Page 72 - The Master Handbook Of Acoustics

47
THE EAR AND THE PERCEPTION OF SOUND

The problem of matching sound in air to sound in the fluid of
the inner ear is beautifully solved by the mechanics of the middle
ear. The evidence that the impedance matching plus the resonance
amplification of Fig. 3-3 really work is that a diaphragm motion com-
parable to molecular dimensions gives a threshold perception.
A schematic of the ear is given in Fig. 3-5. The conical eardrum at
the inner end of the auditory canal forms one side of the air-filled mid-
dle ear. The middle ear is vented to the upper throat behind the nasal
cavity by the Eustachian tube. The eardrum operates as an “acoustic
suspension” system, acting against the compliance of the trapped air
in the middle ear. The Eustachian tube is suitably small and con-
stricted so as not to destroy this compliance. The round window sep-
arates the air-filled middle ear from the practically incompressible
fluid of the inner ear.
The Eustachian tube fulfills a second function by equalizing the
static air pressure of the middle ear with the outside atmospheric pres-
sure so that the eardrum and the delicate membranes of the inner ear
can function properly. Whenever we swallow, the Eustachian tubes

Middle
ear
Oval window
Fluid
Inner ear

Eardrum
Ear canal

Basilar membrane
Round window

Eustachian
tube
FIGURE 3-5
Highly idealized sketch of the human ear showing the unrolled fluid-filled cochlea.
Sound entering the ear canal causes the eardrum to vibrate. This vibration is transmit-
ted to the cochlea through the mechanical linkage of the middle ear. The sound is ana-
lyzed through standing waves set up on the basilar membrane.