The reference sound power in air is normally taken to be 10−12 watt = 0 dB SWL.
Notice: Usable music sound (trumpet) and noise sound (excavator) have both the same sound power of 0.3 watts, but will be judged psycho-acousticly totally different.
Acousticpower, rather than electrical power, is the objective of a sound system.
For perspective, the relative acousticpower levels are shown in Table I in watts.
To illustrate how electrical power requirements are determined from the desired level of acousticalpower, assume an area of 2400 square feet, and a hexagonal pattern with minimum overlap spacing which dictates 26 speakers for coverage.