Chapter 11: Hearing

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 (or compression). • 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 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 • 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.

7. Figure 11-2 p264

8. Figure 11-3 p264

9. 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 p266

14. Figure 11-8 p268

15. Figure 11-9 p269

16. 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.

17. Figure 11-10 p270

18. 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

19. 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.

20. Figure 11-11 p271

21. 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

22. 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.

23. Figure 11-12 p272

24. Figure 11-13 p272

25. Figure 11-14 p272

26. 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

27. 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

28. 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.

29. Figure 11-15 p273

30. Figure 11-16 p273

31. Figure 11-17 p274

32. Figure 11-18 p274

33. Figure 11-19 p275

34. Figure 11-20 p275

35. 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

36. 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.

37. 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 .

38. Figure 11-21 p276

39. 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

40. 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.

41. http://www.dnatube.com/video/603/How-do-We-Hear-The-Cochlea

42. 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.

43. Figure 11-25 p278

44. 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.

45. Figure 11-28 p279

46. Figure 11-30 p281

47. Figure 11-31 p281

48. 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

49. 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

50. Figure 11-34 p284