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3/21/5 Chapter 11 – Hearing Page 280, column 2 has a mistake – low frequency sounds results in greater activity at the apex. Sound Stimulus: Changes of pressure in the medium in which the ear resides. G9 p 262 The ear usually resides in air, but we can hear when ear is in other media, e.g., water. Process Sound Generator – vibrating object, such as a tuning fork. Ear Ear Canal L H L H L Some physical characteristics of Sound H L H Rapidity of vibration: For humans, from 20 vibrations (cycles) per second up to 20,000 per second. Speed of expansion from source: About 700+ miles/hour; about 1100 ft/sec in air at 0o C Velocity increases as temperature increases Travels faster through less compressible media So faster in fluids and solids. 4 times faster in water than in air. Intensity decreases proportionate to square of distance from sound source without walls. Distance Intensity 1 foot 100 2 feet 25 Decrease is 1/22 of the original or 1/4. 3 feet 11.1 Decrease is 1/32 of the original or 1/9. 4 feet 6.25 Decrease is 1/42 of the original or 1/16. 5 feet 4 Decrease is 1/52 of the original or 1/25. . . 10 feet 1 Decrease is 1/102 of the original or 1/100. Distance between successive peaks: 20 CPS: 20,000 CPS: 50 feet 0.5 inches between peaks at 20,000 cps G9 Ch 11 - 1 3/21/5 Absorption / Reflection: When sound in a gas strikes a solid object, some of it is absorbed – passing into the object creating sound within the object – and some is reflected. Absorption is best when the object is most like the originating medium. So soft materials absorb more sound originating in air. This is why “acoustic tiles” are made of soft materials – so they’ll be more like air and absorb sound. H L H Soft surface Reflection is best when substance is least like the originating. So hard wood, steel are good reflectors of sounds in air. H L H Hard surface Implications for “sound proofing” – To decrease sound in the same room as the sound source, make the walls of soft material. To decrease sound in other rooms, make the walls of hard materials. Put a vacuum in the walls. G9 Ch 11 - 2 3/21/5 Representing Sounds visually. (the irony – we have to use vision to understand audition) Two Ways to Visually Represent Sounds. First Way: Waveform: A plot of pressure vs. time at a specific place, e.g., the ear Pressure Average pressure Time --------------------------------------> Two categories of waveform A. Periodic – Repeats forever. B. Aperiodic – Any waveform that does not repeat. G9 Ch 11 - 3 3/21/5 Characteristics of periodic sound waveforms . . . I. Intensity / Amplitude – A. Instantaneous amplitude: The deviation of waveform from average atmospheric pressure at a single point in time. Problem: Instantaneous amplitude varies from millisecond to millisecond. Which one should you use? B. Average Absolute Amplitude: Average of absolute values of all the instantaneous amplitudes. This is a possibility. C. Root Mean Square (RMS) Amplitude: Square root of the average of all the squared instantaneous amplitudes. The last forms the basis for most common measures. Most preferred by researchers, practitioners, makers of instruments. If you purchase sound equipment, you should look for sound output values in RMS Watts. Some advertisers report Peak Watts which is an exaggeration of the capability of an amplifier. G9 Ch 11 - 4 3/21/5 Units of RMS Intensity/Amplitude Measurement 1. Formal: micropascals (Mike note: 1 µPa = .00001 d/cm2) Based on the force exerted by the change in pressure. Minimum audible sound amplitude: 20 micropascals. Maximum safely hearable sound amplitude: 200,000,000 micropascals So the range of sounds in which we must function is 200,000,000 to 20 or 10,000,000 to 1 or 10 million to 1 (G9 Table 11.1, p 265). That is, the loudest sound we can experience without pain is 10 million times more intense than the faintest sound we can experience. The large range requires conversion to a more manageable scale. 