "call." 00:52:25.580 --> 00:52:29.300 That's how speech sounds are formed. 00:52:29.300 --> 00:52:33.010 At least this works very well for vowel sounds. 00:52:33.010 --> 00:52:37.280 It doesn't explain things like consonant sounds, which 00:52:37.280 --> 00:52:39.220 of course are many different kinds. 00:52:39.220 --> 00:52:42.520 There's stop consonants where your lips close down 00:52:42.520 --> 00:52:45.570 before you utter the consonant "p." 00:52:45.570 --> 00:52:49.220 So "p," everybody close their lips down, 00:52:49.220 --> 00:52:51.124 and then all of the sudden you open it up. 00:52:51.124 --> 00:52:52.540 It's a completely different thing. 00:52:52.540 --> 00:52:54.860 That's not modulating the spectrum. 00:52:54.860 --> 00:52:57.420 That's modulating the time pattern. 00:52:57.420 --> 00:53:00.500 These vowels are distinguished by 00:53:00.500 --> 00:53:02.520 their different spectral patterns, 00:53:02.520 --> 00:53:04.530 which is picked up by your years. 00:53:04.530 --> 00:53:06.850 So I just thought you'd want to know about that. 00:53:06.850 --> 00:53:11.420 Speech sounds are among the most complicated acoustical sounds 00:53:11.420 --> 00:53:14.420 because of the number of frequencies involved, 00:53:14.420 --> 00:53:17.320 the formation, and of course the perception 00:53:17.320 --> 00:53:20.850 of telling, for example, one vowel from another. 00:53:26.100 --> 00:53:30.240 Let's shift gears and move on. 00:53:30.240 --> 00:53:32.730 And instead of talking about the physical characteristics 00:53:32.730 --> 00:53:35.240 of sound, let's talk about how we hear sounds. 00:53:36.950 --> 00:53:40.280 We're only going to get as far as the auditory periphery 00:53:40.280 --> 00:53:42.820 today, but let's just define it. 00:53:42.820 --> 00:53:47.260 The auditory periphery is this whole structure indicated here, 00:53:47.260 --> 00:53:50.230 and it's usually separated into three parts-- 00:53:50.230 --> 00:53:58.190 the external ear, the middle ear, and then the inner ear. 00:53:58.190 --> 00:54:01.910 Those are the three very big divisions 00:54:01.910 --> 00:54:03.610 of the auditory periphery. 00:54:03.610 --> 00:54:05.625 In the external ear, you have your pinna. 00:54:07.170 --> 00:54:08.730 Here's your pinna. 00:54:08.730 --> 00:54:12.030 You have the ear canal, which goes down 00:54:12.030 --> 00:54:15.190 about three centimeters inside your head, 00:54:15.190 --> 00:54:18.020 and it ends up at this yellow structure here called 00:54:18.020 --> 00:54:19.370 the ear drum. 00:54:19.370 --> 00:54:22.190 Tympanic membrane is the scientific term 00:54:22.190 --> 00:54:23.010 for the ear drum. 00:54:24.400 --> 00:54:27.000 That's the end of the external ear. 00:54:27.000 --> 00:54:31.810 The middle ear is an air-filled cavity. 00:54:31.810 --> 00:54:34.860 So we're still talking about sound in the ear. 00:54:34.860 --> 00:54:38.930 In that middle ear cavity are three small bones. 00:54:40.220 --> 00:54:42.130 They're called ossicles. 00:54:42.130 --> 00:54:43.640 I think-- yeah, here we go. 00:54:44.730 --> 00:54:46.930 And in high school biology, you probably 00:54:46.930 --> 00:54:52.410 learned them as hammer, anvil, and stirrup. 00:54:52.410 --> 00:54:57.070 But the scientific names are malleus, incus, and stapes. 00:55:00.300 --> 00:55:05.030 And they convey these sound vibrations of the ear drum. 00:55:05.030 --> 00:55:08.230 When sound hits the ear drum, it causes it to move. 00:55:08.230 --> 00:55:12.460 And these bones are linked right onto the eardrum, 00:55:12.460 --> 00:55:13.925 and they're linked one to another. 00:55:15.070 --> 00:55:18.030 The ear drum then moves the bones, 00:55:18.030 --> 00:55:22.390 and the bones finally end up, in the case of the stapes, 00:55:22.390 --> 00:55:23.624 in the inner ear. 00:55:23.624 --> 00:55:25.165 So that's where the inner ear begins. 00:55:27.050 --> 00:55:30.380 I have a demonstration of ossicles, 00:55:30.380 --> 00:55:33.700 and I'll pass them around. 00:55:33.700 --> 00:55:36.880 These are ossicles from a guinea pig, 00:55:36.880 --> 00:55:39.090 and they're glued to the bottom of this little vial. 00:55:41.040 --> 00:55:43.510 And I made a crummy drawing of them. 00:55:44.980 --> 00:55:49.190 But if you hold this vial so that the piece of tape on it 00:55:49.190 --> 00:55:53.380 is downward, you get this view here. 00:55:53.380 --> 00:55:54.240 You have the stapes. 00:55:55.930 --> 00:55:58.270 And I didn't list the other ones. 00:55:59.470 --> 00:56:07.170 But in the guinea pig, the incus and malleus 00:56:07.170 --> 00:56:10.010 are fused, so they can be considered one. 00:56:11.620 --> 00:56:14.150 This is definitely part of the malleus. 