hearing aids consist of a microphone and amplifier, which either sit
behind or within the pinna, and a small tube that directs the amplified
sound into the external ear canal. Ear trumpets and battery-powered
amplifier hearing aids simply increase the volume of sound that is
available to the ear. The tympanic membrane, ossicles, and cochlea all
need to be working, although amplifier-type hearing aids will com-
pensate to some extent for deficiencies in these structures.
Another type of electronic therapeutic device for hearing loss is
the cochlear implant. This involves surgically inserting an array of
electrodes into the inner ear that electrically stimulate the auditory
nerve in the spatial locations corresponding to particular regions of
the sound frequency spectrum. A tiny microphone is attached to the
external ear that picks up sound vibrations. A frequency analyzer
then performs a crude Fourier analysis of the sound and extracts
component frequencies within several ranges, perhaps twenty or so
segments of the frequency spectrum between approximately 200 and
8,000 Hz. This is conveyed to the inside of the skull (by radio trans-
mitter and receiver), and the auditory nerve is electrically stimulated
in the appropriate locations. The result is a partial reconstruction of
the incoming sound, partial in that the Fourier reconstruction uses
only a few selected frequency ranges.
Cochlear implants enable partial recovery of hearing in individuals
with completely nonfunctional cochleas. Such is the case with severe
noise-induced hearing loss and in many cases of congenital deafness.
Cochlear implant technology at present is not even close to restoring
normal hearing. However, it can provide a limited degree of sound de-
tection sufficient in some cases for understanding speech.
In addition to the analysis of sound by the cochlea, something else is
going on in the inner ear. The bony labyrinth (Fig. 15.11) also consists