Book - School of Science and Technology

Sound control 463emphasis to those frequencies which are readily heard by the human ear and less emphasisto those low frequencies and very high frequencies which are less readily heard.The A weighting scale has now become generally accepted for all practical purposessince it appears to give a good measure of human response to the loudness of noise, butother B and C weighting scales also exist although these are now seldom used.Sound analyser/meterSound level analysers and meters are constantly evolving with the latest advances inmicroprocessor technology the most recent meters are equiped with parallel filters thatmeasure the sound pressure across a whole range of frequencies simultaneously as well asthe more common single figure A weighted value. Modern meters are fully automated andalso allow measurement of statistical indices over preset measurement periods.Noise of fansThe noise produced by fans is often quoted in manufacturers' literature in decibels.Sometimes this value will be a single figure value or a range of values across a frequencyrange. The values may be sound power levels or alternatively sound pressure levelsmeasured under certain conditions at a certain distance from the fan. This informationshould refer to a measurement standard by which the data was obtained. Given the soundpower level of the fan at each frequency, and ignoring any difference between suction anddischarge, it is possible to determine the noise from the system knowing the attenuationprovided by ducts, changes of direction, grilles and the like. If the result shows that toohigh a sound level from the fan remains, it is necessary to consider means for reducing thenoise of the fan. This is discussed later.Generalised formulae have been published which enable the likely sound power level ofa fan to be predicted in the absence of manufacturers' data. One such is as follows:35Correction factor3025201510Centrifugal–forward curvedCentrifugal–backward curvedAxial flow51 2 3 4 5 6 7 8Octave bandsFigure 16.24 Correction factors for sound power of fans