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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 • 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 (Xm) Greatest at high frequencies Due to stiffness (Xs) Greatest at low frequencies
Ossicular chain • Stiffness dominated • Axis of movement depends on sound intensity
Eustachian tube • Muscles: Tensor veli palatini and levator veli palatini • Normally closed, opens to equalize pressure
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 Bone conduction: Through bones of the skull Air in the middle ear cavity Through the ossicles: Most effective
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 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 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