Volume 110, Issue 6, December 2001
Index of content:
- UNDERWATER SOUND 
110(2001); http://dx.doi.org/10.1121/1.1413996View Description Hide Description
The spectra of backscattered energy by dispersed anchovies, which were reported by Holliday (1972), reveal several peaks at frequencies that correspond to theoretically calculated resonance frequencies of year classes of anchovies. Theoretical calculations are based on concurrent measurements of distributions of swim bladder dimensions and a modified form of Minnaert’s (1933) equation. Differences between calculated and measured values of the mean lengths of the second-, third-, and fourth-year classes are within experimental uncertainties (±8%). The calculated mean lengths of juvenile anchovies are in good agreement with historical measurements of the bounds on this parameter (Butler, 1989). Matching of theoretical calculations and measurements of backscattered energy level versus frequency yields estimates of the total Q of the spectral line, and the relative number density per year class. The resultant estimate of of adult anchovies is approximately 4.4. This value of is consistent with laboratory measurements of the Q of individual anchovies, (∼7 at 15 m) and measurements of length distributions of year classes and depth distributions. Resultant estimates of relative number densities of year classes were consistent with historical measurements of the relative number densities of year classes of anchovies in the Southern California Bight.
110(2001); http://dx.doi.org/10.1121/1.1405522View Description Hide Description
A simple relation for the rate at which energy is extinguished from the incident wave of a far field point source by an obstacle of arbitrary size and shape in a stratified medium is derived from wave theory. This relation generalizes the classical extinction theorem, or optical theorem, that was originally derived for plane wave scattering in free space and greatly facilitates extinction calculations by eliminating the need to integrate energy flux about the obstacle. The total extinction is shown to be a linear sum of the extinction of each wave guide mode. Each modal extinction involves a sum over all incident modes that are scattered into the extinguished mode and is expressed in terms of the object’s plane wave scatter function in the forward azimuth and equivalent plane wave amplitudes of the modes. The only assumptions are that multiple scattering between the object and wave guide boundaries is negligible, and the object lies within a constant sound speed layer. Modal extinction cross sections of an object for the extinction of the individual modes of a wave guide are then defined. Calculations for a shallow water wave guide show that, after correcting for absorption loss in the medium, the modal cross section of an object for mode 1 in a typical ocean wave guide is very nearly equal to its free space cross section. This new extinction theorem may be applied to estimate the cross section of an object submerged in a wave guide from a measurement of its forward scattered field.
On the relative role of sea-surface roughness and bubble plumes in shallow-water propagation in the low-kilohertz region110(2001); http://dx.doi.org/10.1121/1.1414883View Description Hide Description
In the low-kilohertz frequency range, acoustic transmission in shallow water deteriorates as wind speed increases. Although the losses can be attributed to two environmental factors, the rough sea surface and the bubbles produced when breaking- or spilling waves are present, the relative role of each is still uncertain. For simplicity, in terms of an average bubble population, the time- and space-varying assemblage of microbubbles is usually assumed to be uniform in range and referred to as “the subsurface bubble layer.” However the bubble population is range- and depth-dependent. In this article, results of an experiment [Weston et al., Philos. Trans. R. Soc. London, Ser. A 265, 507–606 (1969)] involving fixed source and receivers, and observations during an extended period of time under varying weather conditions are re-examined by exercising a numerical model that allows for the dissection of the problem. Calculations are made at 2- and 4-kHz. It is shown that at these frequencies and at wind speeds capable of generating breaking waves the main mechanism responsible for the excess loss in the shallow-water waveguide is the patchy nature of the subsurface bubble field. Refraction and attenuation within the pockets of high void fraction are minor contributors to the losses.
110(2001); http://dx.doi.org/10.1121/1.1414704View Description Hide Description
A perturbation model is developed for sound scattering by a poroelastic seafloor having roughness small compared to the acoustic wavelength. The sediment is assumed to be homogeneous and isotropic with wave propagation described by Biot’s equations. When applied to sandy sediments, the model predicts backscattering levels that are substantially lower than those of a fluid model having the same roughness, density, sound speed, and attenuation.