2. Practical: The decibel scale G9 p 265 db = 20log10(Pressure of the sound in micropascals / 20 micropascals) Most sound level meters display RMS amplitude in db. Many also only measure sound whose frequencies would be audible to humans. Range: Minimum audible: Leaves rustling: Average speaking: Loud Music/traffic: Maximum tolerable for any period of time: 0 db 40 db 60 db 80 db 85 db Don’t expose yourself to sounds louder than 85 db for long periods of time. Maximum bearable: 140 db Play VL 11-1 Decibel Scale here. How to destroy your hearing: Always listen to music at the loudest possible intensity. Listen to a lot of it. Unlike vision, for which high intensity light is not pleasant, high intensity sound, sound that can destroy your ability to hear, is often enjoyable. G9 Ch 11 - 5 3/21/5 II. Period / Frequency A. Period: The time between identical points on a periodic waveform. The period of a waveform is a measure that is analogous to wavelength in the description of light. B. Frequency: The number of times a periodic waveform repeats in a given unit of time, typically 1 second, i.e., number of periods per second. F = 1/P and P = 1/F, so we can describe this characteristic of sound using either period or frequency. Frequency is almost always used in the study of audition. Typical measurement: Frequency in cycles per second, cps. Current name for frequency in cycles per second: Hertz or Hz Range of hearing in adult humans: Minimum frequency: 20 Hz Maximum frequency: 20,000 Hz Person with a huge head. Person with a small head. Dogs: Upper limit is between 25,000 and 40,000 Bats: Upper limit is 100,000 Hz. Old Person trick – Driving kids away: Play high frequency sounds. G9 Ch 11 - 6 3/21/5 III. Phase Time at which a waveform begins, relative to some arbitrary time point. Often phase is used to describe the relationship between starting points of two waveforms. For example, here are two identical waveforms, completely “out-of-phase” with each other: Phase relationships between two sounds are typically measured in degrees. Two sounds with same phase: 0. Two sounds with opposite phases: 180. (The illustration above.) Use: If two sounds have a phase relationship of 180, their sum will be complete silence. This is how some noise canceling devices work. A sound with a 180 phase relationship with the offending sound is generated by an amplifier in the headphones and played in the ear, canceling the offending sound. The result is silencing of the noise. Beautiful Music + Signal Reversed phase Headphone combiner ambient noise Ambient Noise Beautiful Music Ear Ambient Noise Reversed Phase Ambient noise Phase reverser Tiny microphone Could use Audacity to demonstrate out-of-phase addition of sounds. IV. Complexity. The number of, amplitudes of, frequencies of, and phases of the collection of sounds that are combined into a single composite sound. The experience of complexity is called timbre. G9 Ch 11 - 7 3/21/5 Most important periodic waveform: The sine wave; G9 p 263. Pictorially Hypotenuse Opposite side Sin of an angle is ratio of length of opposite side to hypotenuse of a right triangle. The waveform of a sine wave is: Pressure = sin(time) As the angle changes, sin changes. Reason for importance: 1. Mathematical: Can be used as a building block for any periodic waveform. ANY other periodic waveform can be created by adding the appropriate combination of sine waves. Converse: ANY complex waveform can be analyzed (broken up) into a collection of sine wave components which would recreate the complex waveform if added together. This analysis is called Fourier (for ee ay) analysis after Joseph Fourier who discovered it. Note also that this means that if we have a bunch of sine wave generators – about 100 would probably do – we can create virtually any sound imaginable. We can reproduce anyone’s voice or any other sound we would like to produce or reproduce, such as voices generated by phones. Think of sound chefs, like food chefs, adding a pinch of 1000 Hz, a pinch of 4000Hz etc. Every sound is made up of sine waves. Fourier told us how to discover what they are. Many artificial voices are created in this way. 2. Psychological: Sine waves sound purer than any other sounds. (This is why G9 labeled the section on Sine Waves, “Pure Tones”.) Leads to the question: What characteristic of our auditory system makes the sine wave sound so pure? Joseph Fourier 1768-1830 G9 Ch 11 - 8 3/21/5 Second Way of