Introduction : The immense range of human hearing, 0.000000000001 watts to 100 watts or more, means using Absolute Sound Power Values is not practical for everyday use, a neat solution is the Sound Power Leveldecibel descriptor.
Sound Power Level : LW : is the Logarithmic ratio of sound power to the Threshold of Hearing and makes the values more manageable i.e. 0 to 160 dB and the symbol is Lw - see the table below.
Lw = 10 lg (W/Wo) dB re 1 pW = 10-12 W
Other sound power terms in current use include LPac, Pac, PWL, PWR, SWL and Acoustic Power
Some Typical Sound Power Levels
Sound Power W
Lw re 1 pW *
* Sound Power Reference Level = 1 x 10-12 W = 1 pW = 1 picowatt
LW uses the 10 lg equation so, as a rule of thumb:
3 dB = a factor of 2 in sound power
10 dB = a factor of 10 in sound power
20 dB = a factor of 100 in sound power
30 dB is a factor of 1000
40 dB is a factor of 10000
Sound Power Level IEC Definition : logarithm of the ratio of a given sound power to the reference sound power. Sound power level in decibels is 20 times the logarithm to the base ten of the ratio.
♦ Note 1 unless otherwise specified the reference sound power is 1 pW.
Sound Power Density IEC Definition : is the Sound Energy Flux in a specified direction and sense through an area perpendicular to that direction, divided by the area.