The instantaneous sound intensity is a vector quantity defined by

where p is the sound pressure in Pascal and u is the particle velocity in meter pr second. For measurement purposes, we will look at the averaged intensity over some time,

By measuring over a closed surface, it is sufficient to measure only one dimension of the sound field. We decompose the sound field into a single dimension and thus simplify the measurement equipment requirements. Rewriting the equation for one dimension, the particle velocity then becomes a scalar and we have

Sound Intensity makes it possible to distinguish between the active and reactive part of the sound field, due to the directional information in the particle velocity u.

In an active field (free-field conditions), p and u is in phase, and the sound intensity can be directly related to the sound pressure. This implies that there are no reflections.

For a fully reactive field, we have u 90 degrees out of phase related to the sound pressure p. This is the same condition as for a completely diffuse field. The net sound energy flow through a surface is zero.

A sound level meter only measures the active part of the field (from I = p(t) * u(t)). This way, the diffuse field is ignored when measuring over a closed surface.

Measurement surface theory

Probe theory

Phase calibration theory

Sound Intensity help index