1. Chapter 11: Hearing

2. Physical Aspects of Sound
Two definitions of “sound”
Physical definition - sound is pressure changes in the air or other medium.
Perceptual definition - sound is the experience we have when we hear.

3. Sound as Pressure Changes
Loud speakers produce sound by:
The diaphragm of the speaker moves out, pushing air molecules together called condensation.
The diaphragm also moves in, pulling the air molecules apart called rarefaction.
The cycle of this process creates alternating high- and low-pressure regions that travel through the air.

4. Figure 11.1 (a) The effect of a vibrating speaker diaphragm on the surrounding air. Dark areas represent regions of high air pressure, and light areas represent areas of low air pressure. (b) When a pebble is dropped into still water, the resulting ripples appear to move outward. However, the water is actually moving up and down, as indicated by movement of the boat. A similar situation exists for the sound waves produced by the speaker in (a).
Figure 11-1 p263

5. Pure Tones
Pure tone - created by a sine wave
Amplitude - difference in pressure between high and low peaks of wave
Perception of amplitude is loudness
Decibel (dB) is used as the measure of loudness
Number of dB = 20 logarithm(p/po)
The decibel scale relates the amplitude of the stimulus with the psychological experience of loudness.

6. Pure Tones - continued
Frequency - number of cycles within a given time period
Measured in Hertz (Hz) - 1 Hz is one cycle per second
Perception of pitch is related to frequency.
Tone height is the increase in pitch that happens when frequency is increased.

7. Figure 11. 2 (a) Plot of sine-wave pressure changes for a pure tone
Figure 11.2 (a) Plot of sine-wave pressure changes for a pure tone. (b) Pressure changes are indicated, as in Figure 11.1, by darkening (pressure increased relative to atmospheric pressure) and lightening (pressure decreased relative to atmospheric pressure).
Figure 11-2 p264

8. Figure 11. 3 Three different frequencies of a pure tone
Figure 11.3 Three different frequencies of a pure tone. Higher frequencies are associated with the perception of higher pitches.
Figure 11-3 p264

9. Figure 11. 4 Three different amplitudes of a pure tone
Figure 11.4 Three different amplitudes of a pure tone. Larger amplitude is associated with the perception of greater loudness.
Figure 11-4 p264

10. Table 11-1 p265

11. Complex Tones and Frequency Spectra
Both pure and some complex tones are periodic tones.
Fundamental frequency is the repetition rate and is called the first harmonic.
Periodic complex tones consist of a number of pure tones called harmonics.
Additional harmonics are multiples of the fundamental frequency.

12. Complex Tones and Frequency Spectra - continued
Additive synthesis - process of adding harmonics to create complex sounds
Frequency spectrum - display of harmonics of a complex sound

13. Figure 11.5 Left: Waveforms of (a) a complex periodic sound with a fundamental frequency of 200 Hz; (b) fundamental (first harmonic) = 200 Hz; (c) second harmonic = 400 Hz; (d) third harmonic = 600 Hz; (e) fourth harmonic = 800 Hz. Right: Frequency spectra for each of the tones on the left.
Figure 11-5 p266

14. Figure 11. 6 (a) The complex tone from Figure 11
Figure 11.6 (a) The complex tone from Figure 11.5a and its frequency spectrum; (b) the same tone with its first harmonic removed.
Figure 11-6 p266

15. Perceptual Aspects of Sound
Loudness is the perceptual quality most closely related to the level or amplitude of an auditory stimulus
Decibels
Audibility curve
Auditory response area

16. Figure 11.7 Loudness of a 100-Hz tone as a function of intensity, determined using magnitude estimation.
Figure 11-7 p267

17. Perceptual Aspects of Sound - continued
Human hearing range - 20 to 20,000 Hz
Audibility curve - shows the threshold of hearing in relation to frequency
Changes on this curve show that humans are most sensitive to 2,000 to 4,000 Hz.
Auditory response area - falls between the audibility curve and and the threshold for feeling
It shows the range of response for human audition.

18. Perceptual Aspects of Sound - continued
Equal loudness curves - determined by using a standard 1,000 Hz tone
Two dB levels are used - 40 and 80
Participants match the perceived loudness of all other tones to the 1,000 Hz standard.
Resulting curves show that tones sound
Almost equal loudness at 80 dB.
Softer at 40 dB for high and low frequencies than the rest of the tones in the range.

