Application of digital cross-spectral analysis to outdoor sound propagation

Digital spectral and associated correlation analysis techniques have been used extensively for time series arising from various physical phenomena. In the field of acoustics, these techniques have found a wide range of application in underwater sound, measurement of acoustical properties of materials, system response functions, noise source location and diagnostics, etc. In this paper, the digital cross-spectral technique is used to study the propagation losses of sound from 22 to 11 314 Hz from a sorce in a given outdoor environment under given meteorological conditions. This technique is particularly applicable when the sound is severely masked by the ambient background noise. The problems encountered in data collection, data processing and digitization, and conversion of power spectral densities to octave-band decibel values are discussed. Results are shown as auto-spectral, cross-spectral, coherence-function, and phase-angle plots. Comparisons of the “environment response function” evaluated using this experimental procedure and predicted attenuations from a simple semianalytical model in octave-bands are made to point out the conservatism of that model. This experimental procedure can be applied as a site-calibration procedure under representative weather conditions for community noise impact evaluation taking into account the complex effects of terrain and atmosphere.