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Sound,
physical phenomenon that stimulates the sense of hearing. In humans,
hearing takes place whenever vibrations of frequencies between about 15
and 20,000 hertz reach the inner ear. The hertz, or Hz, is a unit of
frequency equaling one cycle per second. Such vibrations reach the inner
ear when they are transmitted through air, and the term sound is
something restricted to such airborne vibrational waves. Modern
physicists, however, usually extend the term to include similar vibrations
in liquid or solid media. Sounds of frequencies higher than about 20,000
Hz are called ultrasonic. See Frequency
This
article deals with the major outlines of this field of physics. For the
architectural science of designing rooms and buildings for desirable
properties of sound propagation and reception,.
For the nature of the physiological process of hearing sounds, For the anatomy of the human and animal hearing mechanism,
For the general properties of the generation and propagation of
vibrational waves, including sound waves, see Wave
Motion.
In
general, waves can be propagated transversely or longitudinally. In both
cases, only the energy of wave motion is propagated through the medium; no
portion of the medium itself actually moves very far. As a simple example,
a rope may be tied securely to a post at one end, and the other end pulled
almost taut and then shaken once. A wave will travel down the rope to the
post, and at that point it will be reflected and returned to the hand. No
part of the rope actually moves longitudinally toward the post, but each
successive portion of the rope moves transversely. This type of wave
motion is called a transverse wave. Similarly, if a rock is thrown into a
pool of water, a series of transverse waves moves out from the point of
impact. A cork floating near the point of impact will bob up and down,
that is, move transversely with respect to the direction of wave motion,
but will show little if any outward, or longitudinal, motion. A sound
wave, on the other hand, is a longitudinal wave. As the energy of wave
motion is propagated outward from the center of disturbance, the
individual air molecules that carry the sound move back and forth,
parallel to the direction of wave motion. Thus, a sound wave is a series
of alternate compressions and rarefactions of the air. Each individual
molecule passes the energy on to neighboring molecules, but after the
sound wave has passed, each molecule remains in about the same location.
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