A coded signal technique for the study of environmental noise propagation

Abstract

This paper describes the design of a coded acoustic signal for the study of environmental sound propagation. An increasing awareness of environmental noise problems, and the need to better specify their needs and extent, means that a better knowledge of the effects of weather on sound propagation is necessary. Low signal-to-noise ratio is a common limitation in outdoor propagation investigations. A further problem is time variance. A coded acoustic signal approach consisting of a frequency carrier biphase-modulated by a specially-designed pseudo-random code sequence is proposed to overcome these limitations. This so-called ‘‘inner and outer’’ code sequence is specially designed for environmental sound propagation investigations, combining simultaneous fine time resolution and large range ambiguity. Experiment measurements are used to illustrate how the amplitude of the transfer functions between the receivers together with accurate times of flight may be calculated from the cross-correlation of the measured coded acoustic signal. It is further shown that the mean effect over the total propagation channel of factors such as wind, temperature gradients, turbulence, barrier, and ground reflections can be determined by measurement of the time-varying propagation transfer function using the proposed complex impulse response correlation technique. [Work supported by EPSRC UK.]