00:56:17.010 --> 00:56:22.670 But I don't know where the incus ends and the malleus begins. 00:56:22.670 --> 00:56:27.010 If you had an ear drum, it would be this dashed line here. 00:56:32.010 --> 00:56:33.590 So let me just pass these around. 00:56:35.170 --> 00:56:39.900 And you can probably appreciate from my diagram 00:56:39.900 --> 00:56:44.700 how the high school biology name for the stapes got its name. 00:56:44.700 --> 00:56:45.849 It's the stirrup. 00:56:45.849 --> 00:56:46.640 What's the stirrup? 00:56:53.330 --> 00:56:55.390 Does anybody know what a stirrup is? 00:56:56.781 --> 00:56:57.280 Yeah. 00:56:57.280 --> 00:56:58.972 When you ride horses, what is the-- 00:56:58.972 --> 00:57:00.388 AUDIENCE: You put your foot in it. 00:57:00.388 --> 00:57:02.460 PROFESSOR: You put your foot in it. 00:57:02.460 --> 00:57:06.510 And that's why cowboy boots have a nice big heel, 00:57:06.510 --> 00:57:08.950 so your foot doesn't go all the way through it. 00:57:08.950 --> 00:57:09.920 It sticks in your heel. 00:57:11.590 --> 00:57:13.270 So this is the stirrup. 00:57:13.270 --> 00:57:15.850 You put your cowboy boot right in there 00:57:15.850 --> 00:57:18.130 until your heel hits this foot plate. 00:57:25.740 --> 00:57:28.740 That's pretty obvious how that got its name. 00:57:28.740 --> 00:57:31.095 It's the foot plate where you put your foot. 00:57:31.095 --> 00:57:32.360 Your foot goes right on that. 00:57:33.690 --> 00:57:35.980 And that foot plate is the beginning 00:57:35.980 --> 00:57:39.750 of the next division, which is the inner ear. 00:57:41.240 --> 00:57:43.510 And by the way, I should point out 00:57:43.510 --> 00:57:47.935 before I forget-- what is the smallest bone in the body? 00:57:50.070 --> 00:57:51.260 All answers are given. 00:57:53.140 --> 00:57:55.840 The stapes is the smallest bone in the body. 00:57:55.840 --> 00:57:56.632 Why? 00:57:56.632 --> 00:57:57.340 It's got to move. 00:57:58.410 --> 00:58:01.380 And the lousy little sound-- it's this tiny little ear drum. 00:58:01.380 --> 00:58:04.090 Remember, the ear drum is basically 00:58:04.090 --> 00:58:06.940 a tiny, little thin piece of skin. 00:58:06.940 --> 00:58:09.080 It's like Saran wrap. 00:58:09.080 --> 00:58:13.640 When your doctor looks down your ear canal, 00:58:13.640 --> 00:58:16.500 that doctor can look right through the ear drum. 00:58:16.500 --> 00:58:17.990 It's so thin. 00:58:17.990 --> 00:58:19.870 It's like plastic wrap. 00:58:19.870 --> 00:58:23.930 The doctor can look into the middle ear and say, 00:58:23.930 --> 00:58:25.670 so much fluid in there. 00:58:25.670 --> 00:58:27.700 You've got a middle ear infection. 00:58:27.700 --> 00:58:30.251 Or they can say, middle ear looks good. 00:58:30.251 --> 00:58:31.500 You've got some other problem. 00:58:32.592 --> 00:58:33.800 That's what we're looking at. 00:58:33.800 --> 00:58:36.930 They're looking with their otoscope and a light right 00:58:36.930 --> 00:58:39.130 through the ear drum into the middle ear. 00:58:39.130 --> 00:58:43.380 And that whole middle ear drum and the ossicles 00:58:43.380 --> 00:58:47.985 have to vibrate when there's a tiny little sound like a pin 00:58:47.985 --> 00:58:48.485 drop. 00:58:50.160 --> 00:58:52.710 The pin drops right there, and you can hear it 00:58:52.710 --> 00:58:55.890 because these things are so light and flexible that they 00:58:55.890 --> 00:58:57.095 can vibrate-- and so small. 00:59:01.360 --> 00:59:05.550 The stapes foot plate ends up at the cochlear. 00:59:06.640 --> 00:59:11.530 And the cochlear is the main part of the inner ear. 00:59:12.770 --> 00:59:16.670 And cochlear, as it says here, gets its name 00:59:16.670 --> 00:59:20.090 from the Greek word kochlias, which means snail. 00:59:21.970 --> 00:59:25.870 And certainly, the inner ear looks like a snail shell. 00:59:28.170 --> 00:59:30.710 And in the inner ear, here's where 00:59:30.710 --> 00:59:35.675 sound changes from sound in air, or maybe sound in the bones. 00:59:37.030 --> 00:59:39.605 The inner ear is filled with fluid. 00:59:58.200 --> 01:00:01.840 And inside the inner ear are these wonderful receptor cells 01:00:01.840 --> 01:00:05.304 for hearing and the beginning of the auditory nerve. 01:00:05.304 --> 01:00:06.970 Here's the auditory nerve that's sending 01:00:06.970 --> 01:00:09.110 messages centrally into the brain. 