|This article does not cite any references or sources. (October 2008)|
|Sound pressure||p · SPL|
|Particle velocity||v · SVL|
|Sound intensity||I · SIL|
|Sound power level||SWL|
|Sound exposure level||SEL|
|Sound energy density||E|
|Sound energy flux||q|
|Speed of sound|
Sound intensity or acoustic intensity (I) is defined as the sound power Pac per unit area A. The usual context is the noise measurement of sound intensity in the air at a listener's location as a sound energy quantity.
Sound intensity is not the same physical quantity as sound pressure. Hearing is directly sensitive to sound pressure which is related to sound intensity. In consumer audio electronics, the level differences are called "intensity" differences, but sound intensity is a specifically defined quantity and cannot be sensed by a simple microphone.
The intensity is the product of the sound pressure and the particle velocity
Notice that both and are vectors, which means that both have a direction as well as a magnitude. The direction of the intensity is the average direction in which the energy is flowing. For instantaneous acoustic pressure pinst(t) and particle velocity (t) the average acoustic intensity during time T is given by
|p||pascal||RMS sound pressure|
|ξ||m, metre||particle displacement|
|c||m/s||speed of sound|
|ω = 2πf||radians/s||angular frequency|
|ρ||kg/m3||density of air|
|Z = c ρ||N·s/m³||characteristic acoustic impedance|
|E||W·s/m³||sound energy density|
|Pac||W, watt||sound power or acoustic power|
For a spherical sound source, the intensity in the radial direction as a function of distance r from the centre of the source is:
Here, Pac (upper case) is the sound power and A the surface area of a sphere of radius r. Thus the sound intensity decreases with 1/r2 the distance from an acoustic point source, while the sound pressure decreases only with 1/r from the distance from an acoustic point source after the 1/r-distance law.
= sound intensity at close distance
= sound intensity at far distance
where p (lower case) is the RMS sound pressure (acoustic pressure).
Sound intensity level
Sound intensity level or acoustic intensity level is a logarithmic measure of the sound intensity (measured in W/m2), in comparison to a reference level.
The measure of a ratio of two sound intensities is
If I0 is the standard reference sound intensity
(W = watt), then instead of "dB SPL" we use "dB SIL". (SIL = sound intensity level). The reference value is defined such that a plane wave propagating in a free field has the same value of SPL and SIL as the ratio of the reference pressure squared to the reference intensity is approximately equal to the characteristic impedance of air. In an anechoic chamber, which approximates a free field, the SIL can be taken as being equal to the SPL. This fact is exploited to measure sound power in anechoic conditions.
One method of sound intensity measurement involves the use of two microphones located close to each other, normal to the direction of sound energy flow. A signal analyser is used to compute the crosspower between the measured pressures and the sound intensity is derived from (proportional to) the imaginary part of the crosspower. 
||This article's use of external links may not follow Wikipedia's policies or guidelines. (December 2012)|
- How many decibels is twice as loud? Sound level change and the respective factor of sound pressure or sound intensity
- Acoustic Intensity
- Conversion: Sound intensity level to sound intensity and vice versa
- Ohm's law as acoustic equivalent - calculations
- Relationships of acoustic quantities associated with a plane progressive acoustic sound wave - pdf
- Table of Sound Levels - Corresponding Sound Intensity and Sound Pressure
- What is Sound Intensity Measurement and Analysis?