Book - School of Science and Technology

Noise dispersal 467Table 16.8 Noise attenuation of plain ductworkDuct itemAttenuation(250 Hz)Straight ducts(dB/m run)150 150 mm 0.05600 600 mm 0.31800 1800 mm 0.2600 mm diameter 0.11800 mm diameter 0.03Square bends(dB/bend)150 mm depth 0300 mm 1450 mm 3600 mm 6BranchesAcoustic energy is dividedin ratio of duct areasThe attenuation in ducts depends on length and size: the longer the duct the greater theattenuation; the larger the duct the less the attenuation. The amount of attenuation maybe calculated from the data given in Table 16.8 and the accompanying notes, but for fullertreatment the Guide Section B12 should be consulted.Attenuation of sound energy at bends is a process of reflection back towards the source,rather than the absorption mechanism of straight duct runs. Radius bends, or mitre bendswith turning vanes generally give much less reduction than simple square bends.Where it is clear that residual sound will be too great, ducts may be lined withabsorption material either throughout or for certain lengths, as Figure 16.27. Suchmaterial must, of course, satisfy the requirements for fire protection and not erode whensubject to air flow.An approximate prediction for attenuation by use of an absorption lining to ducts isgiven by the formula:dB/m run ˆ D 1=4A15Absorbent liningAttenuation,dB/m105010 100 1000 10KFrequency, HzFigure 16.27 Acoustic insulation to ductwork