01:00:09.110 --> 01:00:10.980 So the brain would be beginning right here. 01:00:13.460 --> 01:00:16.550 This whole structure here, all this gray stuff, 01:00:16.550 --> 01:00:19.223 and even the shell of the cochlear is bone. 01:00:19.223 --> 01:00:21.800 And it's your temporal bone. 01:00:21.800 --> 01:00:25.280 The temporal bone is the hardest bone in the body. 01:00:25.280 --> 01:00:28.890 You can have a severe blow to the head 01:00:28.890 --> 01:00:31.580 and that temporal bone will keep all these structures intact. 01:00:33.680 --> 01:00:34.920 It's very, very hard bone. 01:00:36.220 --> 01:00:38.200 Surgeons at our hospital do a lot 01:00:38.200 --> 01:00:39.757 of drilling with the dental drill. 01:00:39.757 --> 01:00:41.590 They get down to these important structures, 01:00:41.590 --> 01:00:44.350 because they have to manipulate them. 01:00:44.350 --> 01:00:47.400 These loops here are part of the inner ear, 01:00:47.400 --> 01:00:51.060 but they are part that is sensitive to vestibular 01:00:51.060 --> 01:00:51.560 sensation. 01:00:52.840 --> 01:00:56.070 So those loops are called the semicircular canals. 01:00:56.070 --> 01:00:57.160 They are almost circular. 01:00:57.160 --> 01:01:00.670 They are in the three planes, X, Y, and Z. 01:01:00.670 --> 01:01:05.850 And when you rotate your head, let's say, side to side, 01:01:05.850 --> 01:01:09.500 one of those can move. 01:01:11.190 --> 01:01:15.930 And the receptor cells in it can sense that movement 01:01:15.930 --> 01:01:18.450 and detect that your head had moved. 01:01:18.450 --> 01:01:20.430 And it's very important, because if you 01:01:20.430 --> 01:01:22.900 want to keep your eyes fixated on one point 01:01:22.900 --> 01:01:25.200 but move your head, you can do that 01:01:25.200 --> 01:01:28.615 by the vestibulo-ocular reflex. 01:01:29.790 --> 01:01:32.610 The neurons from this the vestibular system 01:01:32.610 --> 01:01:34.770 send messages into the brain stem, 01:01:34.770 --> 01:01:37.450 and eventually they go through coordinating centers 01:01:37.450 --> 01:01:42.070 into the motor neurons for the extraocular muscles, which 01:01:42.070 --> 01:01:45.030 can, of course, move your eyes when you want 01:01:45.030 --> 01:01:48.710 to do a [? secade ?] or pursuit, or they 01:01:48.710 --> 01:01:52.050 can keep your eyes stabilized, which is moving them 01:01:52.050 --> 01:01:55.090 with respect your head even though your head is moving. 01:01:55.090 --> 01:01:57.372 But we're not going to talk about those. 01:02:05.390 --> 01:02:08.330 Let's talk about the function of the middle ear 01:02:08.330 --> 01:02:09.400 and the external ear. 01:02:09.400 --> 01:02:11.899 That's what we're going to talk about for the rest of today. 01:02:11.899 --> 01:02:12.892 I have a model. 01:02:12.892 --> 01:02:14.350 Let me just pass around this model. 01:02:14.350 --> 01:02:18.200 I think we passed around before on the first day of class, 01:02:18.200 --> 01:02:21.140 but you can look at it again, because we're 01:02:21.140 --> 01:02:23.850 going into more detail today on this structure. 01:02:23.850 --> 01:02:25.630 This comes apart. 01:02:25.630 --> 01:02:26.890 Here's your pinna. 01:02:26.890 --> 01:02:28.045 Here's the long ear canal. 01:02:30.540 --> 01:02:31.820 Here's the ear drum. 01:02:33.460 --> 01:02:38.670 And if I tilt this here, you can see the structures 01:02:38.670 --> 01:02:42.550 we're talking about in the inner ear-- the cochlear, 01:02:42.550 --> 01:02:46.770 the semicircular canals, and this yellow structure here 01:02:46.770 --> 01:02:47.925 is the auditory nerve. 01:02:49.460 --> 01:02:51.370 It's been going into the brain. 01:02:51.370 --> 01:02:52.700 The brain is cut off here. 01:02:54.670 --> 01:02:57.790 This is the eustachian tube, which 01:02:57.790 --> 01:03:04.220 is a way to vent the air-filled middle ear. 01:03:04.220 --> 01:03:06.200 So you want to purge that with air. 01:03:06.200 --> 01:03:08.850 If you go up hiking in a tall mountain, 01:03:08.850 --> 01:03:12.030 the barometric pressure outside gets lower. 01:03:12.030 --> 01:03:13.950 You want to equalize that in your middle ear. 01:03:13.950 --> 01:03:16.625 You open that eustachian tube, usually by swallowing. 01:03:18.880 --> 01:03:20.680 The ossicles are here. 01:03:20.680 --> 01:03:23.090 And if you take out this inner ear, 01:03:23.090 --> 01:03:26.000 the stapes is fixed with it. 01:03:26.000 --> 01:03:27.120 So you can see the stapes. 01:03:28.460 --> 01:03:33.860 In terms of size, this whole inner ear-- the cochlear 01:03:33.860 --> 01:03:38.370 is about the size of an aspirin tablet in a human. 01:03:38.370 --> 01:03:39.500 It's about that size. 01:03:40.916 --> 01:03:41.416 OK. 01:03:41.416 --> 01:03:42.853 Let's pass that around. 01:03:48.981 --> 01:03:49.480 OK. 01:03:49.480 --> 01:03:51.390 What is the function of the middle ear? 01:03:52.610 --> 01:03:54.966 Why do we have these three bones? 