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|Sound pressure p, SPL|
|Particle velocity v, SVL|
|Particle displacement ξ|
|Sound intensity I, SIL|
|Sound power Pac|
|Sound power level SWL|
|Sound energy density E|
|Sound energy flux q|
|Acoustic impedance Z|
|Speed of sound c|
|Audio frequency AF
When the acoustic wave approaches the measurement surface at an angle, the area is taken as the area times the projection of the wave direction upon the normal of the surface.
where , are the sound powers. The sound power level SWL, LW, or LPac of a source is expressed in decibels (dB) relative to a reference sound power. In air this is normally taken to be = 10−12 watt, that is 0 dB SWL.
Unlike sound pressure, sound power is neither room dependent nor distance dependent. Sound power belongs strictly to the sound source. Sound pressure is a measurement at a point in space near the source, while sound power is the total power produced by the source in all directions.
Table of some sound sources 
dB re 10−12 W
Rocket engine 1,000,000 W 180 dB Turbojet engine 10,000 W 160 dB Siren 1,000 W 150 dB Heavy truck engine or
loudspeaker rock concert
100 W 140 dB Machine gun 10 W 130 dB Jackhammer 1 W 120 dB Excavator, trumpet 0.3 W 115 dB Chain saw 0.1 W 110 dB Helicopter 0.01 W 100 dB Loud speech,
0.001 W 90 dB Usual talking,
10−5 W 70 dB Refrigerator 10−7 W 50 dB
Usable music sound (trumpet) and noise sound (excavator) both have the same sound power of 0.3 watts, but will be judged psychoacoustically to be different levels.
Sound power measurement 
- If the source is in free space
- if the source is on the floor or on a wall, such that it radiates into a half sphere.
The sound power estimated this does not diminish or increase with distance, unless reflections are present.
Sound power with plane sound waves 
Between sound power and other important acoustic values there is the following relationship:
|c||m/s||speed of sound|
|ω = 2πf||rad/s||angular frequency|
|ρ||kg/m3||density of air|
|Z = c · ρ||N·s/m3||acoustic impedance|
|E||W·s/m3||sound energy density|
|Pac||W||sound power or acoustic power|
Sound power level 
Sound power level or acoustic power level is a logarithmic measure of the sound power in comparison to a specified reference level. While sound pressure level is given in decibels SPL, or dB SPL, sound power is given in dB SWL. The dimensionless term "SWL" can be thought of as "sound watts level," the acoustic output power measured relative to 10−12 or 0.000000000001 watt (1 pW). As used by architectural acousticians to describe noise inside a building, typical noise measurements in SWL are very small, less than 1 watt of acoustic power.
where W0 is the 0 dB reference level:
The sound power level is given the symbol LW. This is not to be confused with dBW, which is a measure of electrical power, and uses 1 W as a reference level.
In the case of a free field sound source in air at ambient temperature, the sound power level is approximately related to sound pressure level (SPL) at distance r of the source by the equation
- Chadderton, David V. Building services engineering, pp. 301, 306, 309, 322. Taylor & Francis, 2004. ISBN 0-415-31535-2
- Sound Power, Sound Intensity, and the difference between the two - Indiana University's High Energy Physics Department
- Georgia State University Physics Department - Tutorial on Sound Intensity