Representing Sounds: G9 p 266 Sound Spectrum: frequencies. A plot of intensities of sine wave components of the waveform vs. their Note: This is analogous to the spectrum of a light, studied in the chapter on color. Why? The waveform gives only a fraction of a second’s worth of the sound. It’s difficult to discern the frequencies that have been added to create a complex sound. Think: Structuralists. Thus, we need a way of visually representing the frequencies that comprise each sound so we can correlate the actual sound frequencies with auditory system responses. G9 Ch 11 - 9 3/21/5 Some common sounds and their waveforms and spectra (See Class Videos: Hearing Lecture pages 10 thru 12) Individual Sine Waves of various frequencies . .(Sigview: Individual sine waveforms & FFTs . . .) F = 1/.01 = 100 100 Hz waveform 100 Hz Spectrum 500 Hz waveform 500 Hz Spectrum 5000 Hz waveform 5000 Hz spectrum 10000 Hz waveform 10000 Hz spectrum G9 Ch 11 - 10 3/21/5 More importantly, the spectrum gives us the frequencies making up composites of sine waves 100 Hz + 5000 Hz sine wave (Sigview – Two tone combo – 100,5000, and 500,5000.sws) 100+5000 Waveform Spectrum 500 Hz + 5000 Hz sine wave 500+5000 Waveform Spectrum Three tone combinations 100 + 500 + 5000 Hz sine waves 100+500+5000 Waveform Spectrum G9 Ch 11 - 11 3/21/5 Spectra tell us stuff about sounds that we might not suspect A 500 Hz Square Wave (Sigview –Open signal – 500 Hz 1 sec Square.wav; Edit -> Zoom to 512 ) The Spectrum of the 500 Hz Square waveform It can be shown that a square wave can be created by forming the sum of an infinite number of sine waves, with each successive sine wave being the next odd harmonic (1 times, 3 time, 5 time, 7 times, etc) of the fundamental frequency (500 in this example) and intensity equal to the reciprocal of the harmonic number (1/3, 1/5, 1/7, 1/9, etc). G9 Ch 11 - 12 3/21/5 Spectra tell us about different kinds of noise – Pink, Blue, and White (Blue,White,Pink Noise 1 sec Full View.sws) Pink Waveform Pink Spectrum Blue Waveform Blue Spectrum White Waveform White Spectrum G9 Ch 11 - 13 3/21/5 Physical vs. Psychological Characteristics of Sound – G9 p 267 Physical --> Psychological Intensity --> Loudness: The experience of sound intensity. Frequency . Complexity --> Pitch: The experience of tone frequency --> Timbre: The experience of sound complexity Relationship of Loudness to Intensity Loudness measurement has often involved the method of magnitude estimation – Ch 1. Method of Magnitude Estimation: A standard tone intensity is presented. Participants assign it the arbitrary loudness value of 100. A series of randomly selected intensities is presented. Participants assign each a loudness number representing that the tone’s intensity relative to the standard. An often stated equation is L = I0.5 or L = square root of I. 10.00 8.00 Loudness Value l 6.00 4.00 2.00 0.00 100.00 95.00 90.00 85.00 80.00 75.00 70.00 65.00 60.00 55.00 50.00 45.00 40.00 35.00 30.00 25.00 20.00 15.00 10.00 5.00 .00 i Intensity Implications: Suppose you increase the power of an amplifier from 100 Watts to 200 Watts, increasing intensity by a factor of 2. Loudness will increase by only a factor of 1.4. 100% intensity increase leads to a 40% loudness increase. Suppose you wanted to increase the loudness of your stereo system. Intensity Loudness So you would have to make the sound 4 times as intense for it to 1 1 be perceived as 2 times as loud. 2 1.4 3 1.7 If you want to drive the person who lives next to you crazy, 4 2 you’ll have to make your sound system 4 or 16 times as intense. 5 2.2 Twice as intense probably won’t work. G9 Ch 11 - 14 3/21/5 Argh!! Loudness Depends on Frequency – Equal Loudness Contours – G9 p 268 Each line is a line of equal loudness. Height of the line is how intense it must be to achieve the level of loudness it represents. Note that the Intensity (height of line) needed to achieve equal loudness varies across frequencies. Intensity Low and high frequency sounds must be very intense to have a specific loudness. Generally, the least intensity for a given loudness is needed at about 3,000 Hz. This graph illustrates two major results. 