Sound poweroracoustic 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 fluxoracoustic flux through that area.
Sound power level LWA
Regulations[3] control the maximum sound power level LWA that a device (e.g. vacuum cleaner) is allowed to produce. The A-weighting scale is used in the calculation as the regulation is concerned with the loudness as perceived by the human ear. Measurements[4] are taken at several defined points around the device.
The test environment can be located indoors or outdoors. The ideal environment is on the ground in a large open space or hemi-anechoic chamber (free-field over a reflecting plane ). To account for undesired reflections from nearby objects, walls, and the ceiling, and for any residual background noises, measurement corrections are applied.
θ is the angle between the direction of propagation of the sound and the normal to the surface.
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 m2 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.
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 LW and measured in dB, is defined by[7]
The commonly used reference sound power in air is[8]
The proper notations for sound power level using this reference are LW/(1 pW)orLW (re 1 pW), but the suffix notations dB SWL, dB(SWL), dBSWL, or dBSWL are very common, even if they are not accepted by the SI.[9]
The reference sound power P0 is defined as the sound power with the reference sound intensity I0 = 1 pW/m2 passing through a surface of area A0 = 1 m2:
hence the reference value P0 = 1 pW.
Relationship with sound pressure level
The generic calculation of sound power from sound pressure is as follows:
where:
defines the area of a surface that wholly encompasses the source. This surface may be any shape, but it must fully enclose the source.
In the case of a sound source located in free field positioned over a reflecting plane (i.e. the ground), 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[10]
where
Lp is the sound pressure level;
A0 = 1 m2;
defines the surface area of a hemisphere; and
r must be sufficient that the hemisphere fully encloses the source.
and since by definition I0 = p02/z0, where p0 = 20 μPa is the reference sound pressure,
The sound power estimated practically does not depend on distance. The sound pressure used in the calculation may be affected by distance due to viscous effects in the propagation of sound unless this is accounted for.
^Ross Roeser, Michael Valente, Audiology: Diagnosis (Thieme 2007), p. 240.
^Thompson, A. and Taylor, B. N. sec 8.7, "Logarithmic quantities and units: level, neper, bel", Guide for the Use of the International System of Units (SI) 2008 Edition, NIST Special Publication 811, 2nd printing (November 2008), SP811 PDF
^Chadderton, David V. Building services engineering, pp. 301, 306, 309, 322. Taylor & Francis, 2004. ISBN0-415-31535-2