Index of content:
Volume 124, Issue 4, October 2008
- UNDERWATER SOUND 
124(2008); http://dx.doi.org/10.1121/1.2967485View Description Hide Description
In April 1989 an acoustic experiment was carried out over the abyssal plain south of Madeira in which transmissions were made, for about an hour, at 482, 680, and from a ship steaming at to a receiving array towed by another ship away traveling on a parallel course at the same speed. The signals arrived by two paths, an upper path trapped in the surface duct and a lower path via the main sound channel. This paper describes the experiment and analyses of the intensity fluctuations in the signal received by the lower path. This allows the authors to investigate the horizontal structure of acoustic intensity fluctuations in the ocean when these are due principally to internal waves. This aspect of such acoustic intensity fluctuations has received little attention until now. The experimental results are compared with theoretical predictions based on the parabolic moment equations for propagation and scattering in randomly irregular media, and on the standard Garrett–Munk model for oceaninternal waves. The experimental results and theoretical predictions agree quite well but the comparison also raises some new questions, in particular, about the correlation of intensity fluctuations as the acoustic transmission frequency is varied.
124(2008); http://dx.doi.org/10.1121/1.2967840View Description Hide Description
Thin rubber coatings with cavities in a doubly periodic lattice are able to reduce reflections of underwater sound by redistributing normally incident energy such that absorption in the surrounding rubber is enhanced. For spherical scatterers, the anechoic effect can be studied numerically by the layer-multiple-scattering (LMS) method. In comparison to more flexible but also more computer intensive methods, such as finite-element method modeling, there are two important advantages. An improved physical understanding of the anechoic effect can be achieved by simplified semianalytical analysis, and the high computational speed allows modern global optimization techniques to be applied for coating design. In this paper, the flexibility of the LMS method is improved by combination with an efficient algorithm for numerical computation of transition matrices for superellipsoidal scatterers. (A superellipsoid is a generalization of an ellipsoid, allowing more box-filling shapes, for example.) Extensions to mixtures of nonspherical scatterers of different types are also considered, in order to enhance the broadband performance. Symmetry properties are used to reduce the size of the pertinent equation systems. Examples of numerical coating design for underwater acoustic applications are presented, using differential evolution algorithms for the optimization.
A comparison of ambient ocean noise near San Clemente Island: A bathymetrically complex coastal region off Southern California124(2008); http://dx.doi.org/10.1121/1.2967889View Description Hide Description
Repeated ocean ambient noise measurements at a shallow water site near San Clemente Island reveal little increase in noise levels in the absence of local ships. Navy reports document ambient noise levels at this site in 1958–1959 and 1963–1964 and a seafloor recorder documents noise during 2005–2006. When noise from local ships was excluded from the 2005–2006 recordings, median sound levels were essentially the same as were observed in 1958 and 1963. Local shipnoise, however, was present in 31% of the recordings in 1963 but was present in 89% of the recordings in 2005–2006. Median levels including local ships are higher than median levels chosen from times when local shipnoise was absent. Biological sounds and the sound of wind driven waves controlled ambient noise levels in the absence of local ships. The median noise levels at this site are low for an open water site due to the poor acoustic propagation and low average wind speeds. The quiet nature of this site in the absence of local ships allows correlation of wind speed to wave noise across the spectral band of this study.