Sound - Abstract - Definition and Review

a  mechanical disturbance from a state of equilibrium that propagates
through an elastic material medium. A purely subjective definition of sound
is also possible, as that which is perceived by the ear, but it is not
particularly illuminating and is unduly restrictive, for it is useful to
speak of sounds that cannot be heard by the human ear, such as those that
are produced by dog whistles or by sonar equipment.

Sound is treated in a number of articles in the MACROPAEDIA. For the physics
of sound generation, propagation, and detection, see Sound. For a more
general discussion of vibration and wave motion, see Mechanics. For a
discussion of biological mechanisms of sound production and reception, see
Speech; Sensory Reception. For a discussion of musical sounds, see Music,
Art of; Musical Instruments. For a discussion of the use of acoustical waves
as a tool of research, see Earthquakes.

Most solids, liquids, and gases of ordinary experience can serve as media
for sound. A mechanical disturbance may be produced in any number of ways
but will consist of a sudden increase in pressure at some point. Since the
material is elastic, the  compression is not permanent; once the
disturbing influence is removed, the compressed region will rebound, but in
doing so it will compress an adjacent region. The result of this cycle
repeating itself is the generation of a compression wave, followed by a
rarefaction wave as each region of elastic material rebounds. These waves
are longitudinal, i.e., the displacement of a particle of the medium is in
the direction of wave motion (note, however, that there is no net transport
of material). The waves thus generated travel through the medium at a speed
that is a function of the equilibrium pressure and density of the material
and, to various extents, of the specific heat (for a gas), the elasticity
(for liquids and solids), and the temperature of the medium and of the
frequency of the wave. In dry air (at 0[{degree}] C [32[{degree}] F] and a
sea-level pressure of 14.7 pounds per square inch [1013.25 millibars]) the
speed of sound is 331.29 m per second (741.1 miles per hour). (This
calculation, made in 1986, corrected an earlier calculation made about
1942.) The speed of sound in seawater is 1,490 m (4,889 feet) per second and
in steel 5,000 m (16,405 feet) per second.

A great many of the sounds encountered in daily life are periodic, that is,
the sound waves associated with them occur in patterns that repeat with
regularity over time. Such sounds are characterized by a dominant frequency,
or  pitch, which may be defined as the number of waves that pass a
fixed point per unit time. The simplest form of sound wave, called a pure
tone, can be represented by a sine wave; more complex but still relatively
simple forms, such as notes played on various musical instruments, can be
represented by more complex harmonic curves, with numbers of harmonic tones
or overtones superimposed on the fundamental tone. The complexity of a sound
wave can become so great that no particular pitch can be discerned; white
noise, so called by analogy with white light (a mixture of all frequencies,
or colours, of visible light), is an example.

Another parameter of sound, in addition to velocity and frequency, is
 intensity, which is defined as the average flow of energy per unit
time through a unit area of the medium. Intensity is usually measured in
watts per square centimetre. The standard usually used for the quietest
sound audible to the human ear has an intensity of 10{sup -16} watt per
square centimetre. Closely related to the intensity of a sound is the sound
pressure, or pressure excess over equilibrium caused by a sound wave. Sound
pressure is measured in dynes per square centimetre or, in the SI system of
units, in  pascals. The sound pressure of the human voice at ordinary
conversational level, measured directly in front of the mouth, is about 0.1
pascal, which may be compared with the standard pressure of the atmosphere
of about 10[{sup 5}] pascals. Also related to sound intensity, although not
simply, is  loudness. Loudness is a subjective phenomenon and can be
measured only comparatively, by means of a standard reference sound under
specified conditions.

Sound waves behave in many ways as light waves do, and optical analogies are
frequently useful in describing acoustical phenomena. Thus, sound waves can
be reflected, refracted, diffracted, and scattered, and many of the
differences in behaviour between sound and light waves reflect simply the
very great difference in wavelengths ordinarily dealt with.

This abstract and review was obtained from OnLine subscription services -
courtesy of Charles Turley.