Page 73 - The Master Handbook Of Acoustics

48 CHAPTER THREE

open, equalizing the middle ear pressure. When an aircraft (at least
those without pressurized cabins) undergoes rapid changes in alti-
tude, the occupants might experience momentary deafness or pain
until the middle ear pressure is equalized by swallowing. Actually, the
Eustachian tube has a third emergency function of drainage if the mid-
dle ear becomes infected.
The Inner Ear

Only the acoustical amplifiers and the mechanical impedance matching
features of the middle ear have been discussed so far. These are relatively
well understood. The intricate operation of the cochlea is still clouded in
mystery, but extensive research is steadily adding to our knowledge.
Figure 3-1 shows the close proximity of the three mutually-perpen-
dicular, semicircular canals of the vestibular mechanism, the balanc-
ing organ, and the cochlea, the sound-analyzing organ. The same fluid
permeates all, but their functions are independent.
The cochlea, about the size of a pea, is encased in solid bone. It is
coiled up like a cockleshell from which it gets its name. For the pur-
3
poses of illustration, this 2 ⁄4-turn coil has been stretched out its full
length, about one inch, as shown in Fig. 3-5. The fluid-filled inner ear
is divided lengthwise by two membranes, Reissner’s membrane and
the basilar membrane. Of immediate interest is 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 distant end of the basilar membrane, causing an out-
ward movement of the membrane of the round window. Sound actuat-
ing the oval window results in standing waves being set up on 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 near the dis-
tant end of the basilar membrane; high-frequency sound produces
peaks near the oval window. 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