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

13.6 Indoor Noise Criteria 325account the factors of duration and the presence of pure tones, the effective noiseperception level (EPNL) has been defined asL EPN = L PN + C + D. (13.4)Here C is the correction factor for pure tones and D is the duration correction. Thetone correction varies from 0 dB up to a maximum of 6.7 dB. The estimation of Centails a complex procedure (Edge and Cawthorn, 1977) that involves examinationof the band spectra to detect any band whose level exceeds the level of the bands toeither side. The duration correction D, expressed in decibels, which accounts forduration of the noise, may be either positive or negative but it is usually negative.It is calculated from( d/0.5)∑D = 10 log 10 antilog L PNT(k)− 13 − L PNTmax (13.5)10k=0Here d represents the total length of the time elapsed when the noise begins toexceed the background level to the moment when it falls back to the level ofimperceptibility. The number 0.5 represents the increment index, i.e., if the totalduration d of the detectable sound is 5 s, then ten intervals are being consideredin the summation of Equation (13.5). L PNT is the tone-corrected value of L PN (i.e.L PNT = L PN + C).13.6 Indoor Noise CriteriaIn order to render communication possible in both indoor and outdoor areas at work,it is necessary to minimize the speech interference arising from the background.The A-weighted sound level can be utilized to determine the acceptability of indoornoise, but it cannot give an indication as to which part of the frequency spectrumis responsible for interference. A number of noise evaluation curves are availablefor rating the acceptability of noise in indoor situations. The most frequentlyused families of curves are the noise criterion (NC) curves, noise rating (NR)curves, room criterion (RC) curves, and balanced noise criterion (NCB) curves.These curves were developed in order to provide criteria to either determine theacceptable noise levels in buildings or to specify the acceptable noise in buildings.Noise Criterion CurvesThe NC curves of Figure 13.2 were the result of an exhaustive series of interviewswith people in offices, industrial spaces, and public areas. It was found that theprinciple concern is the interference of noise with speech communication andlistening to radio, television, and music. In order to find the NC rating of a particulararea, the octave-band sound pressure levels of the noise are measured and plottedon the family of the NC curves of Figure 13.2. The highest curve penetrated byany octave band and pressure level of the measured noise defines the NC value forthe spectrum.