Abstract
An investigation of the properties of a directly carrier-modulated code sequence modulation signal is presented for the study of outdoor propagation. The audible frequency carrier is biphase-modulated using a pseudorandom code sequence specially designed for simultaneous fine-time resolution and large-range ambiguity. An experiment is described in which the correlation properties of the spread spectrum signal are demonstrated, and the amplitude of the transfer functions between the receivers, together with accurate times-of-flight, are calculated from the cross-correlation of the measured acoustic spread spectrum signals. The results illustrate that an acoustic spread spectrum technique can provide significantly improved ways of measuring sound propagation outdoors. In particular, the fine-time resolution for time-of-flight suggests an application in acoustic tomography, for example in the investigation of nocturnal boundary layers, horizontal gradients, and of turbulence structures in the atmospheric surface layer. [Work supported by EPSRC UK.]
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