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Physiology
Middle ear space stiffness dominated: High-frequency emphasis
ImpedanceImpedance
Opposition to vibrationOpposition to vibration
Two forms:Two forms:
Reactance (X)Reactance (X)
Resistance (R) Resistance (R)
Remains same across frequenciesRemains same across frequencies
Reactance (X)Reactance (X)
Two componentsTwo components
Due to mass (XDue to mass (Xmm))
Greatest at high frequencies Greatest at high frequencies
Due to stiffness (XDue to stiffness (Xss))
Greatest at low frequencies Greatest at low frequencies
0
20
40
60
80
100
120
Frequency (Hz)
Imp
ed
en
ce (
Z)
Stiffnessreactance Mass reactance
Resistance
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 earSound transmission to inner ear Impedance mismatch between air in middle ear and Impedance mismatch between air in middle ear and
fluid in inner earfluid in inner ear
To compensate:To compensate:
Area difference between TM and footplate of the stapes (55/3.2 Area difference between TM and footplate of the stapes (55/3.2 = 17)= 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 pressure increase = 17 * 1.3 * 2 = 44.2
Total dB increase = 31-33 dBTotal dB increase = 31-33 dB
Possible methods of sound transmission into Possible methods of sound transmission into inner earinner ear
Bone conduction: Through bones of the Bone conduction: Through bones of the skullskull
Air in the middle ear cavityAir in the middle ear cavity
Through the ossicles: Most effectiveThrough the ossicles: Most effective
Functions of the middle earFunctions of the middle ear
Effective sound transmission to inner earEffective sound transmission to inner ear
Shields the round window from direct air Shields the round window from direct air pressure variationspressure variations
Protective function: Acoustic reflexProtective function: Acoustic reflex
http://www.neurophys.wisc.edu/animations/
Acoustic reflexAcoustic reflex Protective functionProtective function
Due to muscle contraction in response to intense soundDue 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 10-Reflex results in attenuation of loud sounds by about 10-30 dB30 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 Not very effective for short duration sounds: Latency about 10-150ms about 10-150ms
Measurement of acoustic reflexMeasurement of acoustic reflex
Middle ear muscles contract: Stiffness Middle ear muscles contract: Stiffness increasesincreases
More sound reflected backMore sound reflected back
Lack of acoustic reflex: Implies problem in Lack of acoustic reflex: Implies problem in middle ear muscles/part(s) of acoustic middle ear muscles/part(s) of acoustic reflex pathwayreflex pathway