19. Figure 11. 8 The audibility curve and the auditory response area
Figure 11.8 The audibility curve and the auditory response area. Hearing occurs in the light green area between the audibility curve (the threshold for hearing) and the upper curve (the threshold for feeling). Tones with combinations of dB and frequency that place them in the light red area below the audibility curve cannot be heard. Tones above the threshold of feeling result in pain. The frequencies between the places where the dashed line at 10 dB crosses the audibility function indicate which frequencies can be heard at 10 dB SPL.
Figure 11-8 p268

20. Perceptual Aspects of Sound - continued
Pitch – the perceptual quality we describe as high and low

21. Figure 11.9 A piano keyboard, indicating the frequency associated with each key. Moving up the keyboard to the right increases frequency and tone height. Notes with the same letter, like the A’s (arrows), have the same tone chroma.
Figure 11-9 p269

22. Perceptual Aspects of Sound - continued
Timbre - all other perceptual aspects of a sound besides loudness, pitch, and duration
It is closely related to the harmonics, attack and decay of a tone.
Effect of missing fundamental frequency
Removal of the first harmonic results in a sound with the same perceived pitch, but with a different timbre.
This is called periodicity pitch.

23. Figure Frequency spectra for a guitar, a bassoon, and an alto saxophone playing a tone with a fundamental frequency of 196 Hz. The position of the lines on the horizontal axis indicates the frequencies of the harmonics and their height indicates their intensities.
Figure p270

24. Perceptual Aspects of Sound - continued
Attack of tones - buildup of sound at the beginning of a tone
Decay of tones - decrease in sound at end of tone

25. From Pressure Changes to Electricity
Outer ear - pinna and auditory canal
Pinna helps with sound location.
Auditory canal - tube-like 3 cm long structure
It protects the tympanic membrane at the end of the canal.
The resonant frequency of the canal amplifies frequencies between 1,000 and 5,000 Hz.

26. Figure 11.11 The ear, showing its three subdivisions—outer, middle, and inner.
Figure p271

27. From Pressure Changes to Electricity - continued
Middle ear
Two cubic centimeter cavity separating inner from outer ear
It contains the three ossicles
Malleus - moves due to the vibration of the tympanic membrane
Incus - transmits vibrations of malleus
Stapes - transmit vibrations of incus to the inner ear via the oval window of the cochlea

28. From Pressure Changes to Electricity - continued
Function of Ossicles
Outer and inner ear are filled with air.
Inner ear is filled with fluid that is much denser than air.
Pressure changes in air transmit poorly into the denser medium.
Ossicles act to amplify the vibration for better transmission to the fluid.
Middle ear muscles dampen the ossicles’ vibrations to protect the inner ear from potentially damaging stimuli.

29. Figure The middle ear. The three bones of the middle ear transmit the vibrations of the tympanic membrane to the inner ear.
Figure p272

30. Figure 11. 13 Environments inside the outer, middle, and inner ears
Figure Environments inside the outer, middle, and inner ears. The fact that liquid fills the inner ear poses a problem for the transmission of sound vibrations from the air of the middle ear.
Figure p272

31. Figure (a) A diagrammatic representation of the tympanic membrane and the stapes, showing the difference in size between the two. (b) How lever action can amplify a small force, presented on the right, to lift the large weight on the left. The lever action of the ossicles amplifies the sound vibrations reaching the tympanic inner ear.
Figure p272

32. From Pressure Changes to Electricity - continued
Inner ear
Main structure is the cochlea
Fluid-filled snail-like structure (35 mm long) set into vibration by the stapes
Divided into the scala vestibuli and scala tympani by the cochlear partition
Cochlear partition extends from the base (stapes end) to the apex (far end)
Organ of Corti contained by the cochlear partition

33. ABC Video: Ringtones and the Cochlea33

34. From Pressure Changes to Electricity - continued
Key structures
Basilar membrane vibrates in response to sound and supports the organ of Corti
Inner and outer hair cells are the receptors for hearing
Tectorial membrane extends over the hair cells

35. From Pressure Changes to Electricity - continued
Transduction takes place by:
Cilia bend in response to movement of organ of Corti and the tectorial membrane
Movement in one direction opens ion channels
Movement in the other direction closes the channels
This causes bursts of electrical signals.

