[Obama08]

بسم الله الرحمن الرحيم

Sound pressure level (SPL) or sound level Lp is a logarithmic measure of the rms sound pressure of a sound relative to a reference value. It is measured in decibel (dB).

Lp = 10 log[(Prms)^2/(pref)^2]

where pref is the reference sound pressure and prms is the rms sound pressure being measured.

The commonly used reference sound pressure in air is pref = 20 µPa (rms), which is usually considered the threshold of human hearing
Most measurements of audio equipment will be made relative to this level, meaning 1 pascal will equal 94 dB of sound pressure.
In other media, such as underwater, a reference level of 1 µPa is more often used

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