Biomedical Engineering Reference
In-Depth Information
at the tympanic membrane and the output pressure applied to the stapes footplate that
couples into the vestibular canal. This transfer function is important for prosthetic devices
that drive the ossicles or the oval window electrically if the tympanic membrane has been
destroyed.
The forces involved are very small but can be calculated because the area of the
tympanic membrane is known and the acoustic pressure can be measured. For a sound
signal at the minimum audible threshold, the pressure
P
o
=
10
−
6
N/m
2
is applied to
the tympanic membrane area of 59.4 mm
2
, making the RMS force
20
×
F
=
P
o
A
=
2
×
10
−
5
×
59
.
4
×
10
−
6
=
1
.
2nN
For normal speech levels of 60 dB above the minimum audible threshold, the acoustic
pressure increases to
P
025 N/m
2
, and the force increases to 1.5
=
0
.
μ
N.
6.3.3 The Inner Ear
The inner ear includes the cochlea and the semicircular canals. The cochlea is a fluid-
filled chamber where fluid movement is converted into nerve action potentials by hair
cells. Each hair cell has fine rods of protein, called stereocilia, emerging from the one
end. Some 30,000 of these hair cells, arranged in four rows, are attached to the top of
the basilar membrane in a matrix of cells called the organ of Corti. They operate as
miniature displacement transducers responding to displacements of the basilar membrane
relative to the perilymph.
Deflection of the hairs in one direction increases the release of a chemical transmitter at
the base of the hair cell, while deflection in the other direction inhibits its release. Variations
in the concentration of this chemical transmitter alter the discharge rate of nearby neurons
that make up the spiral ganglion. Changes in this neural activity are transmitted to the
brain via the auditory nerve.
The cochlea consists of three spiral canals, two of which are separated by the basilar
membrane, part of the organ of Corti. The basilar membrane, which gets wider and more
flaccid as the cochlea gets narrower, conveys the vibratory movement along its length as a
traveling wave (much like a snapping rope). This mechanism is illustrated in Figure 6-5.
The amplitude of this wave reaches a peak at a location that depends on frequency, as
shown in the two examples in Figure 6-4. High-frequency peaks occur toward the base
of the basilar membrane, up to 20 kHz (where the membrane is stiffest and narrowest),
whereas the low-frequency peaks occur toward the apex, down to 20 Hz. This spatial
relationship with frequency sensitivity is called tonotopic organization.
The outer hair cells on the basilar membrane also have a contractile function (actin
filaments) and serve as controllable amplifiers for the inner hair cells, which send action
potentials to the auditory cortex via the spiral ganglion. Loss of the outer hair cells results
in about a 60 dB loss in hearing.
6.3.4 Hearing Statistics
The undamaged ear is incredibly sensitive, with the threshold of hearing equating to a
displacement of only 10
−
11
m of the tympanic membrane. It can also accommodate a
dynamic range of 120 dB in sound pressure before damage occurs.
