Sound Power, Intensity and Pressure

An introduction to decibel, sound power, sound intensity and sound pressure

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The Decibel

Decibel is a logarithmic unit used to describe the ratio of the signal level - power, sound pressure, voltage or intensity or several other things.

The decibel can be expressed as:

decibel = 10 log( P / Pref )   (1)

where 

P = signal power (W)

Pref = reference power (W) 

Sound Power Level

Sound power is the energy rate - the energy of sound per unit of time (J/s, W in SI-units) from a sound source.

Sound power can more practically be expressed as a relation to the threshold of hearing - 10-12 W - in a logarithmic scale named Sound Power Level - Lw: 

Lw = 10 log ( N / No)      (2)

where 

Lw = Sound Power Level in Decibel (dB)

N = sound power (W)

Sound Intensity

Sound Intensity is the Acoustic or Sound Power (W) per unit area. The SI-units for Sound Intensity are W/m2

The Sound Intensity Level can be expressed as: 

LI = 10 log( I / Iref )   (3)

where 

LI = sound intensity level (dB)

I = sound intensity (W/m2)

Iref = 10-12 - reference sound intensity (W/m2

Sound Pressure Level

The Sound Pressure is the force (N) of sound on a surface area (m2) perpendicular to the direction of the sound. The SI-units for the Sound Pressure are N/m2 or Pa.

The Sound Pressure Level: 

Lp = 10 log( p2 / pref2 ) = 10 log( p / pref )2  = 20 log ( p / pref )   (1)

where

Lp = sound pressure level (dB)

p = sound pressure (Pa)

pref = 2 10-5 - reference sound pressure (Pa)

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