Chapter 11 Function of the Ear

1. Chapter 11Function of the Ear Perry C. Hanavan, Au.D.

2. Hearing and hearing tests • Hearing range between 20 – 20,000 Hz • Hearing tested between 250 – 8000 Hz • Audiogram • The ear is not equally sensitive to all frequencies. • Different for sound field and ear phones…

3. Outer Ear • The Mirror • The Tube

4. Outer Ear The Mirror: • The pinna is a very interesting part of the body! • No one knows the function of each of the nooks and crannies of the pinna …. such as localization, speech perception, etc. • Pinna serve as an “acoustic mirror”

5. Outer Ear The Mirror: • Higher frequencies (shorter sound wavelengths) are reflected back to the ear canal by the pinna • Sounds above 1500 Hz are enhanced by the “pinna effect”

6. Outer Ear The Mirror: • The pinna effect results in a natural 5-8 dB high-frequency amplification

7. Outer Ear The tube: • The external ear canal is 25-30 mm (1 inch) long, closed at the tympanic membrane and “open” at the entrance • Marshall, et al refer to this tube as “quarter wavelength resonator” • meaning that it has a 17 dB resonance (gain) near 2700 Hz in adults

8. Question The child’s ear canal is adult-like by which age? • 1 • 2 • 3 • 4 • 5

10. Outer Ear • The tube: • People naturally hear through this resonance. • Infants have shorter ear canals and their resonance can be up to 7000 Hz. • Children’s ears are adult-like by age 3.

11. Outer Ear • The tube: • With mastoid cavities and large perforations, the resonance drops to about 1500-2000 Hz. • Small perforations and tubes, have no effect.

12. Outer Ear

13. Outer Ear • The tube: • Ear canal finishes growing by age 9, and entire conchea by age 12.

14. Middle Ear • Ear “Drum”: • Actually refers to the tympanic membrane AND all of the other structures down wind… • Ossicles, muscles, tendons,…

16. Middle Ear • Tympanic Membrane: • Three layers (pars flaccida has two layers) • Vibration pattern is rather complex • Top portion (pars flaccida) does not transduce sounds very effectively.

18. Middle Ear • “Matching transformer”: • The only reason we have a middle ear is to be able to convert sound in AIR to sound in WATER. • Fish do not need (or have) middle ears since they live in water

19. Middle Ear • Matching Transformer: • Allows us to hear 30-40 dB better than if no middle ear

20. Middle Ear • Matching Transformer: • Loss of this natural transformer means that we would have a 30-40 dB loss. • Serous otitis media can give a 30-40 dB loss. • Our voice is attenuated by 30-40 dB when we are under the water. • Still not perfect! The middle ear is only about 66% efficient….

21. Middle Ear • Stapedial Reflex: • Attached to the third stirrup-like bone is the stapedial muscle. • Found in all mammals. • Protection from loud sounds (eg. our voice!)

22. Middle Ear • Stapedial Reflex: • Basis behind acoustic reflex testing in audiometry. • Contracts upon high level inputs. • Own voice, loud music and noise, amplified sound.

23. Middle Ear • Stapedial Reflex: • Usually contracts with inputs of 80-90 dB (ie., above the speech range of intensities). • Only slightly higher for those with hearing loss. • Our ear has a rudimentary high-level compression system for loud sounds.

24. How does this change the speech signal?

27. Some common hearing ranges

28. Problems of the outer and middle ears • Conductive hearing loss • Maximally 60 dB loss • Medically and/or surgically treatable

29. Examples of conductive losses • Wax (cerumen) occlusion • Eardrum perforation • Serous otitis media (ear infection) • Otosclerosis (stiffening of the ossicles)

30. Inner Ear (Cochlea) • Physiology: • Over 100,000 moving parts stuffed into a volume smaller than the tip of your baby finger. • Organ of Corti • Of the 15,000 nerve endings: • ¼ afferent (to the brain) from inner hair cells • ¾ efferent (from the brain) to the outer hair cells

31. Inner Ear (Cochlea) • Frequencies are arranged in the cochlea similar to a piano keyboard… • High frequencies are at the “basal” end near the stapes footplate • Low frequencies are at the “apical” end and are well protected by the 2 ½ turns of the cochlea …. Low frequencies have a longer longevity than higher frequencies (more peripheral).

36. Inner Ear • Outer Hair Cells: • Receives “outgoing” stimuli and this functions to amplify the soft sounds (“motor” units). • Usually first to die off (up to 60 dB HL, all of hair cell damage is outer hair cell…) • Source of oto-acoustic emissions. (OAE)

37. Inner Ear • Outer Hair Cell Damage: • Modern hearing aids helps the “motor function” • BUT… don’t really retune the hair cells. • Will still need something else to help with noise reduction (eg. directional microphones).

38. Inner Ear • Outer Hair Cell Damage: • Loss of “motor function” for amplifying the quiet sounds. • Loss of some “tuning” function. • At more intense levels, “no real hearing loss” • I can hear OK, if you just speak up!

39. Inner Ear • Outer Hair Cells: • Receives “outgoing” stimuli and this functions to amplify the soft sounds (“motor” units). • Usually first to die off (up to 60 dB HL, all of hair cell damage is outer hair cell…) • Source of oto-acoustic emissions. (OAE)

40. Inner Ear • Inner Hair Cell Damage: • Typical damage (in addition to outer hair cell damage) for losses above 60 dB HL.

41. Problems of the inner ear • Sensorineural hearing loss • Permanent • No maximum hearing loss

42. Examples of sensori-neural hearing loss • Presbycusis (age related loss) • Noise/music exposure • Certain drugs • Hereditary conditions

43. Central Processing • VIII cranial nerve takes sound up to the auditory cortex (transverse temporal gyrus) • Several synapses or “weigh stations” along the way.

46. Central Processing • Poorer representation of sound in brain • Loss of tonotopic representation? • Loss of one-to-one correspondence • Slower neural conduction rate • Possible mild demylenization? • Loss of insulation of nerves

47. Natural Gain Characteristics of the Ear • (Pinna effect) high frequency amplification • Ear canal (high frequency amplification) • Stapedial reflex (high level reduction) • Outer hair cells (low level amplification) • Central- binaural summation (3-6 amp.)

48. Ear Canal Resonance

49. Localization

50. Basilar Membrane-Organ of Corti