2.3  The Middle Ear (ME)

malleus

incus

stapes

 

 


"Impedance matching":

 

The ossicles match the impedance at the TM to the higher impedance of the fluid in the cochlea

and the threshold of hearing would be higher

 


To match Z, we need to increase the pressure of the waveform before transmitting it into the cochlea

We want the intensity going into the cochlea to be the same as the intensity that reaches the TM

 

 

You remember that

 

 

(review this?)

 

so the intensities will be equal if

 

 

rearranging,

 

 

Since ZC is higher than ZTM, p2 going into the cochlea must be higher than p2 at the TM

 

 


Three components of pressure gain

 

1) "Area Ratio":

(the footplate is the part of the stapes that fits into the oval window -

the part that actually delivers pressure into the cochlea)

This is obvious because Pressure = F/A (remind me)

 

Think about hitting a nail with a hammer

the same force applied to the head of the nail creates great pressure at the point

the same force drives a small nail farther than a large one - it creates more pressure

 


2) "Lever ratio"

Displacement is reduced

A hammer is used as a lever to remove nails

the handle moves farther than the nail but applies a lot of force

A factor of 1.3, or 2 dB

The stapes is displaced much less than TM :

0.1 mm for stapes (max)

up to 2 mm for TM (max)

 


3) "Buckling" of the TM

 


All together, these factors provide a pressure gain of a factor of 44, or 33 dB

 

17 * 1.3 * 2 = 44,

 

25 dB + 2 dB + 6 dB = 33 dB

 

 


More about the middle ear:

In other words, the pressure gain is not the same at every frequency

the number 33 dB is correct only for frequencies near 1000 Hz, and

the pressure gain is smaller at other frequencies

 

 


The TM-to-cochlea impedance difference is not perfectly matched by the middle ear

The lost intensity is reflected

(Pressure goes up but intensity goes down!!

Is that possible?)

there were no ME

or if the ME did not work properly

Conductive hearing loss occurs because intensity is not transmitted or "conducted" into the cochlea

Conditions that cause a conductive hearing loss:

middle ear infections

diseases that affect the ossicles or their joints (otosclerosis)

 

 


Middle Ear Muscles

attached to the stapes

attached to the malleus


 

The "Middle Ear Reflex"

intense (>80 dB SL)

(dB SL = dB above threshold)

AND low frequency

(low means < 2 kHz)

AND long duration

(long means > 200 msec))

 

low-intensity sounds

(of any frequency)

OR high frequency sounds

(of any intensity)

OR transient sounds

(of any frequency or intensity)

 

 


Eustachian tube

Thresholds may increase when a pressure imbalance exists:

  in an airplane

  when you have a ME infection

 


Next:  inner ear