1) The intensity required to achieve a specific loudness varies as frequency of a tone of constant loudness changes, with the lowest intensity required for a given loudness always around 2000-3000 Hz. Tones of extremely low or extremely high frequencies are not perceived as loud as middle-frequency tones of the same intensity. 2. The curves are flatter at higher intensities – near the top of the graph. The intensity required to create a high loudness sound is roughly constant – top line in graph. But the intensity required to create equally low loudness changes considerably – brown line in graph. This means that at high loudness, it doesn’t what the frequency of the sound is – it’s all loud. But at low loudness, the frequency of the sound matters a lot – middle frequency sounds need not be nearly as intense as low or high frequency sounds. To correct for inequities at low intensities, all modern audio reproduction equipment has built in circuits that boost the intensities of low frequency and high frequency sounds when overall intensity is low. This helps make the perception of sound quality of soft music the same as the perception of loud music. . In the olden days, amplifiers used to have a switch, called the “Loudness” switch. The user had to flip that switch in order to get the boost of low and high frequency sounds. Now, the boost occurs automatically. “Thanks, modern science and engineering!!” G9 Ch 11 - 15 3/21/5 Pitch vs. Frequency G9 p 268 Frequency measured in Hz – cycles per second. Pitch measured in mels. Based on behavioral measures analogous to those used to define loudness. Relationship of Pitch in mels to frequency of pure tones in Hz is not easily characterized, but it’s essentially a straight line relationship. Pitch in mels Frequency Timbre vs. Complexity – G9 p 269 Many sounds are composed of a fundamental frequency and other high frequencies that are integer multiples of the fundamental frequency. In such cases, the fundamental frequency is called the first harmonic. The higher frequencies are named according to their relationship to the fundamental. A frequency twice the fundamental is the 2nd harmonic. 3 times the fundamental is the 3rd harmonic, etc. Many naturally occurring sounds have this fundamental + harmonics structure that we apparently are structured to hear the harmonics as being like the fundamental. This accounts for the fact that musical notes that are integer multiples of other notes sound similar to those other notes. These are the octave similarities so apparent in music. G9 Ch 11 - 16 3/21/5 Timbre and spectrum differences between a sine wave and a square wave. For both sounds, the fundamental frequency is the same – 500 Hz. But the square wave has a ton of higher harmonics – the 3rd, 5th, 7th, 9th, 11th, etc. (500 Hz sine and square waveforms and FFTs.sws) The square wave sounds different from the sine wave – because its timbre is quite different. In the case of a square wave – the extra sounds are the odd harmonics – 3rd, 5th, 7th, etc. We hear those odd harmonics. So the timbre of a square wave is different from that of a sine wave. Important Questions: What’s going on? How can we hear the sine wave components? We can’t see the individual wavelength components of a light – all we can see is the composite. But we CAN hear the sine wave components of a sound. What’s the explanation for these differences? G9 Ch 11 - 17 3/21/5 Auditory Apparatus – G9 p 270 Outer ear Earwig Pinna – Ear flap. Some amplification compared to a simple hole in head. Auditory canal 1” long; About 6 cc in volume Earwigs are fairly abundant and found in many areas of the world. There is no evidence that they transmit diseases to humans or other animals. Their pincers are commonly believed to be dangerous, but in reality even the curved pincers of males cause little harm to humans.[45] It is a common myth that earwigs crawl into the human ear and lay eggs in the brain.[46][47] Finding earwigs in the human ear is rare, as most species do not fly and prefer dark and damp areas (such as basements) rather than typical bedrooms.