# Sound flow

Sound quantities

The sound flow  is a sound field quantity in acoustics . It results as the surface integral of the speed of sound over a sound- penetrated surface. Only the parts of the sound velocity that are perpendicular to the surface, i.e. in the direction of the vectorial surface  element, play a role: ${\ displaystyle q}$ ${\ displaystyle {\ vec {v}}}$${\ displaystyle \ mathrm {d} {\ vec {A}}}$

${\ displaystyle q = \ int {\ vec {v}} \ cdot \ mathrm {d} {\ vec {A}}}$

The sound flow and sound velocity on the surface are therefore always in phase .

The sound flow characterizes the volume of the transmission medium (air) which - caused by the alternating sound pressure - flows back and forth through an area over a period of time. Hence its unit: m 3 / s.

## Simplified formula

If all particles of the transmission medium have the same sound velocity (velocity) on a surface, i. That is, if the rhythmic flow through the surface is in phase everywhere, the formula for the sound flow with the area and the amount of the sound velocity can be simplified to: ${\ displaystyle A}$${\ displaystyle v}$

${\ displaystyle q = v \, A}$

This applies if the surface is chosen in such a way that the speed of sound has the same phase everywhere (e.g. on the surface of a sphere with a spherical sound source in the center) or - and this determines the field of application of the term - if the diameter of a sound The transmitted cross-section is small compared to the wavelength (e.g. in the throat , in the inner ear , in the opening of a Helmholtz resonator ).