The immense range of human hearing, see the table below, means that absolute sound pressure values are impractical for everyday use, a neat solution is the
Logarithmic sound pressure leveldB descriptor.
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, the threshold of hearing. It is measured in decibels (dB).
Lp = 20 lg (p/po) dB re 20 µPa for air ... water, steel, etc., are different.
Some typical sound pressure levels
Sources at 1 m
Lp re 20 µPa *
Threshold of pain
6 dB = double the Pa
Threshold of hearing
* Reference Sound Pressure po in air = 2 x 10-5 Pa = 20 µPa = 0 dB
The Reference Sound Pressure was chosen conventionally to correspond to the quietest sound at 1000 Hz that the human ear can detect.
Lp uses the 20 lg equation so, as a rule of thumb:
6 dB = a factor of 2 in sound pressure
10 dB = a factor of 3 in sound pressure
20 dB = a factor of 10 in sound pressure