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Sound pressure is the pressure deviation from the local ambient pressure caused by a sound wave. Sound pressure can be measured using a microphone in air and a hydrophone in water. The SI unit for sound pressure is the pascal (symbol: Pa). The instantaneous sound pressure is the deviation from the local ambient pressure p0 caused by a sound wave at a given location and given instant in time. The effective sound pressure is the root mean square of the instantaneous sound pressure over a given interval of time. In a sound wave, the complementary variable to sound pressure is the acoustic particle velocity. For small amplitudes, sound pressure and particle velocity are linearly related and their ratio is the acoustic impedance. The acoustic impedance depends on both the characteristics of the wave and the medium. The local instantaneous sound intensity is the product of the sound pressure and the acoustic particle velocity and is, therefore, a vector quantity.
The sound pressure deviation p is
The entire pressure ptotal is
Sound pressure level (SPL) or sound level Lp is a logarithmic measure of the rms pressure of a particular sound relative to a reference sound source. It is usually measured in decibels (dB (SPL), dBSPL, or dBSPL).
where p0 is the reference sound pressure and prms is the rms sound pressure being measured.
The commonly used reference sound pressure in air is p0 = 20 µPa (rms). In underwater acoustics, the reference sound pressure is p0 = 1 µPa (rms).
It can be useful to express sound pressure in this way when dealing with hearing, as the perceived loudness of a sound correlates roughly logarithmically to its sound pressure. See also Weber-Fechner law.
When making measurements in air (and other gases), SPL is almost always expressed in decibels compared to a reference sound pressure of 20 µPa, which is usually considered the threshold of human hearing (roughly the sound of a mosquito flying 3 m away). Thus, most measurements of audio equipment will be made relative to this level. However, in other media, such as underwater, a reference level of 1 µPa is more often used. These references are defined in ANSI S1.1-1994. In general, it is necessary to know the reference level when comparing measurements of SPL. The unit dB (SPL) is often abbreviated to just "dB", which gives some the erroneous notion that a dB is an absolute unit by itself.
The human ear is a sound pressure sensitive detector. It does not have a flat spectral response, sound pressure levels are often frequency weighted so that the measured level will match perceived sound level. The International Electrotechnical Commission (IEC) has defined several weighting schemes. A-weighting attempts to match the response of the human ear to noise, while C-weighting is used to measure peak sound levels. If the actual, as opposed to weighted, SPL is desired, many instruments allow a "flat" or unweighted measurement to be made. See also Weighting filter.
When measuring the sound created by an object, it is important to measure the distance from the object as well, since the SPL decreases in distance from a point source with 1/r (and not with 1/r2, like sound intensity). It often varies in direction from the source, as well, so many measurements may be necessary, depending on the situation. An obvious example of a source that varies in level in different directions is a bullhorn.
Sound pressure p in N/m2 or Pa is
Sound pressure p is connected to particle displacement (or particle amplitude) ξ, in m, by
Sound pressure p is
normally in units of N/m2 = Pa.
The distance law for the sound pressure p is inverse-proportional to the distance r of a punctual sound source.
The assumption of 1/r² with the square is here wrong. That is only correct for sound intensity.
Note: The often used term "intensity of sound pressure" is not correct. Use "magnitude", "strength", "amplitude", or "level" instead. "Sound intensity" is sound power per unit area, while "pressure" is a measure of force per unit area. Intensity is not equivalent to pressure.