Hearing Science: Middle Ear Physiology

What are the four middle ear contributions to hearing?

Impedance

Polarization Function

Ventilation and Pressurization

Protection and Distortion Reduction

What is each middle ear contribution designed to do?

solve a problem

Impedance

to the flow of energy (difference for air vs. fluid)

What problem is the middle ear impedance matching transformer function trying to solve?

impedance mismatch between the outer ear and inner ear

When does impedance mismatch between the outer and inner ear occur?

when sound is transmitted from air to water

What would happen if the middle ear structures were not in place?

sound would not be efficiently transmitted to the fluid filled cochlea

What hearing loss would occur if the middle ear structures were missing?

approximately 30 dB

The middle ear is...

a mechanical device that amplifies sound

What are the two transformer mechanisms?

Lever mechanism

Areal ratio of the TM to the oval window

Lever Mechanism

the increase in force will be proportional to the ratio of the two lengths (length of the malleus and incus)

How long is the malleus?

1.3x as long as the incus

Pressure at the incus will be _____ as great as the force applied to the malleus

1.3x

Theoretically

What is the missing piece of information?

the gain across frequencies

Areal Ratio Mechanism

energy collected over a relatively large surface is transmitted to a smaller surface

How is the energy directed?

the force applied to a large area (TM) is concentrated into a small area (oval window)

The pressure at the stapes footplate is _________ than that at the tympanic membrane.

greater

What did the Aibara et. al study do in 2001?

measured the middle ear sound transfer function for 12 fresh temporal bones

Where was the sound pressure level measured?

near the TM

How was the sound pressure in the scala vestibuli measured?

using a hydrophone

Hydrophone

microphone for recording underwater sound

What is the max gain when comparing the sound level in the middle ear to the sound level in the inner ear?

23.5 dB at 1.2 kHz

What does the y-axis represent?

the gain provided by the middle ear

-GME: middle ear gain)

How do you determine gain?

Why do we need the middle ear to provide gain?

gain = amplified soun

we need to amplify it because there is a problem with the amplification of sound from air to fluid (ear canal to cochlea)

What is the difference between ear canal resonance vs. middle ear resonance?

Ear Canal Resonance: how to amplify high frequency sounds

Middle Ear Transfer Function: how to amplify low frequency sounds

What is the max gain when comparing the sound level of the air around us to the sound level within the outer ear (ear canal resonance)?

16 dB at 2.8 kHz