Sound power

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Sound power or acoustic power P_ac is a measure of sonic energy E per time t unit.

It is measured in watts, or sound intensity I times area A:

The measure of a ratio of two sound powers is

where

P₁, P₀ are the sound powers.

The sound power level SWL, L_W, or L_Pac of a source is expressed in decibels (dB) and is equal to 10 times the logarithm to the base 10 of the ratio of the sound power of the source to a reference sound power. It is thus a logarithmic measure.

The reference sound power in air is normally taken to be 10⁻¹² watt = 0 dB SWL.

Sound power is neither room dependent nor distance dependent. Sound power belongs strictly to the sound source.

Table: Sound power and sound power level of some sound sources

Situation and sound source	sound power Pac watts	sound power level Lw dB re 10−12 W
Rocket engine	1,000,000 W	180 dB
Turbojet engine	10,000 W	160 dB
Siren	1,000 W	150 dB
Heavy truck engine or loudspeaker rock concert	100 W	140 dB
Machine gun	10 W	130 dB
Jackhammer	1 W	120 dB
Excavator, trumpet	0.3 W	115 dB
Chain saw	0.1 W	110 dB
Helicopter	0.01 W	100 dB
Loud speech, vivid children	0.001 W	90 dB
Usual talking, Typewriter	10−5 W	70 dB
Refrigerator	10−7 W	50 dB

Usable music sound (trumpet) and noise sound (excavator) both have the same sound power of 0.3 watts, but will be judged psychoacoustically to be different levels.

Sound level

A frequently used method of defining the sound power level at a particular distance from a point source is the sound level:^{[citation needed]}

Sound Level =

or if the source is on the floor or on a wall.

Sound level is most commonly used as an initial estimate of noise.^{[citation needed]} The measurement does not take account of atmospheric conditions, radiation and absorption, blocking and other smaller factors which will affect real noise conditions.^{[citation needed]}

Sound power with plain sound waves

Between sound power and other important acoustic values there is the following relationship:

where:

External links

• Ohm's law as acoustic equivalent — calculations
• Relationships of acoustic quantities associated with a plane progressive acoustic sound wave — pdf
• NIOSH Powertools Database