![]() ![]() The precise value depends on ambient temperature and pressure, as well as other variables. The speed of sound in air is about 343 m/s and 1485 m/s in water. Its value is related to the compressibility (SI unit: m 2/N or 1/Pa) and the density (SI unit: kg/m 3) of the material in which the waves propagate, by. The speed at which the waves propagate is the speed of sound (SI unit: m/s). Representation of a standing wave (top) and a propagating wave (bottom). This is known as the inverse distance law. As a rule of thumb, the level decreases by 6 dB each time the distance from a source is doubled. ![]() When specifying a level, it is important to specify the reference pressure, distance from the source, bandwidth measured, and possible weighting functions. In underwater acoustics 1 µPa is typically used as reference.Įxamples of the typical (broadband) sound pressure levels associated with different situations are given in the figure below. Where is the root mean square (RMS) acoustic pressure measured and = 20 µPa is the reference pressure (also an RMS value) commonly used in air. The logarithmic scale follows naturally from how the human auditory system experiences loudness. The values described here are often given on the logarithmic decibel scale, relative to the hearing threshold of 20♱0 -6 Pa (or 20 µPa), in units of dB SPL. The amplitude at normal speech levels is about 0.02 Pa. The amplitude of the small pressure variations that can be detected by the human ear vary from roughly 20♱0 -6 Pa at the hearing threshold to 600 Pa for jet engine noise. This static value is the atmospheric pressure (about 100,000 pascals at sea level). This could be elastic waves in solids (vibrations) pressure waves in liquids, like underwater acoustics or the combined propagation in porous materials (poroelastic waves).įor humans, sound is best understood as the sensation, as detected by the ear, of very small rapid changes in the acoustic pressure p above and below a static value. The definition of sound also includes the propagation in media other than air. Sound here means not only what is detected by the human ear but also infrasound and ultrasound that is, wave propagation with frequencies below and above the human auditory range. Acoustics is the physics of sound, including all of the multiphysics disciplines concerned with the production, transmission, and detection of the sound signal. ![]()
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