3-4.
THE MECHANICS OF HEARING
The human ear is composed of three major sections: the external ear, the
middle ear, and the inner ear. Each of these has a distinct function in the hearing
process.
a. The external ear or external auditory canal is a somewhat trumpets shaped
structure that leads to the middle ear. It's bone and cartilage is covered with skin that is
very sensitive. This canal skin is protected and kept moist by the sebaceous glands
(located distally). Sound waves entering the ear travel through the external ear canal,
are funneled to the middle ear, and strike the eardrum, or tympanic membrane.
b. The middle ear consists of the eardrum, the air cavity, filled with a chain of
small bones called the malleus, incus, stapes or the hammer, anvil, and stirrup plus the
oval window. One end of this chain rests against the eardrum, while the other end is
connected to the inner ear. It is within the middle ear that the sound waves strike the
tympanic membrane (eardrum), which in turn, cause vibrations. Note in Figure 3-1 that
the tympanic membrane is catty-corner to the auditory canal. These vibrations are then
bounced off hammer, anvil, stirrup, and oval window as they are carried or transferred
to the inner ear.
c. The inner ear consists of a spiral or curved tube filled with fluid. The spiral
tube contains the cochlea, organ of Corti, and semicircular canals. The cochlea, sea
shell shaped, bone contains the organ of Corti, which consists of many sensory cells
with delicate hairs or hair cells projecting into the fluid. The organ of Corti receives the
sound waves and transfers these impulses to the eighth cranial nerve. This auditory
nerve transmits the impulses to the brain for interpretation by the brain.
3-5.
PROPERTIES OF SOUND
There are several definitions of the physical properties of sound, which you need
to understand and that will be helpful to you in carrying out your responsibilities.
a. Sound. Sound is based upon the sense of hearing. Sound is the effect
produced on the organ of hearing and its central connections by the vibrations of the air
or other medium. The motion, energy, and frequency, of these particles (gas, liquid, or
solid) passing through a medium, like a human ear or tunnel, provide a sensation. This
sensation or vibration reaching the human ear is interpreted by the human brain as
"sound." A frequency of between 8 and 20,000 cycles per second provides the stimulus
for the subjective sensation of hearing. Although sound resembles forms of
electromagnetic energy (light, x-rays, radio waves), it always requires a transport
medium.
b. Frequency. Frequency is the rate per unit time that high and low pressure
regions occur. It is measured as Hertz, where one Hertz is equivalent to one cycle per
second. The range of sound frequencies, which the human ear can perceive, extends
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