01:03:54.966 --> 01:03:56.090 Why do we have the eardrum? 01:03:56.090 --> 01:04:00.321 Why doesn't sound come right in and strike the inner ear 01:04:00.321 --> 01:04:00.820 itself? 01:04:02.040 --> 01:04:05.520 Well, it turns out that if you look 01:04:05.520 --> 01:04:13.590 at the physical characteristics of sound in air, 01:04:13.590 --> 01:04:17.100 and you want to get that airborne sound 01:04:17.100 --> 01:04:19.900 to sound in water, different medium. 01:04:19.900 --> 01:04:21.375 So this is fluid or water. 01:04:24.370 --> 01:04:25.125 This is air. 01:04:27.179 --> 01:04:28.720 Sound is coming in here, and you want 01:04:28.720 --> 01:04:33.490 to get it into the fluid of the inner ear, which is essentially 01:04:33.490 --> 01:04:33.990 water. 01:04:36.020 --> 01:04:40.430 If you don't do anything and you have the sound coming in here, 01:04:40.430 --> 01:04:42.440 most of it bounces back off. 01:04:43.760 --> 01:04:52.320 In fact, 99.5% of the energy of sound 01:04:52.320 --> 01:04:57.020 in air at a fluid boundary is reflected back into the air. 01:04:58.340 --> 01:05:04.210 So if you're in a boat here-- I didn't 01:05:04.210 --> 01:05:08.700 draw this right-- you're in a boat here, you're fishing, 01:05:08.700 --> 01:05:10.980 you're talking to your buddy in the back of the boat 01:05:10.980 --> 01:05:14.950 and you say, pass me another beer, and your buddy says, 01:05:14.950 --> 01:05:19.420 be quiet, you'll scare the fish-- actually, 01:05:19.420 --> 01:05:20.650 the fish can't hear you. 01:05:20.650 --> 01:05:27.230 Because most of the energy in your saying "pass me a beer" 01:05:27.230 --> 01:05:29.200 bounced right back off into the air. 01:05:31.380 --> 01:05:35.910 So how does the auditory system deal with this? 01:05:35.910 --> 01:05:41.690 We want to listen very carefully to a pin drop, 01:05:41.690 --> 01:05:45.030 but most of the energy bounces back off 01:05:45.030 --> 01:05:48.760 at this boundary between air and fluid. 01:05:49.830 --> 01:05:51.320 That's the job of the middle ear. 01:05:57.710 --> 01:05:59.630 Here is how the middle ear moves. 01:05:59.630 --> 01:06:02.680 This is a nice movie made by Heidi Nakajima 01:06:02.680 --> 01:06:04.700 at Mass Eye and Ear Infirmary. 01:06:04.700 --> 01:06:06.500 This orientation is a little bit different, 01:06:06.500 --> 01:06:07.740 but this is the ear drum. 01:06:08.780 --> 01:06:13.080 This is the malleus, the incus, and the stapes. 01:06:14.250 --> 01:06:15.790 Together, they're the middle ear. 01:06:17.810 --> 01:06:20.940 I said this inner ear is the cochlear here, 01:06:20.940 --> 01:06:25.490 and it's encased in bone-- fluid encased in bone. 01:06:25.490 --> 01:06:28.710 So how does this stapes work? 01:06:28.710 --> 01:06:31.240 Well, there's a little window in the bone. 01:06:32.380 --> 01:06:34.170 It's called the oval window. 01:06:36.700 --> 01:06:44.816 And the foot plate of the stapes pushes on that oval window. 01:06:44.816 --> 01:06:46.190 It's not indicated here, but it's 01:06:46.190 --> 01:06:47.665 right underneath this oval part. 01:06:48.970 --> 01:06:51.940 There's another window called the round window. 01:06:53.160 --> 01:06:54.820 That's indicated by blue there. 01:06:59.420 --> 01:07:01.850 And it's just a pressure relief point, 01:07:01.850 --> 01:07:06.490 because if you pushed on fluid, it would push back to you. 01:07:06.490 --> 01:07:08.193 Fluid is relatively incompressible. 01:07:09.730 --> 01:07:14.070 So this pushing in means this membrane over the round window 01:07:14.070 --> 01:07:15.700 can push out easily. 01:07:15.700 --> 01:07:18.240 So it's easy to push in and pull back, 01:07:18.240 --> 01:07:19.720 because this membrane can give. 01:07:21.000 --> 01:07:24.740 As you can see, the motion of these bones 01:07:24.740 --> 01:07:27.660 is coming into the fluid quite nicely 01:07:27.660 --> 01:07:30.990 and changing some membranes inside the inner ear. 01:07:32.150 --> 01:07:37.470 The job of the middle ear is so that most of that sound energy 01:07:37.470 --> 01:07:43.430 gets into the fluids of the water of the inner ear. 01:07:43.430 --> 01:07:45.020 How does it do that? 01:07:45.020 --> 01:07:48.930 The primary way is by changing area. 01:07:48.930 --> 01:07:56.520 The eardrum is this big drum, and the stapes foot plate 01:07:56.520 --> 01:08:00.630 is this much lower in area or smaller structure. 01:08:02.750 --> 01:08:04.455 And there's some formulas here. 01:08:05.750 --> 01:08:12.920 p1, a1, those pressure and area at the tympanic membrane, 01:08:12.920 --> 01:08:18.410 equals p2 a2 where the same characteristics at the stapes 01:08:18.410 --> 01:08:19.149 foot plate. 