THE SCIENCE AND APPLICATIONS OF ACOUSTICS - H. H. Arnold ...

398 14. Machinery Noise ControlFigure 14.18. Lagging or jacketing of a pipe to minimize flow noise transmission.occur in the vicinity of 6.6 kHz. Higher order resonance-like conditions will alsooccur at 2 f r ,3f r , etc., but would be of little concern because those frequenciesare extremely high.Besides increasing wall thicknesses, another method for achieving noise attenuationin pipes is that of wrapping or lagging the pipes. A typical lagging setup isshown in Figure 14.18. A 2.5–8 cm layer of acoustically absorbent material (fiberglass,mineral wools, or polyurethane foam) is wrapped around the pipe wall. Thisabsorbing layer, in turn, is sheathed in sheet metal, dense vinyl, or sheet lead. Theouter dense layer is extremely important in providing a high level of noise reduction.Applying an even denser outer layer and adding additional composite layerscan result in even more noise reduction. The principal disadvantage of using laggingas the only noise reduction measure is that long lengths of piping would needtreatment, so it would be advisable to give priority to noise source reduction (i.e.,utilization of multiple pressure reduction stages or diffusers) and to use lagging asa secondary measure to bring the overall sound levels down to acceptable values.14.14 Mufflers and SilencersIn the preceding section we have discussed some of the elements of silencingair-jet flow. In this section we examine further aspects of reducing the noise fromair-jet flow. The term muffler is commonly applied to the exhaust gas silencer foran internal combustion engine, and silencer usually denotes the noise suppressorinstalled in a duct or air intake. We use these terms interchangeably in this sectionsince the same operational principle applies to both of them. Mufflers and silencerswere developed to reduce noise energy while facilitating gas flow.Both silencers and mufflers fall into one or the other of two categories: dissipativeunits and reactive units. Dissipative mufflers and silencers reduce noise energy byemploying sound-absorption materials that are flow resistive at frequencies inthe audio range. Reactive mufflers and silencers reduce noise through destructive