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# Sound power

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 Title: Sound power Author: World Heritage Encyclopedia Language: English Subject: Collection: Publisher: World Heritage Encyclopedia Publication Date:

### Sound power

Sound measurements
Characteristic
Symbols
Sound pressure  p, SPL
Particle velocity  v, SVL
Particle displacement  δ
Sound intensity  I, SIL
Sound power  P, SWL
Sound energy  W
Sound energy density  w
Sound exposure  E, SEL
Acoustic impedance  Z
Speed of sound  c
Audio frequency  AF
Transmission loss  TL

Sound power or acoustic power is the rate at which sound energy is emitted, reflected, transmitted or received, per unit time. The SI unit of sound power is the watt (W). It is the power of the sound force on a surface of the medium of propagation of the sound wave. For a sound source, unlike sound pressure, sound power is neither room-dependent nor distance-dependent. Sound pressure is a measurement at a point in space near the source, while the sound power of a source is the total power emitted by that source in all directions. Sound power passing through an area is sometimes called sound flux or acoustic flux through that area.

## Contents

• Mathematical definition 1
• Table of selected sound sources 2
• Relationships with other quantities 3
• Sound power level 4
• Relationship with sound pressure level 4.1
• References 5

## Mathematical definition

Sound power, denoted P, is defined by

P = \mathbf f \cdot \mathbf v = Ap\, \mathbf u \cdot \mathbf v = Apv

where

In a medium, the sound power is given by

P = \frac{A p^2}{\rho c} \cos \theta,

where

• A is the area of the surface;
• ρ is the mass density;
• c is the sound velocity;
• θ is the angle between the direction of propagation of the sound and the normal to the surface.

For example a sound at SPL = 85 dB or p = 0.356 Pa in air (ρ = 1.2 kg·m−3 and c = 343 m·s−1) through a surface of area A = 1 m2 normal to the direction of propagation (θ = 0 °) has a sound energy flux P = 0.3 mW.

This is the parameter one would be interested in when converting noise back into usable energy, along with any losses in the capturing device.

## Table of selected sound sources

Situation and
sound source
Sound power
(W)
Sound power level
(dB ref 10−12 W)
Saturn V rocket 100,000,000 200
Turbojet engine 100,000 170
Turbofan aircraft at take-off 1,000 150
Turboprop aircraft at take-off 100 140
Machine gun
Large pipe organ
10 130
Symphony orchestra
Heavy thunder
Sonic boom
1 120
Rock concert
Chain saw
Accelerating motorcycle
0.1 110
Lawn mower
Car at highway speed
Subway steel wheels
0.01 100
Large diesel vehicle 0.001 90
Alarm clock 0.0001 80
Noisy office
Vacuum cleaner
10−5 70
Busy restaurant
Hair dryer
10−6 60
Quiet office
Average home
10−7 50
Refrigerator
low voice
Quiet home
10−8 40
Quiet conversation
10−9 30
Whisper
Wristwatch ticking
10−10 20
Human breath 10−11 10
Reference value 10−12 0



## Relationships with other quantities

Sound power is related to sound intensity:

P = AI,

where

• A is the area;
• I is the sound intensity.

Sound power is related sound energy density:

P = Acw,

where

## Sound power level

Sound power level (SWL) or acoustic power level is a logarithmic measure of the power of a sound relative to a reference value.
Sound power level, denoted LW and measured in dB, is defined by

L_W = \frac{1}{2} \ln\!\left(\frac{P}{P_0}\right)\!~\mathrm{Np} = \log_{10}\!\left(\frac{P}{P_0}\right)\!~\mathrm{B} = 10 \log_{10}\!\left(\frac{P}{P_0}\right)\!~\mathrm{dB},

where

• P is the sound power;
• P0 is the reference sound power;
• 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 power in air is

P_0 = 1~\mathrm{pW}.

The proper notations for sound power level using this reference are LW/(1 pW) or LW (re 1 pW), but the suffix notations dB SWL, dB(SWL), dBSWL, or dBSWL are very common, even if they are not accepted by the SI.

The reference sound power P0 is defined as the sound power with the reference sound intensity I0 = 1 pW/m2 passing through a surface of area A0 = 1 m2:

P_0 = A_0 I_0,

hence the reference value P0 = 1 pW.

### Relationship with sound pressure level

In the case of a free field sound source in air at ambient temperature, the sound power level at distance r from the sound source is approximately related to sound pressure level (SPL) by

L_W = L_p + 10 \log_{10}\!\left(\frac{4\pi r^2}{A_0}\right)\!~\mathrm{dB},

where

• Lp is the sound pressure level;
• A0 = 1 m2.

Derivation of this equation:

\begin{align} L_W &= \frac{1}{2} \ln\!\left(\frac{P}{P_0}\right)\\ &= \frac{1}{2} \ln\!\left(\frac{AI}{A_0 I_0}\right)\\ &= \frac{1}{2} \ln\!\left(\frac{I}{I_0}\right) + \frac{1}{2} \ln\!\left(\frac{A}{A_0}\right)\!. \end{align}

For a progressive spherical wave,

z_0 = \frac{p}{v},
A = 4\pi r^2,

where z0 is the characteristic specific acoustic impedance.

Consequently,

I = pv = \frac{p^2}{z_0},

and since by definition I0 = p02/z0, where p0 = 20 μPa is the reference sound pressure,

\begin{align} L_W &= \frac{1}{2} \ln\!\left(\frac{p^2}{p_0^2}\right) + \frac{1}{2} \ln\!\left(\frac{4\pi r^2}{A_0}\right)\\ &= \ln\!\left(\frac{p}{p_0}\right) + \frac{1}{2} \ln\!\left(\frac{4\pi r^2}{A_0}\right)\\ &= L_p + 10 \log_{10}\!\left(\frac{4\pi r^2}{A_0}\right)\!~\mathrm{dB}. \end{align}

The sound power estimated practically does not depend on distance, though theoretically it may diminish with distance due to viscous effects in the propagation of sound.