Effect of pulse length on low frequency average reverberation intensity in shallow water waveguide

Abstract

The average reverberation intensity (abbreviated as ARI in this paper) is affected by many factors. In general, an active sonar system transmits short pulse signals, and the ARI in shallow water is proportional to the pulse length. If the ARI of a long pulse signal can be treated as the superposition of that of several short pulse signals, then the average intensity and the duration of reverberation of the long pulse signal should increase linearly with the pulse length. To verify this assumption, an experiment was conducted in a shallow-water waveguide with a flat bottom and sandy sediment. Signals were transmitted by a shark's-lip type transducer and received by two vertical linear arrays located respectively at the distance of 1km and 10km away from the source. The experimental result shows that the average intensity and duration of low frequency reverberation both increase with pulse length, though not linearly. The reason is that the sound absorption effect and scattering attenuation effect can be ignored for the ARI of short pulses, but cannot be ignored for long pulses. In this paper, the effect of signal pulse length on low frequency ARI in shallow water was analyzed and discussed according to the data. Signals with different pulse lengths lead to different ARIs through changing the size of scattering area. In order to illustrate the performance in the experiment, a comparison and thorough analysis of the ARI for different pulse lengths, ambient noise levels and propagation ranges has been made. The conclusions of this paper are: a) the ARI in shallow water is affected cumulatively by range, pulse length and background ambient noise; b) the ARI at different ranges in the scattering area will be weighted by two-way sound propagation loss; c) the weighting parameters will be changed when the reverberation-ambient noise ratio is low.