Damping designed products may be required. As an example, large flat areas may require damping so that they do not act like loudspeakers. The damping action can quiet a cabinet that resonates. Various plastics have helped alleviate problems in all types of noisemakers. Different damping approaches can be used, such as applying plastic foam sound insulator or plastic panels that have low damping characteristics.

A popular approach is to use plastic grommets where applicable. Sound-absorbing grommets are used on equipment such as motors' bolt attachments and trash compactors. Testing and all other types of

Figure 4 Grommet replaces a five individual metal assembly (Courtesy of Mobay/Bayer)

Metal linkage assembly Plastic grommet equipment can take advantage of grommets or be redesigned to use plastic. Grommets provide their greatest noise reduction through damping in the octave frequency bands above 500 Hz where the ear is most sensitive and sound most annoying.

Grommets have replaced assembled linkages (Fig. 4.15). In addition to reducing noise, the usual injection molded polyurethane (PUR) grommet eliminated the time consuming/costly metal assembly. During assembly it is snapped into a hole in the steel lever, then a grooved rod is inserted into the grommet. The grommet isolates vibration from the metal parts and eliminates the hardening and cracking that used to shorten the life of the old assembly. The plastic design mechanically is at least as strong as the metal assembly that includes withstanding high load pull-outs and can withstand high cyclic loads applied at different degrees off the rod axis at temperatures up to 300°F (149°C).

To quiet a noise-generating mechanism, the first impulse is often to enclose it. Enclosure can be the best solution, but not always. By determining what is causing the noise, appropriate action can be taken to be more specific and provide a cost-effective fix. A plastic enclosure can be used to suppress noise. Recognize that with a metal enclosure a small noise is transmitted to the metal structure that serves to amplify the sound.

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