When airborne sound cannot be controlled at the source, silencers provide an important means of reduction
of sound on the propagation path
Acoustic silencers are classified according to their types, performance characteristics, and applications. In general, the sound pressure level (A-weighted, in 1/3 octave bands, or in whole octaves) should not exceed a specified value at a specified position (for example at the workplace, in the neighborhood or in break rooms) . The allowable contribution for a sound source can then be determined in terms of the sound power level and directivity index of this source using the laws of sound propagation and the requirements relating to the location of contributions to various partial sound sources. The required insertion loss of the silencer is given by the difference between the allowed and actual sound power levels of the source.