# Acoustic Glossary

## Sound Power Terms, Definitions, Units, Measurements ...

Sound Power is the total acoustic power radiated by a source, in all directions and measured in watts

Sound Power is the sound intensity multiplied by the surface area = p·v·A : where p is the sound pressure, v the particle velocity and A is the surface area

Sound Power measurements are therefore not straight forward, requiring sound intensity measuring equipment or sound level meter measurements in a reverberant or anechoic chambers.

Sound Power is also known as Acoustic Power : LPac : Pac : PWL : PWR : SWL.

The range of human hearing is 0.000000000001 watts to 100 watts or more, so sound power values in watts is not practical for everyday use. However the sound power level in decibels - discussed below, neatly solves this problem.

Sound Power Level in decibels is ten times the logarithm of the ratio of the sound power to the sound power reference level of 1 pico Watt

Sound Power Level (Lw) = 10 lg (W/Wo) dB

.

Sound Power Reference Level (Wo) = 1 pW = 10-12 W ≡ 0 dB

The table shows practical examples of the Sound Power Levels in decibels and sound power Watts
Source Sound Power W Lw re 1 pW
Jet Engine 10,000 W 160 dB
Loudspeaker Rock 100 W 140 dB
2 Pneumatic hammers 2 W 123 dB
Pneumatic hammer   1 W 120 dB
Outboard motor 0.01 W 100 dB
Dishwasher 0.0001 W 80 dB
Office 0.000001 W 60 dB
Fridge 0.00000001 W 40 dB
Whisper 0.0000000001 W 20 dB
Threshold 10-12 W 0 dB
LW uses the 10 lg factor so, as a rule of thumb:
3 dB = a factor of 2, twice or half the sound power
10 dB = a factor of 10 in sound power
20 dB = a factor of 100 in sound power
30 dB is a factor of 1000
40 dB is a factor of 10000

Sound Power Level Definition IEC 801-22-05, logarithm of the ratio of a given sound power to the reference sound power. Such power level in decibels is ten times the logarithm to the base ten of the ratio.
Note : unless otherwise specified, the reference sound power is 1 pW.

Related Terms - listed alphabetically

Acoustic Reference Wind Speed Definition IEC 415-06-06, wind speed of 8 m/s at reference conditions (10 m height, roughness length equal to 0,05 m) used in the calculation of the apparent sound power level to provide a uniform basis for the comparison of apparent sound power levels from different wind turbine generator system.
Note : the acoustic reference speed is generally expressed in meters per second.

Apparent Sound Power Level Definition IEC 415-06-03, A-weighted sound power level referred to 1 pW of a point source at the rotor center with the same emission in the downwind direction as the wind turbine being measured as determined at the acoustic reference wind speed.
Note : the apparent sound power level is generally expressed in decibels.

Sound Power Absorption Coefficient Definition IEC 801-31-02, at a given frequency and for specified conditions, of a surface, fraction of incident sound power not reflected from the surface.
Note : unless otherwise specified, a diffuse sound field is to be understood.

Sound Power Density (I), is the average sound energy flowing through a specified point.

Sound Power Density Definition IEC 801-21-38, sound energy flux in a specified direction and sense through an area perpendicular to that direction, divided by the area. Also known as sound intensity and sound energy flux density.

Sound Power Density SI units are W/m2 (watts per metre-squared).

Sound Power of a Source Definition IEC 801-21-36, is the total sound energy radiated by the source in a specified frequency band over a certain time interval, divided by the interval.

Sound Power Reflection Coefficient Definition IEC 801-31-04, at a given frequency and for specified conditions of a surface, fraction of incident sound power reflected from the surface.
Note : unless otherwise specified, random incidence is presumed.

Sound Power Spectrum Level, the power per unit frequency may be measured for broadband sound, SI units W/Hz (watt/hertz)

Sound Power Through a Surface Element (J), the rate of flow of sound energy through a specified area. The SI Unit is the watt (W).

Sound Power Through A Surface Element Definition IEC 801-21-37, time average product of the in-phase components of the instantaneous sound pressure and the volume velocity through the surface element considered, and is also known as the sound energy flux

Statistical Sound Power Absorption Coefficient Definition IEC 801-31-03, sound power absorption coefficient measured or calculated with plane waves at randomly distributed angles of incidence.

Other Sound Power Related Terms power spectral densitypower spectrumpower spectrum averagingpower spectrum levelsound energysound intensity