Rotary blowers often produce high levels of sound at discrete frequencies, related to the rotational speed of the unit. If these discrete frequencies are at the low end of the spectrum they may be difficult to attenuate.
Traditional absorptive silencers work well at middle and high frequencies but their performance drops off below 250 Hz. Reactive silencers can be used but these can impart significant back pressure on the process and so waste energy. In addition, reactive silencers cannot follow changes in the frequency of the noise and may also contain chambers in which unwanted material can accumulate, which could affect the performance of the silencer.
Active and absorptive attenuation
The introduction of a hybrid active silencer did not impede the air flow and had absorptive packing for the high frequencies and active attenuation for the lower frequencies. The aim of using both methods together is to reduce noise levels from the whole spectrum of frequencies.
From £3000. (1995)
Discharge noise reduced by 42 dB. The attenuation - active and absorptive - is shown in the graph
Equipment manufactured by Digisonix Inc, USA.