01:08:21.350 --> 01:08:27.569 So when you decrease the area a lot, a2 goes way down. 01:08:27.569 --> 01:08:29.560 p2 has to go way up. 01:08:31.260 --> 01:08:36.080 So that's then the main way that the middle ear 01:08:36.080 --> 01:08:41.270 allows sound and air to go into sound and fluid. 01:08:41.270 --> 01:08:45.916 The engineers would call this impedance matching. 01:08:54.350 --> 01:08:57.810 And they would say that when you change media, 01:08:57.810 --> 01:09:01.720 the impedance of one medium being different from another 01:09:01.720 --> 01:09:04.479 means that most of the energy is going to bounce back off. 01:09:04.479 --> 01:09:09.130 If you have a device here like the middle ear 01:09:09.130 --> 01:09:14.040 to make the impedances more matching, much of this energy 01:09:14.040 --> 01:09:16.689 is going to then go through the boundary from one 01:09:16.689 --> 01:09:19.910 medium to the other if you match the impedances. 01:09:19.910 --> 01:09:21.770 And one way of matching the impedances 01:09:21.770 --> 01:09:23.910 is to change the areas. 01:09:25.189 --> 01:09:29.270 Another way-- and this may be the reason we have three 01:09:29.270 --> 01:09:33.989 and not just one middle ear bone-- is by a lever action. 01:09:35.410 --> 01:09:38.819 So this is kind of like a lever where the fulcrum is off 01:09:38.819 --> 01:09:41.420 to one side, not right in the middle. 01:09:41.420 --> 01:09:43.990 And you can obviously get force amplification 01:09:43.990 --> 01:09:45.405 from a lever action. 01:09:47.210 --> 01:09:50.470 A third mechanism might be a buckling 01:09:50.470 --> 01:09:51.795 of the tympanic membrane. 01:09:52.960 --> 01:09:56.090 And you'll have to read-- I'm not an expert on that at all. 01:09:56.090 --> 01:09:59.750 I'm not even sure if that's even in vogue these days. 01:09:59.750 --> 01:10:03.970 But these actions are much less than the change 01:10:03.970 --> 01:10:06.830 in area offered by the eardrum. 01:10:08.610 --> 01:10:13.570 So what happens when a patient comes into the Massachusetts 01:10:13.570 --> 01:10:16.720 Eye and Ear Infirmary, and for some reason, 01:10:16.720 --> 01:10:22.710 either via an accident or a developmental problem, 01:10:22.710 --> 01:10:24.240 they don't have an eardrum, and they 01:10:24.240 --> 01:10:26.640 don't have these three ossicles. 01:10:26.640 --> 01:10:30.940 So the sound goes right in from the outside 01:10:30.940 --> 01:10:34.720 and strikes, let's say, the round window of the cochlear. 01:10:34.720 --> 01:10:36.120 Are they deaf? 01:10:36.120 --> 01:10:36.650 Well, no. 01:10:36.650 --> 01:10:38.080 They have a hearing loss. 01:10:38.080 --> 01:10:41.400 Some of the energy gets through into the fluid. 01:10:43.410 --> 01:10:45.060 How big is their hearing loss? 01:10:45.060 --> 01:10:48.290 Well, this is the so-called audiogram 01:10:48.290 --> 01:10:52.450 that's generated when you visit a hospital 01:10:52.450 --> 01:10:54.710 and you complain that your hearing isn't so good. 01:10:54.710 --> 01:10:57.470 They send you down to the Audiology Department. 01:10:57.470 --> 01:10:59.720 They put you in a testing booth. 01:10:59.720 --> 01:11:04.280 They put earphones on, and the tester goes outside 01:11:04.280 --> 01:11:06.590 so they don't make any extraneous noise. 01:11:07.820 --> 01:11:13.270 And they say, raise your hand when you can hear a sound. 01:11:13.270 --> 01:11:16.810 So they test your hearing-- this is the so-called audiogram-- 01:11:16.810 --> 01:11:20.160 and plot it on the y-axis as the amount 01:11:20.160 --> 01:11:23.300 of hearing loss in decibels. 01:11:25.150 --> 01:11:27.150 It's just the way they plot it. 01:11:27.150 --> 01:11:30.790 And plot it on the x-axis as the frequency. 01:11:30.790 --> 01:11:35.720 And they typically test 2,550, 1,000-- 01:11:35.720 --> 01:11:40.350 which is abbreviated here 1k-- 2k, and 4k. 01:11:40.350 --> 01:11:44.850 They typically don't test the extremes of the human hearing. 01:11:44.850 --> 01:11:46.280 They test the middle range. 01:11:46.280 --> 01:11:51.630 This is the range over which most speech sounds are made. 01:11:51.630 --> 01:11:54.250 And that's the most important for most people. 01:11:54.250 --> 01:11:56.200 When they say, I can't hear very well, 01:11:56.200 --> 01:11:58.890 it means they can't understand somebody when they're speaking. 