Book - School of Science and Technology

468 Ductwork designwhereD ˆ perimeter of lined duct (m)A ˆ free area of duct (m 2 ) ˆ absorption coefficient of lining material (Table 16.9)This expression holds for lined lengths up to about 2 m only and has been found tooverestimate performance at high frequencies.Alternatively, or in addition, specially designed attenuators or silencers may be usedinserted in the run of ducting. These are rated according to dB loss, but the specification ofperformance should be based on `insertion loss' not on a test in free space. Attenuatorsmay be built up by splitters, as in Figure 16.28, and their design may be considered as ifeach section were a small duct lined with absorption material. As attenuation is inverselyproportional to the cross-sectional area of the duct, it follows that the smaller the subdivisionsof the absorber the shorter the length required. On the other hand, any absorptiondevice increases resistance to air flow and hence a reasonable balance must be kept.Where attenuators are fitted in plant rooms, it is particularly important to ensure thatthey are so sited that break in of noise is minimised. It is of little use to fit an expensiveattenuator to a piece of equipment and then allow a bare sheet metal duct connection toTable 16.9 Absorption coefficients of some duct lining materialsAbsorption coefficient at125 Hz 500 Hz 2000 Hz 4000 HzFibre glass resin bonded mat25 mm thick 0.1 0.55 0.8 0.8550 mm thick 0.2 0.7 0.75 0.825 mm thick 0.05 0.45 0.8 0.75Polyurethane foam50 mm thick 0.2 0.65 0.7 0.7Mineral fibre ceiling tiles 0.1 0.6 0.8 0.8RectangularCylindrical (POD)Figure 16.28 In-duct attenuators