Sound measurements

Symbols

 p, SPL, LPA

 v, SVL

 I, SIL

 P, SWL, LWA

 W

 E, SEL

p is the ;

sound pressure

v is the .

particle velocity

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

P is the ;

sound power

A(r) is the of radius r.

surface area of a sphere

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.

I is the sound intensity;

I

neper

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.

Relationships of Acoustic Quantities Associated with a Plane Progressive Acoustic Sound Wave

Table of Sound Levels. Corresponding Sound Intensity and Sound Pressure

What Is Sound Intensity Measurement and Analysis?