Sound intensity, also known as acoustic intensity, is defined as the power carried by sound waves per unit area in a direction perpendicular to that area. The SI unit of intensity, which includes sound intensity, is the watt per square meter (W/m²). One application is the noise measurement of sound intensity in the air at a listener's location as a sound energy quantity.^[1]

Sound intensity is not the same physical quantity as sound pressure. Human hearing is sensitive to sound pressure which is related to sound intensity. In consumer audio electronics, the level differences are called "intensity" differences, but sound intensity is a specifically defined quantity and cannot be sensed by a simple microphone.

Sound intensity level is a logarithmic expression of sound intensity relative to a reference intensity.

Mathematical definition

Sound intensity, denoted I, is defined by

$${\displaystyle \mathbf {I} =p\mathbf {v} }$$

• p is the sound pressure;
• v is the particle velocity.

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

$${\displaystyle \langle \mathbf {I} \rangle ={\frac {1}{T))\int _{0}^{T}p(t)\mathbf {v} (t)\,\mathrm {d} t.}$$

$${\displaystyle \mathrm {I} =2\pi ^{2}\nu ^{2}\delta ^{2}\rho c}$$

• ${\displaystyle \nu }$ is frequency of sound,
• ${\displaystyle \delta }$ is the amplitude of the sound wave particle displacement,
• ${\displaystyle \rho }$ is density of medium in which sound is traveling, and
• ${\displaystyle c}$ is speed of sound.

Inverse-square law

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

$${\displaystyle I(r)={\frac {P}{A(r)))={\frac {P}{4\pi r^{2))},}$$

• P is the sound power;
• A(r) is the surface area of a sphere of radius r.

Thus sound intensity decreases as 1/r² from the centre of the sphere:

$${\displaystyle I(r)\propto {\frac {1}{r^{2))}.}$$

This relationship is an inverse-square law.

Sound intensity level

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 L_I, expressed in nepers, bels, or decibels, and defined by^[2]

$${\displaystyle L_{I}={\frac {1}{2))\ln \left({\frac {I}{I_{0))}\right)\mathrm {Np} =\log _{10}\left({\frac {I}{I_{0))}\right)\mathrm {B} =10\log _{10}\left({\frac {I}{I_{0))}\right)\mathrm {dB} ,}$$

• I is the sound intensity;
• I₀ is the reference sound intensity;
  • 1 Np = 1 is the neper;
  • 1 B = 1/2 ln(10) is the bel;
  • 1 dB = 1/20 ln(10) is the decibel.

The commonly used reference sound intensity in air is^[3]

$${\displaystyle I_{0}=1~\mathrm {pW/m^{2)) .}$$

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 L_{I /(1 pW/m²)} or L_I (re 1 pW/m²), but the notations dB SIL, dB(SIL), dBSIL, or dB_SIL 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

$${\displaystyle I\propto p^{2}.}$$

The equality of SIL and SPL requires that

$${\displaystyle {\frac {I}{I_{0))}={\frac {p^{2)){p_{0}^{2))},}$$

For a progressive spherical wave,

$${\displaystyle {\frac {p}{c))=z_{0},}$$

$${\displaystyle I_{0}={\frac {p_{0}^{2}I}{p^{2))}={\frac {p_{0}^{2}pc}{p^{2))}={\frac {p_{0}^{2)){z_{0))}.}$$

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.

Measurement

Sound intensity is defined as the time averaged product of sound pressure and acoustic particle velocity.^[6] Both quantities can be directly measured by using a sound intensity p-u probe comprising a microphone and a particle velocity sensor, or estimated indirectly by using a p-p probe that approximates the particle velocity by integrating the pressure gradient between two closely spaced microphones.^[7]

Pressure-based measurement methods are widely used in anechoic conditions for noise quantification purposes. The bias error introduced by a p-p probe can be approximated by^[8]

$${\displaystyle {\widehat {I))_{n}^{p-p}\simeq I_{n}-{\frac {\varphi _{\text{pe))\,p_{\text{rms))^{2)){k\Delta r\rho c))=I_{n}\left(1-{\frac {\varphi _{\text{pe))}{k\Delta r)){\frac {p_{\text{rms))^{2}/\rho c}{I_{r))}\right),}$$

${\displaystyle I_{n))$

${\displaystyle {\hat {I))_{n}^{p-p))$

${\displaystyle p_{\text{rms))}$

${\displaystyle k}$

${\displaystyle \rho }$

${\displaystyle c}$

${\displaystyle \Delta r}$

On the other hand, the bias error introduced by a p-u probe can be approximated by^[8]

$${\displaystyle {\hat {I))_{n}^{p-u}={\frac {1}{2))\operatorname {Re} \left\((P{\hat {V))_{n}^{*))\right\}={\frac {1}{2))\operatorname {Re} \left\((PV_{n}^{*}e^{-j\varphi _{\text{ue))))\right\}\simeq I_{n}+\varphi _{\text{ue))J_{n}\,,}$$

${\displaystyle {\hat {I))_{n}^{p-u))$

${\displaystyle P}$

${\displaystyle V_{n))$

${\displaystyle J_{n))$

${\displaystyle \varphi _{\text{ue))}$