Page 74 - Master Handbook of Acoustics

The Eustachian tube fulfills a second function by equalizing the static air pressure of the middle
ear with the outside atmospheric pressure so that the eardrum and the delicate membranes of the inner
ear can function properly. Whenever we swallow, the Eustachian tubes open, equalizing the middle
ear pressure. Changes in external air pressure (such as when an aircraft without a pressurized cabin

undergoes rapid changes in altitude) may cause momentary deafness or pain until the middle ear
pressure is equalized by swallowing. Finally, the Eustachian tube has a third emergency function of
drainage if the middle ear becomes infected.

The Inner Ear

The acoustical amplifiers and mechanical impedance matching features of the middle ear, discussed
so far, are relatively well understood. The intricate operation of the inner ear, containing the cochlea,
is not as explicit.

The cochlea is the sound-analyzing organ. It is in close proximity to the three mutually
perpendicular, semicircular canals of the vestibular mechanism, the balancing organ (see Fig. 4-1).
The same fluid permeates both organs, but their functions are independent. The cochlea is about the
size of a pea and is encased in solid bone. It is coiled up like a cockleshell from which it gets its
name. For the purposes of illustration, as shown in Fig. 4-5, this 2-3/4 turn coil has been stretched out

its full length, about 1 in. The fluid-filled inner ear is divided lengthwise by two membranes:
Reissner’s membrane and the basilar membrane. Of immediate interest are the basilar membrane and
its response to sound vibrations in the fluid.
Vibration of the eardrum activates the ossicles. The motion of the stapes, attached to the oval
window, causes the fluid of the inner ear to vibrate. An inward movement of the oval window results
in a flow of fluid around the basilar membrane, causing an outward movement of the membrane of the

round window; the round window thus provides pressure release. Sound actuating the oval window
results in standing waves being set up along the basilar membrane. The position of the amplitude peak
of the standing wave on the basilar membrane changes as the frequency of the exciting sound is
changed.

Low-frequency sound results in maximum amplitude nearer the distant end of the basilar
membrane; high-frequency sound produces peaks nearer the oval window; mid frequencies are in
between. For a complex signal such as music or speech, many momentary peaks are produced,
constantly shifting in amplitude and position along the basilar membrane. These resonant peaks on the
basilar membrane were originally thought to be so broad as to be unable to explain the sharpness of
frequency discrimination displayed by the human ear. Subsequent research showed that at low sound
intensities, the basilar membrane tuning curves are very sharp, broadening only for intense sound. It

appears that the sharpness of the basilar membrane’s mechanical tuning curves is comparable to the
sharpness of single auditory nerve fibers, which innervate it.

Stereocilia

Waves set up on the basilar membrane in the fluid-filled duct of the inner ear stimulate hairlike nerve
terminals that convey signals to the brain in the form of neuron discharges. There is one row of inner
hair cells and three to five rows of outer hair cells. Each hair cell contains a bundle of tiny hairs

called stereocilia. As sound causes the cochlear fluid and the basilar membrane to move, the
stereocilia vibrate according to the vibrations around them. Stereocilia at various locations along the