A mill that ground products into a powder and dropped them under gravity into an orbital sifter was producing high levels of noise at operator positions nearby. The powder was abrasive and would destroy a conventional silencer approach, not to mention clogging it with dust.
A nylon "sock" similar to shower curtain material was used to direct the powder to the sifters, which was flexible to allow the vigorous sifter movement and which was tough but also disposable.
This "sock" was also acoustically transparent which meant that it let noise bleed out almost unattenuated but, by the same token, also did not direct noise towards the sifter inlet.
This allowed over-sized silencers to be fitted around it, which allowed for movement in the sifter and also made no contact with the "sock", thus avoiding complex acoustic interaction effects.
The sock thus protected the silencer while allowing noise to bleed out into the silencer's absorptive surfaces.
The noise levels were reduced by 8dB to 82dB.
The total cost was circa £750 (2005 rates).
Information supplied by
ISP Alginates (UK) Ltd. & PDA Ltd.