36. Figure 11. 15 (a) A partially uncoiled cochlea
Figure (a) A partially uncoiled cochlea. (b) A fully uncoiled cochlea. The cochlear partition, which is indicated here by a line, actually contains the basilar membrane and the organ of Corti, as shown in Figure
Figure p273

37. Figure 11. 16 (a) Cross- section of the cochlea
Figure (a) Cross- section of the cochlea. (b) Close-up of the organ of Corti, showing how it rests on the basilar membrane. Arrows indicate the motions of the basilar membrane and tectorial membrane that are caused by vibration of the cochlear partition.
Figure p273

38. Figure Scanning electron micrograph showing inner hair cells (top) and the three rows of outer hair cells (bottom). The hair cells have been colored to stand out.
Figure p274

39. Figure (a) How movement of the hair cell cilia causes an electrical change in the hair cell. When the cilia are bent to the right, the tip links are stretched and ion channels are opened. Positively charged potassium ions (K+) enter the cell, causing the interior of the cell to become more positive. (b) When the cilia move to the left, the tip links slacken, and the channels close.
Figure p274

40. Figure How hair cell activation and auditory nerve fiber firing are synchronized with pressure changes of the stimulus. The auditory nerve fiber fires when the cilia are bent to the right. This occurs at the peak of the sine-wave change in pressure.
Figure p275

41. Figure 11. 20 (a) Pressure changes for a 250-Hz tone
Figure (a) Pressure changes for a 250-Hz tone. (b) Pattern of nerve spikes produced by two separate nerve fibers. Notice that the spikes always occur at the peak of the pressure wave. (c) The combined spikes produced by 500 nerve fibers. Although there is some variability in the single neuron response, the response of the large group of neurons represents the periodicity of the 250-Hz tone.
Figure p275

42. Vibrations of the Basilar Membrane
There are two ways nerve fibers signal frequency:
Which fibers are responding
Specific groups of hair cells on basilar membrane activate a specific set of nerve fibers;
How fibers are firing
Rate or pattern of firing of nerve impulses

43. Vibrations of the Basilar Membrane - continued
Békésys’ Place Theory of Hearing
Frequency of sound is indicated by the place on the organ of Corti that has the highest firing rate.
Békésy determined this in two ways:
Direct observation of the basilar membrane in cadavers.
Building a model of the cochlea using the physical properties of the basilar membrane.

44. Vibrations of the Basilar Membrane - continued
Physical properties of the basilar membrane
Base of the membrane (by stapes) is:
Three to four times narrower than at the apex.
100 times stiffer than at the apex.
Both the model and direct observation showed that the vibrating motion of the membrane is a traveling wave .

45. Vibrations of the Basilar Membrane - continued
Envelope of the traveling wave
Indicates the point of maximum displacement of the basilar membrane
Hair cells at this point are stimulated the most strongly leading to the nerve fibers firing the most strongly at this location.
Position of the peak is a function of frequency.

46. Figure 11. 21 (a) A traveling wave like the one observed by Békésy
Figure (a) A traveling wave like the one observed by Békésy. This picture shows what the membrane looks like when the vibration is “frozen” with the wave about two-thirds of the way down the membrane. (b) Side views of the traveling wave caused by a pure tone, showing the position of the membrane at three instants in time as the wave moves from the base to the apex of the cochlear partition.
Figure p276

47. Figure The amount of vibration at different locations along the basilar membrane is indicated by the size of the arrows at each location, with the place of maximum vibration indicated in red. When the frequency is 25 Hz, maximum vibration occurs at the apex of the cochlear partition. As the frequency is increased, the location of the maximum vibration moves toward the base of the cochlear partition.
Figure p276

48. Evidence for Place Theory
Tonotopic map
Cochlea shows an orderly map of frequencies along its length
Apex responds best to low frequencies
Base responds best to high frequencies

49. Figure 11. 23 Tonotopic map of the guinea pig cochlea
Figure Tonotopic map of the guinea pig cochlea. Numbers indicate the location of the maximum electrical response for each frequency.
Figure p277

50. Evidence for Place Theory - continued
Neural frequency tuning curves
Pure tones are used to determine the threshold for specific frequencies measured at single neurons.
Plotting thresholds for frequencies results in tuning curves.
Frequency to which the neuron is most sensitive is the characteristic frequency.

51. Figure 11. 24 Frequency tuning curves of cat auditory nerve fibers
Figure Frequency tuning curves of cat auditory nerve fibers. The characteristic frequency of each fiber is indicated by the arrows along the frequency axis. The frequency scale is in kilohertz (kHz), where 1 kHz = 1,000 Hz.
Figure p277

52. A Practical Application
Cochlear Implants
Electrodes are inserted into the cochlea to electrically stimulate auditory nerve fibers.
The device is made up of:
a microphone worn behind the ear,
a sound processor,
a transmitter mounted on the mastoid bone,
and a receiver surgically mounted on the mastoid bone.

