Balanced noise-criterion (NCB) curves

Abstract

The widely used noise-criterion (NC) curves are not based on the American National Standard (four-band) definition for speech-interference level (SIL) and do not extend down to the 31.5- and 16-Hz octave bands. In addition, experience has shown that the procedure originally recommended for using the curves to determine whether a measured noise in a space is acceptable to occupants should be replaced. The premises underlying the new balanced noise-criterion (NCB) curves are: (1) Use the American National Standard SIL as the rating number for each curve; (2) achieve spectral balance by making equal the calculated octave-band loudnesses for those bands with midfrequencies between 16 and 8000 Hz that contain the same number of critical bands—otherwise, weight downward the loudnesses in proportion to the number of critical bands contained; and (3) satisfy earlier studies that the difference between the overall calculated loudness level and the SIL for each curve must not exceed approximately 24 units. The NCB curves are used (1) before construction, to specify acceptable octave-band noise levels, and (2) after construction, to rate the noise as to its effect on speech communication; as to any spectral imbalance (e.g., rumble or hiss), including how many decibels of noise reduction are needed in each octave band to eliminate spectral imbalance; and as to excess low-band levels that might cause feelable or audible structural vibrations. The NCB curves have successfully predicted all cases of sizable complaints by occupants of existing building spaces accessed for this study.