01:12:00.010 --> 01:12:02.530 And this is the audiogram from someone 01:12:02.530 --> 01:12:04.620 who lacked a middle ear. 01:12:04.620 --> 01:12:09.150 And this 40 dB here-- across all the different frequencies, 01:12:09.150 --> 01:12:11.860 approximately 40 db-- is the amount of hearing loss 01:12:11.860 --> 01:12:12.360 they have. 01:12:13.630 --> 01:12:15.680 So if you go back to the audiogram 01:12:15.680 --> 01:12:18.370 that we had in the first slide of today's lecture, 01:12:18.370 --> 01:12:21.270 everything would be lifted up by 40 dB. 01:12:21.270 --> 01:12:23.500 You have a 40 dB hearing loss. 01:12:23.500 --> 01:12:26.420 You're not deaf at all, but that's 01:12:26.420 --> 01:12:30.670 a moderate to severe hearing loss, a 40 dB hearing loss. 01:12:31.870 --> 01:12:35.500 You might have problems-- you certainly 01:12:35.500 --> 01:12:38.000 would have problems hearing a pin drop. 01:12:38.000 --> 01:12:42.400 You might have problems hearing a telephone ring 01:12:42.400 --> 01:12:44.500 if it were on the other side of the room. 01:12:46.180 --> 01:12:48.563 You might have problems with conversation. 01:12:50.580 --> 01:12:54.390 A treatment to that would be several types of treatment. 01:12:54.390 --> 01:12:57.580 The surgeons in the Ear, Nose, and Throat Department 01:12:57.580 --> 01:13:01.200 at Mass Eye and Ear could reconstruct 01:13:01.200 --> 01:13:03.820 your middle ear and your eardrum. 01:13:03.820 --> 01:13:06.510 They could use a skin flap, a piece of skin taken 01:13:06.510 --> 01:13:08.490 from somewhere else on your body, 01:13:08.490 --> 01:13:10.130 put it in the place of the eardrum. 01:13:10.130 --> 01:13:15.200 They could use some either wire or Teflon or plastic pieces 01:13:15.200 --> 01:13:20.510 that could connect that eardrum into the oval window 01:13:20.510 --> 01:13:21.220 of the cochlear. 01:13:21.220 --> 01:13:26.879 So they can reconstruct the middle ear fairly easily. 01:13:26.879 --> 01:13:28.670 If the person doesn't want to have surgery, 01:13:28.670 --> 01:13:31.310 they can have a hearing aid. 01:13:31.310 --> 01:13:35.390 Essentially, you have a flat frequency loss here. 01:13:35.390 --> 01:13:37.470 So put a device in the ear canal that 01:13:37.470 --> 01:13:43.850 boosts every single frequency by 40 dB, amplify the sound. 01:13:43.850 --> 01:13:47.000 So a hearing aid works pretty well 01:13:47.000 --> 01:13:49.750 for these people with this type of a hearing loss. 01:13:49.750 --> 01:14:00.490 This type of a hearing loss is called a conductive hearing 01:14:00.490 --> 01:14:04.420 loss, because it's in the conductive mechanism 01:14:04.420 --> 01:14:08.060 to conduct the sound from outside your body 01:14:08.060 --> 01:14:09.350 into the inside of your body. 01:14:09.350 --> 01:14:10.780 It's a conductive hearing loss. 01:14:12.390 --> 01:14:14.645 So that is the job of the middle ear-- 01:14:14.645 --> 01:14:18.150 to ensure efficient transmission of sound 01:14:18.150 --> 01:14:20.410 in air into the fluids of your body. 01:14:20.410 --> 01:14:22.881 And without it, you have a moderate to severe hearing 01:14:22.881 --> 01:14:23.380 loss. 01:14:26.250 --> 01:14:28.290 There's a disease called otosclerosis. 01:14:45.170 --> 01:14:46.752 "Oto" meaning hearing. 01:14:50.270 --> 01:14:52.490 My department at Harvard Med School 01:14:52.490 --> 01:14:54.520 is otology and laryngology. 01:14:55.690 --> 01:14:57.310 Otology and laryngology. 01:14:58.920 --> 01:15:03.550 And sclerosis means hardening or rocky or bony growths. 01:15:09.060 --> 01:15:13.980 And the surgery that happens-- sometimes 01:15:13.980 --> 01:15:18.170 around the stapes, bony growths can grow around it 01:15:18.170 --> 01:15:21.480 and fix the foot plate so that it can't vibrate anymore. 01:15:23.480 --> 01:15:26.720 So what's done for that is you take out the stapes, 01:15:26.720 --> 01:15:29.710 you take off the bony growths, and if you just 01:15:29.710 --> 01:15:33.760 put the stapes back in, often these bony growths grow again. 01:15:33.760 --> 01:15:35.910 So actually you take it out and replace it 01:15:35.910 --> 01:15:37.780 with an artificial stapes. 01:15:37.780 --> 01:15:40.015 And the operation is called a stapedectomy. 01:15:46.010 --> 01:15:49.230 The "stape" and "ectomy" means taking it out. 01:15:49.230 --> 01:15:51.770 You replace it with a prosthesis. 01:15:51.770 --> 01:15:56.020 It's a very successful surgery for otosclerosis, 01:15:56.020 --> 01:15:57.700 which is a conductive hearing loss. 01:15:59.950 --> 01:16:01.560 That's the job of the middle ear, 01:16:01.560 --> 01:16:07.190 and that's relatively easy to treat when there's a problem. 01:16:09.300 --> 01:16:12.110 Is there a function of the external ear? 01:16:13.980 --> 01:16:17.980 Well, a lot of textbooks say the external ear funnels the sound 01:16:17.980 --> 01:16:19.075 into your ear canal. 