Sound energy flux
Encyclopedia
Sound energy flux results from the integral of the acoustic pressure p times the particle velocity
v over a surface A , and is given by the integral below.
Particle velocity
Particle velocity is the velocity v of a particle in a medium as it transmits a wave. In many cases this is a longitudinal wave of pressure as with sound, but it can also be a transverse wave as with the vibration of a taut string....
v over a surface A , and is given by the integral below.
-
The sound energy flux is the average rate of flow of sound energy for one period through any specified area A and is usually referred to as acoustic intensity.
In a mediumTransmission mediumA transmission medium is a material substance that can propagate energy waves...
of densityDensityThe mass density or density of a material is defined as its mass per unit volume. The symbol most often used for density is ρ . In some cases , density is also defined as its weight per unit volume; although, this quantity is more properly called specific weight...
ρ for a plane or spherical free waveWaveIn physics, a wave is a disturbance that travels through space and time, accompanied by the transfer of energy.Waves travel and the wave motion transfers energy from one point to another, often with no permanent displacement of the particles of the medium—that is, with little or no associated mass...
having a velocity of propagation v, the sound energy fluxFluxIn the various subfields of physics, there exist two common usages of the term flux, both with rigorous mathematical frameworks.* In the study of transport phenomena , flux is defined as flow per unit area, where flow is the movement of some quantity per time...
through the area A corresponding to an effective sound pressureSound pressureSound pressure or acoustic pressure is the local pressure deviation from the ambient atmospheric pressure caused by a sound wave. Sound pressure can be measured using a microphone in air and a hydrophone in water...
p is
- J = (p2A / ρ v) cos θ
where θ = the angle between the direction of propagation of the sound and the normal to the area A.
This is the parameter one would be interested in when converting noise back into usable enery, along with any losses in the capturing device.
For example a sound at 85 dB or 0,356 Pa in air (ρ=1,2 kg/m2; v=343 m/s; A=1m2;cos θ =1 ) has a sound energy flux of 0,3 mW