- This article is about compression waves. For other meanings, see sound (disambiguation)
Sound is defined as a mechanical compression and rarefaction or a longitudinal
displacement wave that propagates through a medium (solid, liquid or gas). The speed of this
propagation depends on the type and temperature of the medium. In dry air at sea level at 20° C (68° F) the speed
of sound is approximately 343.6 m/s. For estimation purposes it is often convenient to use 1 m/ms. Sounds can often be
thought of as having two components: frequency and amplitude. The frequency refers to the rate of change within the wave and is measured in hertz (Hz). The amplitude refers to the magnitude of pressure change within the wave. While the pressure can be measured in pascals, the amplitude is more often referred to as sound pressure level and measured in decibels, or dB SPL.
Sounds that are sine waves with a single pair of frequency and amplitude components are generally perceived as a pure tone. (While sound waves tend to be thought of as being sine waves, that is not always true. Square waves, sawtooth waves, triangle
waves, and many other types are possible and they all have a distinct sound. For convenience in this article, however, it is
best to think of sine waves.) Sounds are usually much more complex than that, consisting of multiple tone components and harmonics. While the sound may still be referred to in general use as a single frequency
(for example, a piano striking the A above middle C is said to be playing a note at 440 Hz) the sound you perceive will
be colored by all of the frequency components and their relative amplitudes (see timbre.)
The frequency range of sound audible to humans is approximately between 20 and 20,000 Hz. This range varies by individual
and generally shrinks with age. It is also an uneven curve -- sounds near 3000 Hz are often perceived as louder than a sound
with the same amplitude at a much lower or higher frequency. Above and below this range are ultrasound and infrasound, respectively. The amplitude range
of sound for humans has a lower limit of 0 dB SPL (sometimes called the threshold of hearing). While there is
technically no upper limit, sounds begin to do damage to ears at 85 dB SPL sounds above 130 dB SPL can cause
pain. Again, this range varies by individual and changes with age.
The perception of sound is the sense of hearing. In humans and many animals this is accomplished by the ears,
but loud sounds and low frequency sounds can be perceived by other parts of the body through the sense of touch. Sounds are used in several ways, most notably for communication through speech or, for example, music. Sound perception can also be used for
acquiring information about the surrounding environment in properties such as spatial characterics and presence of other animals
or objects. For example, bats use one sort of echolocation, ships and submarines use sonar, and humans can
determine spatial information by the way in which they perceive sounds.
The study of sound is called acoustics and is performed by acousticians. A
notable subset is psychoacoustics, which combines acoustics and
psychology to study how people react to sounds.
See Also
Sound Measurement
External Links
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