01:16:21.000 --> 01:16:26.320 But there is another function of the external ear that's 01:16:26.320 --> 01:16:32.050 more on the lines of localizing sounds using your external ear. 01:16:34.410 --> 01:16:36.920 These are examples of external ears-- our pinna. 01:16:38.400 --> 01:16:41.070 Everybody has a slightly different one. 01:16:41.070 --> 01:16:42.688 Who is this historical figure? 01:16:42.688 --> 01:16:43.188 Anybody? 01:16:49.180 --> 01:16:51.490 He was a president of the United States. 01:16:51.490 --> 01:16:54.600 LBJ, President Johnson. 01:16:54.600 --> 01:16:59.220 He was always caricatured by the political cartoon guys 01:16:59.220 --> 01:17:02.707 with these huge ears, big pinnae. 01:17:02.707 --> 01:17:04.290 Everybody has different shaped pinnae. 01:17:06.940 --> 01:17:10.820 It turns out that the external ear can help you localize 01:17:10.820 --> 01:17:12.140 where sound is coming from. 01:17:14.590 --> 01:17:15.895 Well, how can it do that? 01:17:19.030 --> 01:17:20.930 Well, if you have a pinna and you 01:17:20.930 --> 01:17:36.270 do this interesting experiment-- you take a microphone 01:17:36.270 --> 01:17:38.820 and put the microphone inside here. 01:17:38.820 --> 01:17:39.795 So here's the pinna. 01:17:41.200 --> 01:17:42.290 Here's the ear canal. 01:17:43.500 --> 01:17:46.200 Put the microphone out here, and start out 01:17:46.200 --> 01:17:50.565 with a completely flat spectrum, broadband noise. 01:17:52.910 --> 01:17:55.810 The noise is absolutely flat so that it 01:17:55.810 --> 01:17:58.040 has equal energy at all the frequencies. 01:17:59.720 --> 01:18:03.110 You measure it out there, and then you move your microphone 01:18:03.110 --> 01:18:09.130 down here in the ear canal, maybe near the eardrum , 01:18:09.130 --> 01:18:12.120 and measure the spectrum again. 01:18:13.500 --> 01:18:16.830 So this is plotted in terms of gain 01:18:16.830 --> 01:18:19.250 with respect to free field. 01:18:19.250 --> 01:18:20.540 Free field is out here. 01:18:25.840 --> 01:18:28.860 Free field means basically in the room or in the environment. 01:18:29.970 --> 01:18:32.000 Now we're going to measure the spectrum down 01:18:32.000 --> 01:18:35.150 here and plot the gain. 01:18:35.150 --> 01:18:39.080 So anything above 0 is going to be higher than in the ear, 01:18:39.080 --> 01:18:41.270 and everything below 0 is going to be lower. 01:18:43.750 --> 01:18:47.450 Let's look at this solid curve here, 01:18:47.450 --> 01:18:50.690 which is minus 15 degrees elevation. 01:18:52.200 --> 01:18:56.590 Elevation of a sound source-- if it's straight ahead, it's zero. 01:18:56.590 --> 01:18:59.725 If it's minus 15, it's 15 degrees below zero. 01:19:01.040 --> 01:19:04.510 If it's above zero, it could be 15 degrees. 01:19:04.510 --> 01:19:06.730 On this case, it's 7.5 and 30. 01:19:06.730 --> 01:19:09.360 So elevations that are positive are above you. 01:19:11.280 --> 01:19:13.640 As that sound source moves around 01:19:13.640 --> 01:19:19.630 from being below you to above you, its spectrum changes, 01:19:19.630 --> 01:19:21.780 the spectrum way down here at the ear drum. 01:19:22.980 --> 01:19:27.200 And in particular, there are some very sharp dips 01:19:27.200 --> 01:19:29.705 or nulls in the spectrum that move around. 01:19:30.760 --> 01:19:35.860 It's thought that you can use those nulls as a cue 01:19:35.860 --> 01:19:37.530 to where this sound is. 01:19:39.810 --> 01:19:41.870 Now, what causes those nulls? 01:19:41.870 --> 01:19:46.390 Well, because the pinna is very complicated, 01:19:46.390 --> 01:19:51.640 you can imagine that some sound comes in and strikes the pinna 01:19:51.640 --> 01:19:53.490 and reflects off it. 01:19:53.490 --> 01:19:56.960 And maybe it reflects-- excuse my artistic abilities 01:19:56.960 --> 01:20:00.770 here-- maybe it reflects into the ear canal. 01:20:00.770 --> 01:20:05.260 Contrast that with other sound that comes straight in. 01:20:06.830 --> 01:20:10.640 Eventually, these two sounds are going to meet up at a point. 01:20:10.640 --> 01:20:15.650 And let's say this sound taking a longer time path 01:20:15.650 --> 01:20:17.780 went through half of its cycle. 01:20:18.790 --> 01:20:22.230 So now this sound, when it's starting 01:20:22.230 --> 01:20:25.360 to go a negative pressure, meets up 01:20:25.360 --> 01:20:28.290 with this sound, which came straight in and is starting 01:20:28.290 --> 01:20:30.450 to go in a positive pressure. 01:20:30.450 --> 01:20:33.165 Positive plus negative could sum to zero. 01:20:34.730 --> 01:20:36.990 And the geometry has to be just right, 01:20:36.990 --> 01:20:39.540 and the frequency has to be just right. 