Sound intensity

The sound intensity is defined as the sound power P_ac per unit area A. The usual context is the measurement of sound intensity in the air at a listener's location. The basic units are watts/m².

<math>

J = \frac{1}{T} \int_{0}^{T}p(t) \cdot v(t)\,dt <math>

For a spherical sound source, the intensity as a function of distance r is:

<math>

J_r = \frac{P_{ac}}{A} = \frac{P_{ac}}{4 \cdot \pi \cdot r^2} <math>

P_ac is the acoustic power.

The sound intensity J in W/m² is:

<math>

J = p \cdot v = \frac{p^2}{Z} = Z \cdot v^2 <math>

Sound pressure p in N/m² = Pa
Sound velocity (particle velocity) v in m/s
Acoustic impedance Z in N·s/m³
Area A in m²

The amplitude of sound intensity (not sound pressure!) decreases in the free field (direct field) with 1/r² of the distance of a point source.

Sound intensity level is a different measure used in acoustics. Unlike sound intensity, sound intensity level is logarithmic. Neither ear drums nor microphones can convert sound intensity to voltage modulation.

Note: The term "intensity" is not used for anything else than the measure of sound in watts per area. To describe the strenght of sound not regarding to strict intensity, one can use "magnitude" "strength", "amplitude", or "level" instead.