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Sound power or acoustic power is the rate at which sound energy is emitted, reflected, transmitted or received, per unit time.^{[1]} The SI unit of sound power is the watt (W).^{[1]} It is the power of the sound force on a surface of the medium of propagation of the sound wave. For a sound source, unlike sound pressure, sound power is neither room-dependent nor distance-dependent. Sound pressure is a measurement at a point in space near the source, while the sound power of a source is the total power emitted by that source in all directions. Sound power passing through an area is sometimes called sound flux or acoustic flux through that area.
Sound power, denoted P, is defined by^{[2]}
where
In a medium, the sound power is given by
For example a sound at SPL = 85 dB or p = 0.356 Pa in air (ρ = 1.2 kg·m^{−3} and c = 343 m·s^{−1}) through a surface of area A = 1 m^{2} normal to the direction of propagation (θ = 0 °) has a sound energy flux P = 0.3 mW.
This is the parameter one would be interested in when converting noise back into usable energy, along with any losses in the capturing device.
^{[3]}
Sound power is related to sound intensity:
Sound power is related sound energy density:
Sound power level (SWL) or acoustic power level is a logarithmic measure of the power of a sound relative to a reference value. Sound power level, denoted L_{W} and measured in dB, is defined by^{[4]}
The commonly used reference sound power in air is^{[5]}
The proper notations for sound power level using this reference are L_{W/(1 pW)} or L_{W} (re 1 pW), but the suffix notations dB SWL, dB(SWL), dBSWL, or dB_{SWL} are very common, even if they are not accepted by the SI.^{[6]}
The reference sound power P_{0} is defined as the sound power with the reference sound intensity I_{0} = 1 pW/m^{2} passing through a surface of area A_{0} = 1 m^{2}:
hence the reference value P_{0} = 1 pW.
In the case of a free field sound source in air at ambient temperature, the sound power level at distance r from the sound source is approximately related to sound pressure level (SPL) by^{[7]}
Derivation of this equation:
For a progressive spherical wave,
where z_{0} is the characteristic specific acoustic impedance.
Consequently,
and since by definition I_{0} = p_{0}^{2}/z_{0}, where p_{0} = 20 μPa is the reference sound pressure,
The sound power estimated practically does not depend on distance, though theoretically it may diminish with distance due to viscous effects in the propagation of sound.
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