Middle Ear Physiology

acoustic impedance

the opposition of sound flow through an acoustic system

Acoustic admittance

the ease of sound flow through an acoustic system

A system’s resonant frequency

when mass reactance = stiffness reactance

The volume of air in the middle ear is not easily compressed and is governed by:

stiffness

Massive objects

Impede high frequency sounds

Low-pass

Massive objects, impede high-frequency sounds

Stiff objects

Impede low frequency sounds

High-pass

Stiff objects, impede low frequency sounds

Stiffness governed system

Middle ear

Middle ear resonant frequency

~1200 Hz

Stiffness reactance (negative reactance)

Dominates at frequencies below 800 Hz

Mass reactance (positive reactance)

Dominates at frequencies above 5000 Hz

Pinna resonance

5000 Hz

Ear canal resonance

3000-4000 Hz

Middle ear resonance

Increase gain at 1200 Hz, decrease gain below 800 Hz

Minimum audibility curve, lowest can hear in db SPL. According to this we can hear better in mid frequencies than high and low. Everyone would have a hearing loss based on SPL. 

Audiometric zero

the lowest detectable sound intensity for a given frequency in a person with normal hearing

Tympanometry

measures changes in stiffness at the TM and middle ear

Tymp probe tone for adults

226 Hz at 80 dB SPL

Types of tympanograms

A, As, Ad, B, C

How does wide band tymp differ from standard?

WBT uses clicks or chirps from 220 Hz up to 1000 Hz, WBT measures absorbance, 3D model of pressure, absorbance, and frequency

WBT sensitive to

Ear canal or ME maturation

changes in middle ear status caused by different disorders

Acoustic impedance is greater in ME or IE and why

Greater in IE because it is filled with fluid

How much energy is lost when sound travels from air to water (sound from ME to IE)?

35 dB

Energy transduction of ear

Acoustic, mechanical, hydraulic

ME mechanisms for impedance matching

Buckling effect, pressure transformer, ossicular lever

Buckling effect

TM curves to help amplify sound pressure, increasing force.

Increases gain by 6dB

Surface area of TM

55mm2

Surface area of stapes footplate

3.2mm2

Size ratio of TM vs stapes footplate

17:1

Explain pressure transformer

Increase in sound pressure to transfer energy from air filled ME to fluid filled IE

Pressure from the TM is moved through the ossicles and into the stapes footplate in the round window. Pressure is amplified due to the TM having a greater surface area than the stapes footplate

Ossicular lever

Malleus and incus form a lever and provide further increase in pressure at the oval window

Amount of gain from ossicular lever

2 dB

Amount of gain from Areal transformation

25 dB

How to force Eustachian tube open

Valsalva maneuver

Acoustic reflex

Muscle/tendon contraction in response to intense stimuli

Contraction of muscle/tendon causes ossicular fixation, increasing impedance of the middle

ART increase or decrease stiffness of ME

Increase. Protects against low frequency sounds

How much protection does ART provide

15 dB

air conduction

sound waves travel from outer ear, middle ear, to the inner ear

bone conduction

vibrations go through the skull, bypass the outer ear and middle ear, directly stimulate the inner ear

How would bone conduction play a role in hearing or in audio testing?

Figure out the type of hearing loss

Occlusion effect

Sensation of increased loudness of self generated sound, mostly in the low frequencies. 

Why are low frequency sounds louder with occlusion effect?

When plugging your ears, you lose the resonance frequency of the ear, which is high frequency. 

Why is occlusion effect a concern to audiologists?

Masking for bone conduction

Occlusion effect for inserts

15 dB @ 500

10 dB @ 1k

0 dB @ 2k