Index of content:
Volume 119, Issue 4, April 2006
- UNDERWATER SOUND 
119(2006); http://dx.doi.org/10.1121/1.2167148View Description Hide Description
Recently, J. D. Achenbach [J. Acoust. Soc. Am.116, 1481–1487 (2004)] put forward, on heuristic grounds, an elegant technique to calculate amplitudes of guided waves in an elastic solid. The technique is based on application of the reciprocity principle. In this article, the technique is applied to acoustic waveguides. A mathematical justification of the technique is obtained. The technique is extended to enable calculation of excitation coefficients of modes of both discrete and continuous spectra by a given sound source. The results are shown to be identical to those derived with the traditional methods that rely on integral transforms.
119(2006); http://dx.doi.org/10.1121/1.2177591View Description Hide Description
Statistical characteristics of low-frequency sound waves propagating over long distances in a fluctuating ocean are important for many practical problems. In this paper, using the theory of multiple scattering, the mean field of a low-frequency sound wave was analytically calculated. In these calculations, the ratio of the sound wavelength and the scale of random inhomogeneities can be arbitrary. Furthermore, the correlation function of inhomogeneities is expressed in terms of a modal spectrum (e.g., internal waves modes). The obtained mean sound field is expressed as a sum of normal modes that attenuate exponentially. It is shown that the extinction coefficients of the modes are linearly related to the spectrum of random inhomogeneities in the ocean. Measurements of the extinction coefficients can therefore be used for retrieving this spectrum. The mean sound field is calculated for both 3D and 2D geometries of sound propagation. The results obtained can be used to study the range of applicability of the 2D propagation model.
119(2006); http://dx.doi.org/10.1121/1.2169913View Description Hide Description
Noise degrades the accuracy of sonar systems. We demonstrate a practical method for increasing the effective signal-to-noise ratio (SNR) by fusing time delay information from a burst of multiple sonar pings. This approach can be useful when there is no relative motion between the sonar and the target during the burst of sonar pinging. Otherwise, the relative motion degrades the fusion and therefore, has to be addressed before fusion can be used. In this paper, we present a robust motion estimation algorithm which uses information from multiple receivers to estimate the relative motion between pings in the burst. We then compensate for motion, and show that the fusion of information from the burst of motion compensated pings improves both the resilience to noise and sonar accuracy, consequently increasing the operating range of the sonar system.