What is a decibel (dB) ?

The decibel is a logarithmic unit which is used in a number of
scientific disciplines. In all cases it is used to compare some
quantity with some reference value. Usually the reference value is the
smallest likely value of the quantity. Sometimes it can be an
approximate average value.

In acoustics the decibel is most often used to compare sound pressure,
in air, with a reference pressure. References for sound intensity,
sound power and sound pressure in water are amongst others which are
also commonly in use.

Reference sound pressure (in air) = 0.00002 = 2E-5 Pa (rms)
” ” intensity = 0.000000000001 = 1E-12 W/m^2
” ” power = 0.000000000001 = 1E-12 W
” ” pressure (water) = 0.000001 = 1E-6 Pa

Acousticians use the dB scale for the following reasons:

1) Quantities of interest often exhibit such huge ranges of
variation that a dB scale is more convenient than a linear
scale. For example, sound pressure radiated by a submarine may
vary by eight orders of magnitude depending on direction.

2) The human ear interprets loudness on a scale much closer to
a logarithmic scale than a linear scale.