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Sounds are generally audible to the human ear if their frequency (number of vibrations per second) lies between 20 and 20,000 vibrations per second, but the range varies considerably with the individual. Sound waves with frequencies less than those of audible waves are called subsonic; those with frequencies above the audible range are called ultrasonic (see ultrasonics).
A sound wave is usually represented graphically by a wavy, horizontal line; the upper part of the wave (the crest) indicates a condensation and the lower part (the trough) indicates a rarefaction. This graph, however, is merely a representation and is not an actual picture of a wave. The length of a sound wave, or the wavelength, is measured as the distance from one point of greatest condensation to the next following it or from any point on one wave to the corresponding point on the next in a train of waves. The wavelength depends upon the velocity of sound in a given medium at a given temperature and upon the frequency of vibration. The wavelength of a sound can be determined by dividing the numerical value for the velocity of sound in the given medium at the given temperature by the frequency of vibration. For example, if the velocity of sound in air is 1,130 ft per second and the frequency of vibration is 256, then the wave length is approximately 4.4 ft.
The velocity of sound is not constant, however, for it varies in different media and in the same medium at different temperatures. For example, in air at 0°C. it is approximately 1,089 ft per second, but at 20°C. it is increased to about 1,130 ft per second, or an increase of about 2 ft per second for every centigrade degree rise in temperature. Sound travels more slowly in gases than in liquids, and more slowly in liquids than in solids. Since the ability to conduct sound is dependent on the density of the medium, solids are better conductors than liquids, liquids are better conductors than gases.
Sound waves can be reflected, refracted (or bent), and absorbed as light waves can be. The reflection of sound waves can result in an echo : an important factor in the acoustics of theaters and auditoriums. A sound wave can be reinforced with waves from a body having the same frequency of vibration, but the combination of waves of different frequencies of vibration may produce "beats" or pulsations or may result in other forms of interference.
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