Sound power is the total energy that an audio source broadcasts audio. Sound power can be measured in a few different ways and with different accuracy. Highest accuracy is obtained in special measuring rooms, which can be reverberation, half or completely non-free space. You then have control over the effect of the room on the sound field that the source sends out. The sound power is then the same no matter where you place its source, which makes it a good measure to study noise levels in the local / space source or count to an adjacent local area. The sound power level is given with the device dB re 1 pW.
Laboratory measurement of the sound power level of a sound source is simplified so that you have a reverberation sound where you measure the reverberation time with the source placed in the room, switched off. Then you measure its source of sound in operation for one or more operating instances and by means of the reverberation time you can then calculate the source sound effect.
There are many measurement standards for sound effect and what is often used depends on the product. The acoustics workshop has access to a reverberation room where sound effect can be obtained with great accuracy. One of the standards we have recently used is SS-EN ISO 3741: 2010.