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|Sound pressure p|
|Sound pressure level (SPL)|
|Particle velocity v|
|Particle velocity level (SVL)|
|(Sound velocity level)|
|Particle displacement ξ|
|Sound intensity I|
|Sound intensity level (SIL)|
|Sound power Pac|
|Sound power level (SWL)|
|Sound energy density E|
|Sound energy flux q|
|Acoustic impedance Z|
|Speed of sound c|
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.
Acoustic intensity Edit
The intensity is the product of the sound pressure and the particle velocity
Notice that both v and I 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 v(t) the average acoustic intensity during time T is given by
|p||pascals||RMS sound pressure|
|ξ||m, metres||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, watts||sound power or acoustic power|
Spatial expansion Edit
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 Edit
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
where I1 and I0 are the intensities.
The sound intensity level is given the letter "LI" and is measured in "dB". The decibel is a dimensionless quantity.
If I0 is the standard reference sound intensity
(W = watt), then instead of "dB SPL" we use "dB SIL". (SIL = sound intensity level).
- 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?