Physiology Middle ear space stiffness dominated: High- frequency emphasis

Impedance Opposition to vibration Opposition to vibration Two forms: Two forms: Reactance (X) Reactance (X) Resistance (R) Resistance (R) Remains same across frequencies Remains same across frequencies

Reactance (X) Two components Two components Due to mass (X m ) Due to mass (X m ) Greatest at high frequencies Greatest at high frequencies Due to stiffness (X s ) Due to stiffness (X s ) Greatest at low frequencies Greatest at low frequencies

Tympanic membrane has non-uniform displacement

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 Impedance mismatch between air in middle ear and fluid in inner ear To compensate: To compensate: Area difference between TM and footplate of the stapes (55/3.2 = 17) Area difference between TM and footplate of the stapes (55/3.2 = 17) Ossicular lever action (1.3/1 = 1.3) Ossicular lever action (1.3/1 = 1.3) Buckling movement of TM (2/1 = 2) Buckling movement of TM (2/1 = 2) Total pressure increase = 17 * 1.3 * 2 = 44.2 Total dB increase = dB

Possible methods of sound transmission into inner ear Bone conduction: Through bones of the skull Bone conduction: Through bones of the skull Air in the middle ear cavity Air in the middle ear cavity Through the ossicles: Most effective Through the ossicles: Most effective

Functions of the middle ear Effective sound transmission to inner ear Effective sound transmission to inner ear Shields the round window from direct air pressure variations Shields the round window from direct air pressure variations Protective function: Acoustic reflex Protective function: Acoustic reflex

Acoustic reflex Protective function Protective function Due to muscle contraction in response to intense sound Due to muscle contraction in response to intense sound Threshold of reflex: Around 80 dB SL (sensation level). Threshold of reflex: Around 80 dB SL (sensation level). Reflex results in attenuation of loud sounds by about dB Reflex results in attenuation of loud sounds by about dB More effective at low frequencies (less than 2 kHz) More effective at low frequencies (less than 2 kHz) Not very effective for short duration sounds: Latency about ms Not very effective for short duration sounds: Latency about ms

Measurement of acoustic reflex Middle ear muscles contract: Stiffness increases Middle ear muscles contract: Stiffness increases More sound reflected back More sound reflected back Lack of acoustic reflex: Implies problem in middle ear muscles/part(s) of acoustic reflex pathway Lack of acoustic reflex: Implies problem in middle ear muscles/part(s) of acoustic reflex pathway