Sound measurements
p, SPL, LPA
v, SVL
δ
I, SIL
P, SWL, LWA
W
w
E, SEL
Z
AF
TL
Sound intensity, denoted I, is defined by
Both I and v are vectors, which means that both have a direction as well as a magnitude. The direction of sound intensity is the average direction in which energy is flowing.
The average sound intensity during time T is given by
For a spherical sound wave, the intensity in the radial direction as a function of distance r from the centre of the sphere is given by
Thus sound intensity decreases as 1/r2 from the centre of the sphere:
This relationship is an inverse-square law.
Sound intensity level (SIL) or acoustic intensity level is the level (a logarithmic quantity) of the intensity of a sound relative to a reference value.
It is denoted LI, expressed in nepers, bels, or decibels, and defined by[2]
The commonly used reference sound intensity in air is[3]
being approximately the lowest sound intensity hearable by an undamaged human ear under room conditions.
The proper notations for sound intensity level using this reference are LI /(1 pW/m2) or LI (re 1 pW/m2), but the notations dB SIL, dB(SIL), dBSIL, or dBSIL are very common, even if they are not accepted by the SI.[4]
The reference sound intensity I0 is defined such that a progressive plane wave has the same value of sound intensity level (SIL) and sound pressure level (SPL), since
The equality of SIL and SPL requires that
For a progressive spherical wave,
In air at ambient temperature, z0 = 410 Pa·s/m, hence the reference value I0 = 1 pW/m2.[5]
In an anechoic chamber which approximates a free field (no reflection) with a single source, measurements in the far field in SPL can be considered to be equal to measurements in SIL. This fact is exploited to measure sound power in anechoic conditions.