Digital determination of third-octave and full-octave spectra of acoustical noise

The spectrum of the acoustical noise produced by typical sources such as business machines is usually measured with analog filter banks. The objective of this study was to perform spectral analysis using digital rather than analog methods. The Cooley-Tukey algorithm has been used to compute the Fourier coefficients of noise samples obtained with the aid of a digital recording system described previously [4]. The resulting coefficients have been used to compute octave band and third-octave band spectra of the noise samples. The digital spectral estimates are in good agreement with analog measurements. Emphasis is placed on the filter characteristic or "window" used for the analysis, and the spectral fluctuations which occur in short (65-250 ms) samples of the noise. For one machine studied, the spectral fluctuations are larger than would be expected for samples of random noise. Spectral analysis of impulsive noise produced by a second machine (a manual, key-entry card punch) indicates that spectral fluctuations occur even when the envelope of the pressure-time pattern is observed to be repetitive.