[4] Auditory canal resonates at about 3400 Hz This means that sounds at 3400 Hz are more intense – about 5-10 db - than sounds of other frequencies Intensity of sound at inner end of ear canal 10 db Tympanic membrane - Ear drum Vibrates in unison with the air pressure changes. Transforms air pressure changes into movement. Vibrates in unison with the vibration of the device causing the sound. Punctures leave danger of intrusion. G9 Ch 11 - 18 3/21/5 Middle ear A Rube Goldberg-seeming device that transmits vibration of the eardrum to the inner ear. Example of a Rube Goldberg cartoon G9 Ch 11 - 19 3/21/5 Middle ear continued . . . Three smallest bones of body. Eardrum connects to malleus Malleus connects to incus. Incus connect to stapes. Stapes is attached to a membrane that is stretched across a hole in the cochlea called the Oval Window. Goldstein’s Yantis’s – note, the cochlea is a cave in bone. Round window Functions of middle ear 1. To increase force of vibrations of eardrum. G9 p. 272 a. By concentrating the area of force from the huge eardrum to the small stapes footplate – the high heel shoe effect. b. By increasing the force through a lever action These two factors results in a 22:1 increase in the force of movement from the eardrum movement to the movement of the oval window membrane. That 22:1 ratio is approximately a 30 db increase in sound level. That is, sounds would be 30 db lower than they are if we didn’t have the middle ear. Mike – demo this if you have the sound equipment in the room. G9 Ch 11 - 20 3/21/5 Functions of middle ear continued . . . G9 p272 2. To control intensity of sound reaching inner ear through the acoustic reflex There are two sets of muscles attached to the bones of the middle ear . . . Tensor tympani muscle connects to malleus Stapedius muscle connect to stapes When these muscles contract . . . Malleus is pulled to one side – so doesn’t impart as much movement to incus Stapes is forced to move from side-to-side rather than back and forth The effect of contraction of these muscles is to reduce the sound pressure reaching the inner ear by several db. They contract in two types of situations. 2 A) In response to loud sounds, but the latency is about .150 seconds, too long to prevent damage. .150 is about 1/6 of a second. In that time, 83 pressure changes of a 500 Hz tone will affect the inner ear. 833 pressure changes of a 5,000 Hz tone of a 5000 Hz tone would get through. Here’s a combination tone 500+5000 – in the first .002 seconds (2 milliseconds), 1 low frequency and 11 high frequency pressure changes occurred. So, in .150 second, that would be 75 low and 8250 high frequency changes. 2 B) In response to self-produced noises in the mouth. So there are two possible reasons for the acoustic reflex . . . 2 A) To reduce levels of loud sounds, although latency is too long to protect against high frequency sounds of high intensity. It’s like having an anti-missile defense system that has to be taken out of storage before it’ll work. 2 B) To reduce noise of self-produced sounds, the distraction of eating and talking, This is probably the major function of these muscles. G9 Ch 11 - 21 3/21/5 Inner ear G9 p 272 Structure is cochlea A cave in the skull. Filled with fluid Two windows covered with membranes – oval window and round window. Semi-circular canals Auditory Nerve Stapes attached to oval window Round window The whole auditory apparatus, so you can see the relative sizes of the structures. From Y1 p 297 G9 Ch 11 - 22 3/21/5 Interior structures of the cochlea Two membranes run its length – Reissner’s membrane and the basilar membrane Close-up of the cochlea Vestibular Canal Reissner’s Membrane Close-up of a cross-section of the cochlea Tympanic Canal What happens in the cochlea Movement of the stapes causes the membrane covering the oval window to move in and out. This, in turn causes fluid movement in the Vestibular canal which leads to fluid movement in the Tympanic canal. That fluid movement causes the basilar membrane to move – vibrating like a sheet or rug having the dirt shaken out of it. G9 Ch 11 - 23 3/21/5 Here is an end-view of the basilar membrane and Organ of Corti – analogous to looking south down Lookout Mountain – C:\Users\Michael\Desktop\Desktop folders\Class Videos\Going South on Lookout Mountain.mp4 Receptors Two sets of cells with hairlike extensions (steriocilia) run the length of the