Sound pressure level (SPL) or sound level is a logarithmic measure of the effective sound pressure of a sound relative to a reference value.

Sound refers to small-amplitude, propagating pressure traveling in a compressible medium. This pressure is related to the corresponding density of the medium, distance of the source and the way how it is governed by a wave equation. Sound pressure is the incremental variation in the static pressure of a medium when a sound wave is propagated through it and sound pressure level is the logarithmic value of it. Sound pressures are often frequency weighted so that the measured level will match perceived levels more closely.

## Units and Measures

The sound pressure level at a point is measured in decibels (dB) and is equal to 20 times the logarithm to the base 10 of the ratio of root mean square of sound pressure to the reference sound pressure. The reference sound pressure in air is taken to be 2 x 10-5 Pa

### Example Units

- Atmosphere environmental pressure – 191 dB
- Threshold of pain – 100 dB
- Auditory threshold – 0 dB

## Useful References

- http://www.sengpielaudio.com/TableOfSoundPressureLevels.htm
- http://www.rane.com/par-s.html#SPL
- http://www.answers.com/topic/sound

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