# Sound Power Level

Of airborne sound, ten times the common logarithm of the ratio of the sound power under consideration of the standard reference power of 1 pW. The quantity so obtained is expressed in decibels.

- For a simple monopole in free space the sound power is expressed as:
- where
- Π = sound power [W]
*ρ*_{0}= density of air [kg m^{-3}]*c*= speed of sound [ms^{-1}]*Q*= volume velocity [m^{3}s^{-1}]*λ*= wavelength [m]

- The volume velocity of the source is the surface area of the source multiplied by the surface normal velocity.

**See also: **Decibel, Sound Intensity, Sound Pressure Level.

**Subjects: ** Noise & Vibration