basilar membrane. The cells are called hair cells. One set is a single row running down one side of the membrane. These are the inner hair cells – about 3500. The other set is a triple row running down the other side. They’re called the outer hair cells – about 12,000 total. We now know that it is the inner hair cells that are the receptors that transduce the movement of the basilar membrane into action potentials. The outer hair cells modulate the responses of the inner hair calls. G9 Ch 11 - 24 3/21/5 More on the Action of basilar membrane and receptors (Possible test question.) Movement of stapes causes pressure changes in the vestibular/tympanic canals. These pressure waves cause ripples on the basilar membrane – like an earthquake moving down Lookout Mt. toward Fort Payne This movement causes the stereocilia attached to the hair cells to bend. (As the trees on Lookout Mt. would bend if an earthquake moved down the mountain.) Inner hair cells The bending of the cilia causes inner hair cells to release neurotransmitter substance which triggers action potentials in auditory nerve neurons whose axons make up the auditory nerve and whose dendrites are located near the base of the inner hair cells. This is illustrated and described in G9 p 274 Figure 11.18. So the inner hair cells are the receptors for audition. G9 Ch 11 - 25 3/21/5 Outer hair cells The movement of stereocilia of outer hair cells causes the outer hair cells to become slightly longer, with the result that the inner hair cells release more neurotransmitter substance, in effect amplifying the response of the inner hair cells. The outer hair cells increase the inner hair cells response only to specific frequencies, in effect, sharpening the response of the inner hair cells to whatever frequency they are responding to. Stimulation of the outer hair cells from higher brain centers causes an attenuation of the responses of the inner hair cells. So the outer hair cells act kind of like a volume control for the inner hair cells – amplifying the responses of the inner hair cells at some times and attenuating them at other times. Show Virtual Lab 11-10 (Cilia Movement) and Virtual Lab 11-13 (Cochlear Amplifier) here to show the action of the basilar membrane along with the amplification associated with the outer hair cells described on p. 277. Show how the hair cells are changing their lengths in VL 11-13. G9 Ch 11 - 26 3/21/5 Coding Frequency – how do we perceive the frequency of a sound? Helmholtz’s Place Theory In the late 1800s Helmholtz believed that the basilar membrane is composed of fibers running at right angles to the length of the membrane. Helmholtz believed that these fibers are strung tautly, like the strings of a harp. High frequency string Low frequency string Sound caused them to vibrate, just as the strings of a harp vibrate in the presence of sounds. Short fibers vibrate most to high frequency sounds. Long fibers vibrate most to low frequency sounds. So he believed that place of vibration is the signal for frequency. The brain will know what the frequency of sound is by knowing where the vibration is occurring. G9 Ch 11 - 27 3/21/5 von Bekesy’s Traveling Wave Theory G9 p 275 Georg Von Békésy carefully examined inner ears of cadavers and built a model of the basilar membrane based on his examinations. . He found that the membrane was a continuous sheet – not a collection of strings. He also found that it was loosely bound, not strung tightly. He proposed that the response of the membrane to sound is a wave that travels from the base of the cochlea to the apex. Like a sheet or rug being snapped to shake off dirt. Shape of the membrane during its “shaking” is illustrate by the figure below Point of maximum movement Point of maximum movement Play C:\Users\Michael\Desktop\Desktop Folders\Classes\Basilar Membrane Animation.mp4 here (Original at https://www.youtube.com/watch?v=dyenMluFaUw ) G9 Ch 11 - 28 3/21/5 The bottom line: The point at which membrane movement is greatest depends on frequency of the sound. So the pattern of vibration of the basilar membrane represents a kind of spectrum of the sound. High frequency sounds: Movement is greatest near the base – near the oval window end. Hair cells at the base release the most neurotransmitter substance. Low frequency sounds: Movement is greatest near the apex. Hair cells near the apex release the most neurotransmitter. Complex sounds: There are various amplitude peaks on the membrane as it vibrates – with the location of each peak corresponding to a frequency component of the complex sound. G9 Ch 11 - 29 3/21/5 Implications 1) The location of point of maximum vibration depends on frequency. 