Sound Power, Intensity and Pressure

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

The Decibel

Decibel is a logarithmic unit used to describe physical values like the ratio of a signal level - like power, sound pressure, voltage or intensity.

The decibel can be expressed as:

Decibel = 10 log(P / Pref)           (1)


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)


Lw = Sound Power Level in Decibel (dB)

N = sound power (W)

  • The lowest sound level that people of excellent hearing can discern has an acoustic sound power about 10-12 W, defined as 0 dB
  • The loudest sound generally encountered is that of a jet aircraft with a sound power of 105 W, or 170 dB

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)


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 in decibel can be expressed as

Lp = 10 log(p2 / pref2) = 10 log(p / pref)2  = 20 log (p / pref)           (4)


Lp = sound pressure level (dB)

p = sound pressure (Pa)

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

  • If the pressure is doubled, the sound pressure level is increased with 6 dB (20 log (2))

Related Topics

  • Acoustics - Room acoustics, acoustic properties - decibel A, B and C - Noise Rating (NR) curves, sound transmission, sound pressure, sound intensity, attenuation and more
  • Physiology - Human physiology, air quality and comfort temperatures, activity and metabolic rates, health effects of carbon monoxide and more
  • Noise and Attenuation - Noise is usually defined as unwanted sound - noise, noise generation, silencers and attenuation in HVAC systems

Related Documents

  • Decibel - Decibel is a logarithmic unit used to describe the ratio of a signal level - like power, sound pressure, voltage, intensity etc.
  • Sound Intensity - Sound intensity level is the acoustic power of sound per unit of area in relation to a fixed reference
  • Sound Power - Sound Power Level and the Sound Power from some common sources as fans, jet engines, cars, humans and more ..
  • Blade Dampers - Noise Generated - Noise levels generated in blade dampers
  • Sound Pressure - recommended Maximum Level in Rooms - Maximum recommended sound pressure levels in rooms with activities as kindergartens, auditoriums, libraries, cinemas and more ...

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