THE SCIENCE AND APPLICATIONS OF ACOUSTICS - H. H. Arnold ...

13.6 Indoor Noise Criteria 327Room Criterion CurvesFigure 13.3. Noise rating (NR) curves.The difficulty with NC curves is that they are not defined in the low frequencyrange—i.e, the 160 and 31.5 Hz octave bands—and complaints have been registeredthat they allow too much noise above the 2 kHz range. Accordingly, Blazierdeveloped a set of room criterion curves on the basis of an extensive study conductedfor ASHRAE of generally acceptable background spectra in 68 unoccupiedoffices. Most of the A-weighted sound pressure levels lie in the range of 40–50 dB.Blazier determined that the curve he derived from the measure data had a slope ofapproximately –5 dB/octave, and he developed a family of straight lines with thisslope (cf. Figure 13.4). It was also established that intense low-frequency noise75 dB or more in region A of Figure 13.5 is apt to cause mechanical vibrationsand rattles in lightweight structures. Noise in region B has a low probability togenerate such vibration. The RC value of a measured spectrum is defined as thearithmetic average of the sound levels at 500, 1000, and 2000 Hz. These curves arebased on measurements made with air conditioning noise only, so they are mainlyused in rating the noise of HVAC systems.Balanced Criterion CurveBeranek (1989a,b) modified the NC curves by adding the 16- and 31.5-Hz octavebands and modifying the slope of the curves so that it became –3.33 dB/octavebetween 500 and 8000 Hz. He also incorporated regions A and B of the RC curves