Volume 120, Issue 4, October 2006
Index of content:
- UNDERWATER SOUND 
120(2006); http://dx.doi.org/10.1121/1.2338811View Description Hide Description
Measurements of long-range (order ) shallow-water reverberation in the Straits of Sicily at 900 and are compared with theoretical predictions. All of the required environmental inputs for the theory are obtained independently, that is to say there are no free parameters. The reflection coefficient and the scattering strength are measured by direct path methods; both quantities show strong frequency dependence. The theoretical reverberation predictions using these measurements are in good agreement with directional reverberation data, i.e., within the expected uncertainty bounds. The good agreement suggests that the supporting environmental measurement techniques are robust and that the physics associated with reverberation in a waveguide is reasonably well understood, at least in simple environments. The ability to independently measure the seabed scattering strength and reflection coefficient is a crucial step for the advancement of inverse methods using reverberation (e.g., rapid environmental assessment) inasmuch as it provides the means for quantitatively measuring the robustness of those methods.
Validation of statistical estimation of transmission loss in the presence of geoacoustic inversion uncertainty120(2006); http://dx.doi.org/10.1121/1.2261356View Description Hide Description
Often the ocean acoustic environment is not well known and sonar performance prediction will be affected by this uncertainty. Here, a method for estimating transmission loss (TL) is proposed which incorporates these environmental uncertainties. Specifically, we derive an approach for the statistical estimation of TL based on the posterior probability density of environmental parameters obtained from the geoacoustic inversion process. First, a Markov chain Monte Carlo procedure is employed in the inversion process to sample the posterior probability density of the geoacoustic parameters. Then, these sampled parameters are mapped to the transmission loss domain where a full multidimensional probability distribution of TL as a function of range and depth is obtained. In addition, TL is also characterized by its summary statistics including the median, percentiles, and correlation coefficients. The approach is illustrated using a data set obtained from the ASIAEX 2001 East China Sea experiment. Based on the geoacoustic inversion results, the predicted TL and its variability are estimated and then compared with the measured TL. In general, there is a good agreement with the percentage of observed number of data points inside the credibility interval.
120(2006); http://dx.doi.org/10.1121/1.2338802View Description Hide Description
This paper describes the characteristics of an underwater biological chorus recorded in the midfrequency band in the Southern California Bight. The recordings were made in July, 2002 by a large-vertical-aperture, 131-element, 2D billboard array. The chorus, observed predominantly on two consecutive nights during the experiment, is composed of two bands of energy centered around and between 4 and . It causes a complete reversal in the vertical directional characteristics of the mid-frequency ambient sound field between day and nighttime periods; whereas the vertical structure during the day shows a notch in the horizontal direction with levels more than below those at higher angles, the nighttime levels in the horizontal can exceed those at other vertical angles by more than . These nighttime sounds also are strongly anisotropic in azimuth; they appear to come mainly from a popular Southern California fishing spot where the water depths exceed . Vertical beam-to-beam coherence squared estimates suggest the chorus source region exists on spatial scales greater than the multipath interference wavelengths of this environment. Individual sounds comprising the chorus, although difficult to separate from the background din, have a fluttering, rasping character.
Acoustic detection of North Atlantic right whale contact calls using the generalized likelihood ratio test120(2006); http://dx.doi.org/10.1121/1.2257385View Description Hide Description
This paper addresses the problem of passive acoustic detection of contact calls produced by the highly endangered North Atlantic right whale Eubalaena glacialis. The proposed solution is based on using a generalized likelihood ratio test detector of polynomial-phase signals with unknown amplitude and polynomial coefficients observed in the presence of locally stationary Gaussian noise. The closed form representation for a minimal sufficient statistic is derived and a realizable detection scheme is developed. The receiver operation characteristic curves are calculated using empirical data recordings containing known right whale calls. The curves demonstrate that the proposed detector provides superior detection performance as compared with other known detection techniques for northern right whale contact calls.