Book - School of Science and Technology

462 Ductwork designNoise criteriaIt is common experience that the human ear can tolerate a noise of low frequency muchmore than one of high frequency, even though of equal intensity. Thus it is necessary toconsider frequency in relation to sound measurement and for this purpose the spectrum offrequencies has been split up into eight octave bands as previously shown in Figure 16.20.Noise criteria curves and noise rating curves (NC and NR) have been established on asubjective basis and each is given a number corresponding to the sound pressure level indecibels in the sixth octave band (1000 Hz). NC curves were much used in the past but nowNR curves are more generally used. For most practical purposes the two sets of curves maybe regarded as interchangeable. Figure 16.23 shows NR plots of equal tolerance for theeight frequency bands. Acceptable NR values may be classified as follows:NR 25 very quietNR 30 normal living spaceNR 35 spaces with some activityNR 40 busy spacesNR 45 light industryNR 50 heavy industryThe Guide Section Al gives recommended NR values for the design of ventilationsystems for various specific applications.A weighting scaleAs pointed out earlier, the ear does not evaluate sounds of different pitch in a linearmanner and hence weighting scales have been adopted in an attempt to account for this.In practical terms this means a filter is used within a sound level meter which gives more1 2 3 4 5 6 7 880Sound pressure level (db RE 20 N/m )2μ70605040302010062.5 125 250 500 1000 2000 4000 8000Mid-frequencies of octave bands Hz5045403530252015Noise rating (NR)Figure 16.23 Noise rating curves