01:20:39.540 --> 01:20:42.700 But it can be just right at a particular frequency, 01:20:42.700 --> 01:20:44.230 and that's what causes the nulls. 01:20:44.230 --> 01:20:46.480 It's just a physical characteristic 01:20:46.480 --> 01:20:49.160 of two sound sources meeting up. 01:20:49.160 --> 01:20:51.580 It is thought, then, that you can 01:20:51.580 --> 01:20:55.060 learn the position of those nulls, 01:20:55.060 --> 01:20:59.830 especially, to be associated with positions of sound 01:20:59.830 --> 01:21:00.690 in space. 01:21:00.690 --> 01:21:04.399 And that's what was done in the researchers' report for today. 01:21:09.450 --> 01:21:11.685 These are some data from four different subjects. 01:21:14.030 --> 01:21:17.230 They tested the subject to localize 01:21:17.230 --> 01:21:20.180 sounds coming from in front of them. 01:21:20.180 --> 01:21:21.570 Left and right would be azimuth. 01:21:23.070 --> 01:21:24.350 That's plotted on the x-axis. 01:21:26.450 --> 01:21:28.728 Up and down would be elevation. 01:21:28.728 --> 01:21:29.936 That's plotted on the y-axis. 01:21:31.950 --> 01:21:35.660 And they move to sounds around to different places, 01:21:35.660 --> 01:21:38.150 and they said to the person, tell me 01:21:38.150 --> 01:21:39.910 where the sound is coming from. 01:21:42.600 --> 01:21:44.720 The answers that the subjects gave 01:21:44.720 --> 01:21:47.680 are in these solid, thick lines. 01:21:47.680 --> 01:21:50.180 The real positions were on the thinner lines. 01:21:51.200 --> 01:21:55.830 And each big individual data points are the small points, 01:21:55.830 --> 01:21:57.990 and the average data points from the subject 01:21:57.990 --> 01:21:59.370 are the big points here. 01:22:00.620 --> 01:22:05.880 So these subjects, when given a checkerboard of locations, 01:22:05.880 --> 01:22:11.210 they could pretty faithfully tell the investigators 01:22:11.210 --> 01:22:13.990 where a sound was coming from, both in elevation 01:22:13.990 --> 01:22:14.770 and in azimuth. 01:22:16.007 --> 01:22:17.840 These are data from four different subjects. 01:22:19.330 --> 01:22:23.427 What was done in the experiment is distort the pinna. 01:22:23.427 --> 01:22:24.510 How are we going to do it? 01:22:24.510 --> 01:22:27.800 Well, we could move our ear a little bit. 01:22:27.800 --> 01:22:30.620 What they did was they put in a little clay 01:22:30.620 --> 01:22:33.997 mold in parts of the pinna to change the shape, 01:22:33.997 --> 01:22:35.330 and they did that on both sides. 01:22:37.290 --> 01:22:40.510 As soon as they did that, these are now 01:22:40.510 --> 01:22:41.840 the answers from the subjects. 01:22:43.740 --> 01:22:47.840 Terrible in terms of elevation sensitivity, 01:22:47.840 --> 01:22:49.510 determining where a sound is coming 01:22:49.510 --> 01:22:51.980 from in terms of different elevations. 01:22:51.980 --> 01:22:53.350 Still pretty good in azimuth. 01:22:54.890 --> 01:22:57.490 There are other queues for sound azimuth 01:22:57.490 --> 01:23:00.250 that involve using two ears, which we're 01:23:00.250 --> 01:23:02.490 going to talk about extensively later this semester. 01:23:04.060 --> 01:23:06.670 The elevational localization was completely 01:23:06.670 --> 01:23:10.170 disrupted when the pinna shape was disrupted. 01:23:12.480 --> 01:23:15.540 Have these subjects go out for a few weeks, come back, get 01:23:15.540 --> 01:23:16.380 tested again. 01:23:17.420 --> 01:23:21.210 They re-learned with the pinna molds 01:23:21.210 --> 01:23:24.905 in how to localize sounds. 01:23:26.320 --> 01:23:29.340 This is an example of re-learning or plasticity. 01:23:31.670 --> 01:23:35.030 Now the pinna cues had different nulls 01:23:35.030 --> 01:23:36.900 because the pinnas were shaped differently. 01:23:36.900 --> 01:23:38.940 They could re-learn these new cues 01:23:38.940 --> 01:23:43.170 and associate them with the same old changes in elevation 01:23:43.170 --> 01:23:44.470 that we had before. 01:23:44.470 --> 01:23:47.650 So that's why it's called re-learning sound localization 01:23:47.650 --> 01:23:50.970 with new ears or new or distorted ears. 01:23:50.970 --> 01:23:56.790 So this is an example then, of subjects learning 01:23:56.790 --> 01:24:01.240 to associate these new cues with the old sound localization 01:24:01.240 --> 01:24:02.420 positions. 01:24:02.420 --> 01:24:11.400 So that's the take home message from this research report 01:24:11.400 --> 01:24:17.830 OK questions I can also do I get on Wednesday, 01:24:17.830 --> 01:24:21.020 we'll talk about the inner ear.