2) Inner hair cells at a specific place on the basilar membrane respond the most when the basilar membrane at their location vibrates the most. 3) Since each auditory nerve synapses only with hair cells at a specific place on the basilar membrane, this means that each auditory nerve responds to a specific frequency in the sound stimulus. Each auditory nerve is “tuned” to a different frequency. The collection of responses of the several thousand auditory nerves is like a spectrum. They perform a rough Fourier analysis of the incoming sound. Spectrum of a high frequency sound Sound Intensity Frequency Basilar membrane Base Apex Auditory nerves. Only the red one is active Spectrum of a low frequency sound Sound Intensity Basilar membrane Base Apex Auditory nerves. Only the red one is active. G9 Ch 11 - 30 3/21/5 Spectrum of a complex sound Sound Intensity Frequency Basilar membrane Base Apex Auditory nerves. Red ones are active Practical Applications . . . This means that if you wanted to give a person the experience of hearing by direct stimulation, you could not stimulate just one auditory nerve cell. Because doing so would give the person sound experience of only one frequency. And you can’t just stimulate all the auditory nerves equally. Instead, you have to stimulate multiple auditory nerves, and the ones you stimulate must be distributed along the basilar membrane, and you must stimulate each nerve with just the appropriate frequency. This fact has been used by developers of cochlear implants – devices used to help persons who have lost the use of their hair cell receptors hear. In this figure, the bluish cord labeled electrode is really a collection of electrodes that is laid along the basilar membrane. This way, each metallic band can stimululate nerve fibers at a different places on the membrane each with the appropriate frequency. The result of stimulation by a whole group of such electrodes is that a whole collection of auditory nerve fibers are activated, such as they are in normal hearing. Play Desktop folders\Class Videos\Implanting cochlear electrodes.mp4 What do persons with cochlear implants hear? PlayDesktop folders\Class Videos\Cochlear Implant Simulation - Hearing Speech.mp4 minutes. G9 Ch 11 - 31 2+ 3/21/5 More on Coding Frequency - Temporal theory, aka Frequency Theory, aka Telephone Theory In this theory it was assumed the basilar membrane vibrated as a whole, like the membrane of a telephone microphone It has been called the telephone theory. Assumed the basilar membrane vibrated as a whole, in unison with sound – higher the sound frequency, faster the membrane vibrated. Assumed that somehow, the whole-membrane vibration was transmitted to higher neural centers. For example, by neurons that fired each time the membrane moved. Problem 1: Von Bekesy discovered that the basilar membrane does not vibrate “as a whole”, as assumed by telephone theory. Problem 2: We can perceive sounds whose frequencies are as high as 20,000 Hz, but neurons cannot respond at rates higher than 1000 action potentials per second, if that high. So the theory, unaltered, cannot account for our ability to hear sounds above 1000 Hz. Volley principle: To account for the 1000 Hz limit, researchers proposed that no single neuron responded with the membrane, but that neurons “took turns” responding, so that each individual neuron responded, say, every 10th vibration or every 20th. This would allow the collection of neurons to signal frequency while not requiring any one to fire at a rate greater than 1000 APs/sec. We now know that : Apparently, low frequency sounds do cause movement of the whole membrane, in unison with the sound. (This “whole-movement” of the membrane is not shown in the animations played earlier.) Recent research has shown that, in fact, many neurons in the auditory chain DO respond in unison with the sound wave – responding only at the highest pressure point of each wave, for example. And, it now appears that neurons do trade off with each other, just as supposed by the proponents of the volley principal many years ago. G9 Ch 11 - 32 3/21/5 Current Theory – So the current theory of how sound frequency is processed is one that suggests that 1) our perception of frequencies of low frequency sounds (say those < 5000 Hz) may be due to neurons responding in unison with the gross movement of the basilar membrane. 