1. Chapter 11 Function 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 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 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 dB better than if no middle ear

20. Middle Ear Matching Transformer:
Loss of this natural transformer means that we would have a dB loss.
Serous otitis media can give a dB loss.
Our voice is attenuated by 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?
Frequency
Mouth
Inner Ear
F1 (500 Hz)
65 dB
56 dB
F2 (1500 Hz)
60 dB
57 dB
F3 (2500 Hz)
55 dB
64 dB

27. Some common hearing ranges
SPECIES
Low (Hz)
High (Hz)
Human 20
20,000
Chimpanzee
100
Dog 50
46,000
Dolphin
1000
130,000
Bat
3000
120,000
Goldfish
2000

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

51. Tonotopic

52. Tonotopic

53. Auditory Nerve Tuning curve for single VIII cranial nerve fiber
The sharper the tuning curve the greater the frequency specificity

54. Central Auditory Mechanism