Book - School of Science and Technology

464 Ductwork designL w ˆ 10 ‡ 10 log 10 Q ‡ 20 log 10 PwhereQ ˆ volume flow (litre/s)P ˆ static pressure (resistance) (Pa)L w ˆ overall sound power level (dB re 10 12 W)To establish the individual sound power levels in the various frequency bands, correctionfactors must be subtracted as read from Figure 16.24.Noise in roomsThe measurement of noise in rooms should be in conformity with a standard such as thatset down by the Association of Noise Consultants in `noise from Building Services Part 1Measurements in Buildings'. Readings are commonly taken:. at a normally occupied position within a room. not less than 1 m distance from any room surface, grille or other ventilation device.There are two components to the sound pressure at a point in a room; the direct soundpropagating to the receiving point from the source and sound reaching the receiving pointafter being reflected from the surrounding room surfaces. This is usually termed thereverberant component. The magnitude of the sound pressure level is the decibel sum ofthe two components, which can be calculated knowing the sound power emitted from thesource(s), the location and radiation characteristics of the source(s) and the acousticproperties of the room. When air enters the room from, say, a grille, it expands into freespace. The difference in the volume of the duct and the room results in an impedancemismatch as seen by the sound wave and this accounts for some low frequency attenuation.Consequently, a large number of small outlets will transmit less low frequency noiseinto a space than a single large grille.There is the question of background noise which is never absent, albeit at very lowlevels even in a sound-proof room. In order to be sure of measuring a noise correctly, it isnecessary to measure the dB of the background, and then of the background plus thenoise in question. By determining the absolute pressure level in Pa of each source of noiseusing antilogarithms and the logarithm of the difference, the dB level of the noise can befound (see Figure 16.25). If there is a difference of more than 10 dB the background issufficiently quieter than the source under consideration that it does not numerically addto it and can therefore be ignored.The ear must, of course, always remain the final arbiter when making a judgement about anoise level, as instrumental measurement, however careful, may be misleading. There mayfor instance be a single monotonous tone which has only a slight effect on the absolute noiselevel in terms of dB, or there may be an intermittent noise which goes quite unrecorded butboth influence the subjective impression about the acceptability of the noise.Noise dispersalThe less the noise produced, the easier is the problem of mitigating it. Nevertheless, allplant is liable to cause noise to a greater or lesser degree.