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100 1000 10000 Frequency, Hz

FIGURE 10 Noise reduction for piping with fiberglass insulation (1 in = 2.54 cm)

100 1000 10000 Frequency, Hz

FIGURE 10 Noise reduction for piping with fiberglass insulation (1 in = 2.54 cm)

FIGURE 11 Design of an effective sound-isolating enclosure requiring air circulation for cooling (Reference 8)

with, for example, overheating, formation of explosive mixtures, or difficulty in getting to the pump for maintenance.

Although partial barriers around a pump usually cannot provide the degree of noise reduction afforded by total enclosures, they may be quite useful. Such barriers normally redirect the noise away from the listener and work best when they are located very near either the source or the listener. They also work better in an open or acoustically dead room than in a highly reverberant one. Their performance can often be enhanced by treating the interior (pump-side) surface with absorbing material or by reflecting the noise into an absorbing surface. The transmission loss of partial barriers is usually not critical as long as it is 20 dB or greater, as reverberation and flanking noise paths will normally limit overall reduction to less than 20 dB.

Exterior control techniques for pipe noise usually consist of pipe wrapping materials in addition to the resilient pipe supports and hangers already mentioned. The function of such materials is identical to that of total enclosures; hence, pipe wraps should be

1. Massive (high area density)

2. Mechanically decoupled from the pipe (loosely sprung)

3. Airtight

For all the very-low-frequency noise, decoupling between a heavy, airtight outer coating and the vibrating pipe wall can be achieved by a compliant acoustic absorbing material, such as preformed fiberglass pipe insulation. This material provides some acoustic absorption of noise as it "passes through," but, more important, it provides vibratory decoupling between the vibrating pipe wall and the external pipe wrap shell or layer.

Multiple-layered panels or pipe wrap normally provide more attenuation than single homogenous layers, as long as the layers are mechanically decoupled. Examples of several pipe wrap configurations are given in Figure 12. Alternative lead-free materials are replacing leaded vinyl.

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