**Sound intensity** also known as **acoustic intensity** is defined as the Energy carried by the sound waves per unit area. The SI unit of intensity, which includes sound intensity, is the watt per square meter (W/m^{2}). One application is the noise measurement of sound intensity in the air at a listener's location as a sound energy quantity.

Sound intensity is not the same physical quantity as sound pressure. Hearing is directly 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. The rate at which sound energy passes through a unit area held perpendicular to the direction of propagation of sound waves is called intensity of sound.

Sound intensity, denoted **I**, is defined by

I
=
p
v
where

*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

⟨
I
⟩
=
1
T
∫
0
T
p
(
t
)
v
(
t
)
d
t
.
Also,

**Intensity of Sound = 2π²n²A²ρv**
Where,
n is frequency of sound, A is the Amplitude of sound wave, v is velocity of sound, and ρ is density of medium in which sound is traveling

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

I
(
r
)
=
P
A
(
r
)
=
P
4
π
r
2
,
where

*P* is the sound power;
*A*(*r*) is the area of a sphere of radius *r*.
Thus sound intensity decreases as 1/*r*^{2} from the centre of the sphere:

I
(
r
)
∝
1
r
2
.
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 *L*_{I}, expressed in dB, and defined by

L
I
=
1
2
ln
(
I
I
0
)
N
p
=
log
10
(
I
I
0
)
B
=
10
log
10
(
I
I
0
)
d
B
,
where

*I* is the sound intensity;
*I*_{0} 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

I
0
=
1
p
W
/
m
2
.

The proper notations for sound intensity level using this reference are *L*_{I /(1 pW/m2)} or *L*_{I} (re 1 pW/m^{2}), but the notations dB SIL, dB(SIL), dBSIL, or dB_{SIL} are very common, even if they are not accepted by the SI.

The reference sound intensity *I*_{0} is defined such that a progressive plane wave has the same value of sound intensity level (SIL) and sound pressure level (SPL), since

I
∝
p
2
.
The equality of SIL and SPL requires that

I
I
0
=
p
2
p
0
2
,
where *p*_{0} = 20 μPa is the reference sound pressure.

For a *progressive* spherical wave,

p
v
=
z
0
,
where *z*_{0} is the characteristic specific acoustic impedance. Thus,

I
0
=
p
0
2
I
p
2
=
p
0
2
p
v
p
2
=
p
0
2
z
0
.
In air at ambient temperature, *z*_{0} = 410 Pa·s/m, hence the reference value *I*_{0} = 1 pW/m^{2}.

In an anechoic chamber, which approximates a free field (no reflection), the SIL can be taken as being equal to the SPL. This fact is exploited to measure sound power in anechoic conditions.

One method of sound intensity measurement involves the use of two microphones located close to each other, normal to the direction of sound energy flow. A signal analyser is used to compute the crosspower between the measured pressures and the sound intensity is derived from (proportional to) the imaginary part of the crosspower.