# Volume Velocity

Within acoustics the volume velocity U of a source is the pressure p divided by the acoustic impedance Z. (U is not a vector and as such should not be described as a velocity.)

The volume velocity may also be described as the surface area of the source multiplied by the normal surface velocity. This is particularly useful for airborne route tracking as volume velocity to pressure is a reciprocal function. This means that the transfer function from the observer to the source may be measured by placing a known volume velocity at the position of the observer and measuring the resultant pressure at the source. Multiplying this transfer function by the volume velocity of the source in operation gives the contribution of the source at the observer location.

For a source to be used as a volume velocity source for the reciprocal measurement of transfer functions it must satisfy three basic criteria:

1. The source must approximate a point source. This limits the size of the acoustic drive unit and thus the volume of the final source.
2. The source has to produce high enough levels so as to be able to measure the pressure or acceleration at the response point.
3. It must be possible to accurately measure the effective volume velocity of the source.
The sound power of a volume velocity is:
where
Π = sound power [Watts]
ω = angular frequency [rad s-1]
Q = volume velocity [m3s-1]
ρ = density of air [kgm-3]
c = speed of sound [ms-1]

### Acknowledgement

We would like to thank Graham Franks sovibtech.com for pointing out the error in the original equation.