Acoustic performance of silencers (at room temperature: ventilation, air conditioning, various industrial processes or at high temperature: various industrial processes, stacks, exhausts ...) is very frequency dependent and is primarily related on the one hand to the performance of the absorbing filling given the spacing of the airways and the length of the silencer (characterized by the propagation loss) and also where appropriate (for silencers other than with simple dissipation) to the geometry of the splitters (characterized by the reflection loss) and also to bypass phenomena: acoustic energy transmission through the casing of the silencer as well as, if appropriate (for silencers other than with simple dissipation) through splitters themselves, and finally to noise regenerated in relation to the speed of passage of the fluid.
This performance can be expressed in terms of difference (with and without silencer) of overall A weighted sound pressure levels or of sound pressure levels per octave bands at specified locations (such as maximum value at 1 m from the outlet plane of the silencer, average value of an enveloping surface) - also known as insertion difference of sound pressure level - or in terms of difference of overall A-weighted sound power levels or of sound power levels per octave bands of the outlet of the silencer (or of the mouth, or of the silenced noise source) - also known as insertion loss of the silencer -.
Dissipative silencers (or: dissipation silencers) are devices allowing an attenuation of sounds on a wide frequency range.
For orders of magnitude (and with respect to a noise spectrum like "pink noise"), a level difference of up to 10 dBA can usually be obtained without special requirements, while a level difference from 10 to 20 dBA requires a standard silencer without significant by-pass, while a level difference of 20 to 30 dBA requires a standard silencer with transverse partitionning devices of the absorbing filling and a resilient mounting, and while that a level difference of 30 to 40 dB involves high performance silencers carefully designed and installed (a higher level difference shall involve a special achievement or 2 silencers installed in series with sufficient spacing).
The prediction of acoustic performance of dissipative silencers can be performed with the simulation software SILDIS®®.
Measurements on silencers can be made (as applicable) according to NF EN SO 7235 Acoustics - Laboratory measurement procedures for ducted silencers and air-terminal units - Insertion loss, flow noise and total pressure loss or according to standard NF EN ISO 11820 Acoustics - Measurements on silencers in situ