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Physiology Middle ear space stiffness dominated: High-frequency emphasis

Physiology Middle ear space stiffness dominated: High- frequency emphasis

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Page 1: Physiology Middle ear space stiffness dominated: High- frequency emphasis

Physiology

Middle ear space stiffness dominated: High-frequency emphasis

Page 2: 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

Page 3: Physiology Middle ear space stiffness dominated: High- frequency emphasis

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

Page 4: Physiology Middle ear space stiffness dominated: High- frequency emphasis

0

20

40

60

80

100

120

Frequency (Hz)

Imp

ed

en

ce (

Z)

Stiffnessreactance Mass reactance

Resistance

Page 5: Physiology Middle ear space stiffness dominated: High- frequency emphasis

Tympanic membrane has non-uniform displacement

Page 6: Physiology Middle ear space stiffness dominated: High- frequency emphasis

Ossicular chain

Stiffness dominated

Axis of movement depends on sound intensity

Page 7: Physiology Middle ear space stiffness dominated: High- frequency emphasis

Eustachian tube

Muscles: Tensor veli palatini and levator veli palatini

Normally closed, opens to equalize pressure

Page 8: Physiology Middle ear space stiffness dominated: High- frequency emphasis

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

Page 9: Physiology Middle ear space stiffness dominated: High- frequency emphasis
Page 10: Physiology Middle ear space stiffness dominated: High- frequency emphasis

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

Page 11: Physiology Middle ear space stiffness dominated: High- frequency emphasis

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/

Page 12: Physiology Middle ear space stiffness dominated: High- frequency emphasis

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

Page 13: Physiology Middle ear space stiffness dominated: High- frequency emphasis

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