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

13.4 Perception of Noise 323inspectors have the discretion to levy fines for violations that are not removed afteran initial visit to the facility.13.4 Perception of NoiseLoudness is not the only characteristic of noise that determines the degree ofannoyance. There are other factors, acoustical as well as nonacoustical, that areimportant. In a classic series of laboratory studies conducted by Kryter and hisassociates (1959, 1963), human subjects rated sounds of equal duration accordingto their noisiness, annoyance, or unacceptability. Through the use of octave bandsof noise Kryter with others established equal noisiness contours, which resemblethose for equal loudness, but less acoustic energy is required at higher frequenciesto produce equal noisiness and more is needed at low frequencies.The unit of noisiness index is the noy N (this term apparently derives from thesecond syllable of the word annoy). Figure 13.1 displays the equal loudness indexcurves. In order to determine the logarithmic measure of the perceived noise level(PNL), the following standardized procedural steps need to be taken:1. Obtain and tabulate the octave band (or 1/3 octave band) sound pressure levelsproduced by the noise.2. Use Figure 13.1 to calculate the noisiness index for each octave (or 1/3 octave)band. Then determine the total noisiness index from either one of the followingtwo expressions:( )∑N t = N max + 0.3 N i − N max (13.2a)iN t = N max + 0.15( ∑iN i − N max)(13.2b)Equation (13.2a) applies to one-octave bands and (13.2b) to one-third octavebands. For each octave (or 1/3 octave) band, N max represents the maximumvalue of N i found within that band.3. The total perceived noisiness index N t , which is summed over all frequencybands, is converted to the perceived noise level PNL (or L PN ) through therelationshipL PN = 40 + (33.22) log N t (13.3)There have been some questions raised regarding the validity of this procedurebecause the listeners in the laboratory trials do not always seem to distinguishthe difference between loudness and noisiness and annoyance. But this procedureis now being used to evaluate single-event aircraft noise. The U.S. FederalAviation Administration (FAA) adopted the PNL method to certify newaircraft.