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by
Danlee Mitchell
and
Jack Logan, Ph.D.
There are three requirements for sound to "occur" in an environment: (1)
a vibrating source to initiate sound, (2) a medium to transmit sound vibrations
throughout the environment and (3) a receiver to hear or record sound
vibrations.
Sound is initiated in an environment by a vibrating source.
Vibrating sources are many and varied in the World -- vocal cords, a membrane
of animal hide or synthetic material, a stretched string that is plucked
or bowed, objects such as wood, stone, clay, metal and glass that are
struck, rattling of beads in a small enclosure, clapping of hands, singing of birds, grunts and groans of animals, buzzing of lips
in a small resonating tube, splitting of an air stream, small pieces of
reed attached to a tube and set in motion by the action of human breath,
and many, many other natural vibrating sources. Sound may also be produced
artificially by electronic synthesis.
To
create vibration there must be a certain amount of surface tension in
the vibrating body. Solid objects or reeds possess inherent tension. Strings
or membranes must first be stretched to sustain vibration.
A medium of sound transmission must be present to transmit
vibrations of a sound source to a receiver. Two efficient mediums of sound
transmission are gases (such as air) and liquids (such as water). Sound
is not capable of being transmitted in a vacuum. Water is a more efficient
transmitter of sound compared to air as sound travels faster and further
in water.
A vibrating source transmits its vibrations through a medium by causing
the medium to move, or vibrate, at exactly the same speed of vibration
as the source itself. The movement of the gas or liquid medium is identical
to surface waves found on any large body of water. Surface waves on water
move up and down, and they transmit energy from one point to another &emdash;
from a source (tidal action, wind, a passing ship, an earthquake) to receiver
(the shoreline). Sound transmission through the air is accomplished by
a similar physical process. The sound source initiates waves in the air,
and the air moves up and down (like surface water waves) at the same rate
of speed as the sound source. This motion of the medium is sensed by a
receiver, such as the human ear or a recording microphone. One complete
up-and-down movement of the sound source is called a cycle and the rate
of speed of the vibrations of a particular sound source is measured in
the number of complete cycles that the source moves per second (cycles
per second or "cps"). In recent years the expression of cps has been assigned
to a proper name, Hertz (Hz), after Heinrich Hertz (1857-1894). Hertz
generated and detected electromagnetic waves across the length of his
laboratory on a wavelength of approximately one metre. To detect the electromagnetic
waves Hertz employed a simple form of oscillator, which he termed a resonator.
Cps is now expressed as Hz (i.e. 1000 Hz, rather than 1000 cps).
A Sound receiver senses vibrating motion from a source
which is transmitted through a medium. The human hearing organ, the ear,
is a sound receiver, as is a recording microphone. The human hearing network
consists of the outer ear chamber, the ear canal, the eardrum (the tympanic
membrane), and the inner ear (cochlea) in the shape of a spiral. The inner
ear contains innumerable minute hair (cilia) outgrowths of graduated sizes
that respond to different speeds of sound vibrations transmitted by the
tympanic membrane. The network of minute cilia receptors is directly connected
to the nervous system, which sends the information sensed by the cilia
directly to the brain, where it is processed and reacted to by different
parts of the body.
In summary, sound is a phenomenon of vibration &emdash; from a source,
through a medium, and to a receiver. Sound is ultimately processed by
the nervous system of the hearer.
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