Biomedical Engineering Reference
In-Depth Information
6.3
HOW HEARING WORKS
6.3.1 The Outer Ear
The ear provides one of the five senses of the human body. The outer ear, comprising
the pinna and the outer ear canal, is the sound-gathering portion of the anatomy, as seen
in Figure 6-3. The pinna performs a number of important functions, including the first
of a number of impedance transformations, sound amplification, and direction finding.
Our sense of directionality is determined by the differential arrival times at the outer ear
in conjunction with the binaural difference in arrival times and loudness at the auditory
complex of the brain.
6.3.2 The Middle Ear
The outer ear is connected to the middle ear by the tympanic membrane (eardrum) through
the ear canal. The middle ear houses the ear ossicles (malleus, incus, and stapes), which
couple sound vibrations through from the tympanic membrane via the oval window into
the cochlea.
The handle of the malleus is attached to the tympanic membrane, and the tensor
tympani muscle, also attached to the malleus, regulates the tension on this membrane.
The incus is attached to the malleus and to the third ossicle in the chain, the stapes, which
is in turn attached via its footplate to the oval window of the cochlea. The stapedius
muscle regulates the range of motion of the stapes. The two muscles (tensor tympani and
stapedius) adjust the amplitude sensitivity of the ear and are instrumental in achieving its
incredible dynamic range as well as reducing the perceived amplitude of our own voices
as we speak.
It was shown earlier in this chapter than only a small fraction of the acoustic power
would be transmitted from the air through to a fluid due to the large difference in their
relative acoustic impedances (
T
dB
=
10 log
10
0
.
0011
=−
29
.
6 dB) unless the impedances
are matched in some way.
The middle ear converts variations in air pressure into variations in fluid pressure in
the inner ear (perilymph in the cochlea). This impedance change is accomplished by the
FIGURE 6-3
Schematic diagram
showing the various
parts of the ear.
