Can Explosion Sources Give an Apparent Transmission Gain

Abstract

The total pressure wave generated by an underwater explosion gradually changes in shape as it propagates out to long ranges. As a result of different attenuation rates in various segments of the high-amplitude explosion pressure wave, the waveform begins to approach a damped sinusoid at some distance from the charge. The effect of varying waveforms on spectral energy distributions has been studied theoretically, with the explosion pressure-time pulse modeled by the function p(t) = C exp(−αt) cosβt. Values of C, α, and β were derived from empirical functions of explosive charge weight and depth, and range. This analysis shows that the changing waveform may serve to sustain spectrum levels for frequencies in the neighborhood of the explosion's bubble pulse fundamental frequency. Consequently, when the theoretical values for a given charge configuration are combined in the customary sonar equation format, the expected transmission loss can become an apparent transmission gain for selected frequencies. These theoretical results raise the interesting question of whether some of the variability in reported attenuation coefficients might be attributable to the particular explosion sources used in the measurements. [This work sponsored by the Office of Naval Research.]