Page 49 - Occupational Health & Safety, November/December 2019

or more intermediate limp, air-impervious barrier layers, and a heavy air-impervious outer jacket.
The most common jacket material con- sists of a protective aluminum or stainless- steel cladding, with limp mass-loaded vinyl laminated to the back side, facing the insu- lation, and is available from noise control suppliers. For complex shaped equipment, there are mastic products available for the outer jacket, which can be trowel applied over the insulation layer, and which then cure in place. The fibrous or porous layers provide acoustical absorption, but also act as a resilient support for the outer sound- containing jacket in order to partially de- couple it from the vibrating surface of the pipe or vessel.
It is important to note that the maxi- mum noise reduction possible from acous- tical lagging is less than that of a fully de- coupled acoustical enclosure because of the contact between the surface of the noise source and the outer jacket. Addition- ally, while acoustical lagging is effective for high frequency noise, in the 500 to 5000 Hz range, it tends to amplify sound at low frequencies and is therefore unsuitable for sources that produce appreciable low-fre- quency noise.
Silencers and Acoustical Louvres. A si- lencer is a common and effective noise con- trol measure which allows flow-through of air or other gases but which removes some portion of the acoustic energy from the gas flow which passes through it. Silencers can be fitted on the intake or outlet of a fan, or in a ventilation opening in an enclosure.
All silencers will constrict the airflow to some degree, resulting in additional backpressure to the flow path. Typically, the additional backpressure increases with increasing acoustical attenuation, so the acoustical performance must be balanced against the allowable incremental back- pressure that the system can accommodate. The additional backpressure can gener- ally be minimized by using silencers with greater cross-sectional dimensions and/or length, if the available space allows.
Concluding Summary
The best noise control solution always de- pends on the specifics of the noisy equip- ment, the setting in which it is located, and the way that workers need to interact with the production processes. In most
situations, a combination of the methods discussed above is necessary. The key to re- solving an existing noise issue—or prevent- ing one at the design stage—is to identify the dominant noise sources and transmis- sion paths, rank them in terms of signifi- cance in contributing to the overall sound levels in the work environment, and select the best combination of engineered noise control solutions based on an understand- ing of the fundamentals of acoustics out- lined above.
Rob Stevens, MASc, PEng, is a principal
acoustical engineer at HGC Engineering, one of North America’s largest engineering consulting firm specializing exclusively in noise, vibration and acoustics. Rob has thir- ty years’ experience in measuring, assessing, and mitigating environmental noise impact and workplace noise exposure in all types of industry throughout the US, Canada, and abroad. He has conducted acoustical assessments and noise/vibration studies for hundreds of industrial sites, developing noise control recommendations to meet local regu- latory requirements and internal corporate noise standards.
www.ohsonline.com
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