2) our perception of frequencies of high frequency sounds (say those > 5000 Hz) may be due to activity of different neurons at different locations on the basilar membrane. Interestingly, as noted on G9 p 280-281, the responses in unison of neurons with frequencies less than 5,000 Hz may explain why we only perceive melodies when they consist of frequencies below 5,000 Hz. Our sense of pitch may change in some fundamental way at 5,000 Hz. G9 Ch 11 - 33 3/21/5 Hearing Loss: Measuring Auditory Ability Audiogram: A plot of thresholds in reverse scale on the vertical axis vs. frequency on the horizontal axis. Reverse scale means that high thresholds (poor hearing) are represented at the bottom of the vertical axis. Normal hearing thresholds are at the top of the vertical axis. Normal hearing 0 10 High-frequency loss 0 10 Note that the high frequency loss is about the same in both hears. This means that whatever caused it – listening to loud music, shooting guns – affected both ears. G9 Ch 11 - 34 3/21/5 Categories of hearing impairment Conductive Loss due to impairment of the mechanisms taking sound to the cochlea The pinna, ear canal, tympanic membrane, ossicles Maximum loss is about 30 db, which means you can still hear, even though you’ve completely lost your outer and middle ear function. An audiogram (from the web) showing a common result of conductive hearing loss. The dotted line represents normal hearing with no conductive loss. The solid line represents conduction hearing loss. Generally, conductive loss is at all frequencies. G9 Ch 11 - 35 3/21/5 Sensorineural Loss due to damage to the cochlea or parts of it. The basilar membrane, the hair cells, the auditory nerve. Typically, loss is greatest for high frequency sounds From the web – Audiogram of a person with high frequency loss – presumably due to sensorineural damage. As in the above example, note that the hearing loss is binaural – affecting hearing through both ears. G9 Ch 11 - 36 3/21/5 Why is ability to hear high frequencies lost first? Answer: Almost all sounds contain high frequency components. Noise is the worst, but even simple waveforms like square waves contain high frequencies. Look at the spectrum of a square wave below. Note that though the fundamental frequency is 500 Hz, the wave contains energy at 1500, 2500, 3500, 4500, 5500, etc. Square Wave Spectrum As mentioned before, many many sounds that we hear are composed of the combination of a fundamental frequency and higher harmonics. So our ears are continually exposed to high frequency sounds even though the fundamental frequencies of those sounds are not so high. Now look at the spectrum of white noise – such as the sound of an explosion or gunshot. Note that MOST of the energy in white noise is at frequencies that we would call high frequencies. Since much of the noise we encounter in real life is like white noise, much of the noise we encounter has lots of high frequency components. Gunshots and explosions are the worst. But even music has LOTS of high frequency components – that’s why it sounds so good. This means that the hair calls near the base of the basilar membrane are exercised the most and are the most likely to “wear out”. G9 Ch 11 - 37 3/21/5 Play VL 11-14 Hearing Loss here. Mike – It’s now an .mp4 in the Class Videos folder VL 11-14 Children, Beethoven, two speakers.mp4. “Classic: Beethoven” is most striking, showing the loss of ability to hear high frequencies “Children singing” also illustrates high frequency loss. Summary of the hearing process – 7 minutes https://www.youtube.com/watch?v=PeTriGTENoc G9 Ch 11 - 38