Page 74 - The Master Handbook Of Acoustics

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THE EAR AND THE PERCEPTION OF SOUND

as to be unable to explain the sharpness of frequency discrimination
displayed by the human ear. Recent research is showing that at low
sound intensities, the basilar membrane tuning curves are very sharp,
broadening only for intense sound. It now 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, about 15,000 outer hair cells
with about 140 tiny hairs called stereocilia jutting from each one. In
addition, there are about 3,500 inner hair cells, each having about 40
stereocilia attached. These stereocilia are the true transducers of
sound energy to electrical discharges. There are two types of hair cells,
inner and outer, so-called by their placement and arrangement. As
sound causes the cochlear fluid and the basilar membrane to move, the
stereocilia on the hair cells are bent, initiating neural discharges to the
auditory cortex.
When sound excites the fluid of the inner ear, membrane and hair
cells are stimulated, sending an electrical wave through the surround-
ing tissue. These so-called microphonic potentials (analog) can be
picked up and amplified, reproducing the sound falling on the ear,
which acts as a biological microphone. These potentials are propor-
tional to the sound pressure and linear in their response over an 80-dB
range. While interesting, this microphonic potential must not be con-
fused with the action potentials of the auditory nerve, which convey
information to the brain.
Bending the stereocilia triggers the nerve impulses that are carried
by the auditory nerve to the brain. While the microphonic signals are
analog, the impulses sent to the acoustic cortex are impulses gener-
ated by neuron discharges. A single nerve fiber is either firing or not
firing (binary!). When it fires, it causes an adjoining one to fire, and so
on. Physiologists liken the process to a burning gunpowder fuse. The
rate of travel bears no relationship to how the fuse was lighted. Pre-
sumably the loudness of the sound is related to the number of nerve
fibers excited and the repetition rates of such excitation. When all the
nerve fibers (some 15,000 of them) are excited, this is the maximum