53. Figure 11.25 Cochlear implant device. See text for details.
Figure p278

54. Updating Békésy’s: The Cochlear Amplifier
Békésy used basilar membranes isolated from cadavers and his results showed no difference in response for close frequencies that people can distinguish.
New research with live membranes shows that the entire outer hair cells respond to sound by slight tilting and a change in length.
For this reason these cells are called the cochlear amplifier.

55. Figure The outer hair cell cochlear amplifier mechanism occurs when the cells (a) elongate when cilia bend in one direction and (b) contract when the cilia bend in the other direction. This results in an amplifying effect on the motion of the basilar membrane.
Figure p278

56. Figure Effect of outer hair cell damage on the frequency tuning curve. The solid curve is the frequency tuning curve of a neuron with a characteristic frequency of about 8,000 Hz (arrow). The dashed curve is the frequency tuning curve for the same neuron after the outer hair cells were destroyed by injection of a chemical.
Figure p279

57. Complex Tones and Vibration of the Basilar Membrane
Basilar membrane can be described as an acoustic prism.
There are peaks in the membrane’s vibration that correspond to each harmonic in a complex tone.
Each peak is associated with the frequency of a harmonic.

58. Figure (a) Waveform of a complex tone consisting of three harmonics. (b) Basilar membrane. The shaded areas indicate locations of peak vibration associated with each harmonic in the complex tone.
Figure p279

59. The Physiology of Pitch Perception
Phase locking
Nerve fibers fire in bursts.
Firing bursts happen at or near the peak of the sine-wave stimulus.
Thus, they are “locked in phase” with the wave.
Groups of fibers fire with periods of silent intervals creating a pattern of firing.

60. Figure 11.29 Perceptual process showing relationships that are related to the perception of pitch.

61. Pitch and the Brain
A shift for earlier emphasis on the cochlea and the auditory nerve to research on the brain
Place coding is effective for the entire range of hearing.
Temporal coding with phase locking is effective up to 5,000 Hz.

62. Figure The human brain, showing the location of the primary auditory receiving area, A1, which also extends inside the temporal lobe. Pulling the temporal lobe back reveals additional auditory areas. These areas will be described in Chapter 12.
Figure p281

63. Figure The tonotopic map on the primary auditory receiving area, A1, which is contained within the core area of the auditory cortex, shown in outline. The numbers represent the characteristic frequencies (CF) of neurons in thousands of Hz. Low CFs are on the left, and high CFs are on the right.
Figure p281

64. Figure Records from a pitch neuron recorded from the auditory cortex of marmoset monkeys. (a) Frequency spectra for tones with a fundamental frequency of 182 Hz. Each tone contains three harmonic components of the 182-Hz fundamental frequency. (b) Response of the neuron to each stimulus.
Figure p282

65. How to Damage your Hair Cells
Presbycusis
Greatest loss is at high frequencies
Affects males more severely than females
Appears to be caused by exposure to damaging noises or drugs

66. Figure 11. 33 Hearing loss in presbycusis as a function of age
Figure Hearing loss in presbycusis as a function of age. All of the curves are plotted relative to the 20-year-old curve, which is taken as the standard.
Figure p283

67. How to Damage your Hair Cells - continued
Noise-induced hearing loss
Loud noise can severely damage the Organ of Corti
OSHA standards for noise levels at work are set to protect workers
Leisure noise can also cause hearing loss

68. Figure Sound level of game 3 of the 2006 Stanley Cup finals between the Edmunton Oilers (the home team) and the Carolina Hurricanes. Sound levels were recorded by a small microphone in a spectator’s ear. The red line indicates a “safe” level for a 3-hour game.
Figure p284

69. Infant Hearing Olsho et al (1988) Audibility curves
DeCasper and Fifer (1980)
2-day old infants can recognize their mothers voice

70. Figure 11. 35 (a) Data obtained by Olsho et al
Figure (a) Data obtained by Olsho et al. (1987), showing the percentage of trials on which the observer indicated that a 3-month-old infant heard 2,000-Hz tones presented at different intensities. NS indicates no sound. (b) Audibility curves for 3- and 6-month-old infants determined from functions like the one in (a). The curve for 12-month-olds, not shown here, is similar to the curve for 6-month-olds. The adult curve is shown for comparison.
Figure p285

71. Figure This baby, from DeCasper and Fifer’s (1980) study, could control whether she heard a recording of her mother’s voice or a stranger’s voice by the way she sucked on the nipple.
Figure p285

72. Video: Infancy: Sensation and Perception72