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Physiology. Middle ear space stiffness dominated: High-frequency emphasis. Impedance. Opposition to vibration Two forms: Reactance (X) Resistance (R) Remains same across frequencies. Reactance (X). Two components Due to mass (X m ) Greatest at high frequencies Due to stiffness (X s )

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physiology
Physiology
  • Middle ear space stiffness dominated: High-frequency emphasis
impedance
Impedance

Opposition to vibration

Two forms:

Reactance (X)

Resistance (R)

Remains same across frequencies

reactance x
Reactance (X)

Two components

Due to mass (Xm)

Greatest at high frequencies

Due to stiffness (Xs)

Greatest at low frequencies

ossicular chain
Ossicular chain
  • Stiffness dominated
  • Axis of movement depends on sound intensity
eustachian tube
Eustachian tube
  • Muscles: Tensor veli palatini and levator veli palatini
  • Normally closed, opens to equalize pressure
sound transmission to inner ear
Sound transmission to inner ear

Impedance mismatch between air in middle ear and fluid in inner ear

To compensate:

Area difference between TM and footplate of the stapes (55/3.2 = 17)

Ossicular lever action (1.3/1 = 1.3)

Buckling movement of TM (2/1 = 2)

Total pressure increase = 17 * 1.3 * 2 = 44.2

Total dB increase = 31-33 dB

possible methods of sound transmission into inner ear
Possible methods of sound transmission into inner ear

Bone conduction: Through bones of the skull

Air in the middle ear cavity

Through the ossicles: Most effective

functions of the middle ear
Functions of the middle ear

Effective sound transmission to inner ear

Shields the round window from direct air pressure variations

Protective function: Acoustic reflex

http://www.neurophys.wisc.edu/animations/

acoustic reflex
Acoustic reflex

Protective function

Due to muscle contraction in response to intense sound

Threshold of reflex: Around 80 dB SL (sensation level).

Reflex results in attenuation of loud sounds by about 10-30 dB

More effective at low frequencies (less than 2 kHz)

Not very effective for short duration sounds: Latency about 10-150ms

measurement of acoustic reflex
Measurement of acoustic reflex

Middle ear muscles contract: Stiffness increases

More sound reflected back

Lack of acoustic reflex: Implies problem in middle ear muscles/part(s) of acoustic reflex pathway

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