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The Littoral Acoustic Demonstration Center (LADC) is an Office of Naval Research funded consortium of scientists from the University of Southern Mississippi (USM), the University of New Orleans (UNO), the Naval Research Laboratory at Stennis Space Center (NRL-SSC), and the University of Louisiana at Lafayette (ULL). LADC was formed to conduct ambient noise and marine mammal acoustic measurements and analyses. Some of the marine mammal goals are to (1) investigate detection, identification, and tracking using bottom-moored hydrophones; (2) coordinate with scientists making visual observations and other acoustic measurements; and (3) evaluate, and modify if necessary, automatic detection and characterization computer algorithms when applied to marine mammal signals received on bottom-mounted hydrophones. The underlying desire is to provide products that will help to assess the effects of anthropogenic noise on marine mammal behavior and the development of mitigation procedures. Due to recent court rulings, the Navy has become concerned with the possible negative effects of sonar on marine mammals and the development of mitigation procedures. Not only do mitigation procedures have Navy applications, but they also apply to some aspects of commercial or scientific exploration of the oceans. LADC has conducted experiments in the northern Gulf of Mexico and in the Ligurian Sea in cooperation with other organizations. Environmental Acoustic Recording System (EARS) buoys were deployed at sites with historically abundant sightings of marine mammals. EARS buoys are autonomous self-recording, single-channel, bottom-moored acoustic buoys developed by the Naval Oceanographic Office. Analysis approaches have included click production analysis, propagation modeling of sperm whale clicks, and spectral and wavelet transforms of sperm whale clicks., Published in NRL Review 2005, p101-103, 2005.

For many reasons. the three dimensional (3-D) arrival structure at the undersea ambient noise field is at interest to the research and development community. One reason is that the arrival structure can be used to estimate the beam noise at an array, which may be required to estimate the performance at the array as an operational Navy asset or a scientific measurement tool. Another reason is that there are clues inherent in the vertical arrival structure that relate to the nature at the acoustic propagation along the azimuths at the noise sources. Similarly, there are also clues in the horizontal arrival structure of the undersea ambient noise field that relate to the azimuthal distribution at the noise sources. Both at these classes at clues are important in the verification and validation at undersea ambient noise models. The ideal measurement tool to measure the 3-D arrival structure at the noise field is a high resolution volumetric array sonar system. Unfortunately, such a system is not generally available. However towed horizontal line arrays are available and can be used. even though they are far from ideal. The beam patterns at a line array are conically symmetric about the axis at the array. When the axis of a horizontal line array is tilted slightly from an elevation angle at 0 deg the vertical character at the beam cones can be used to discriminate between various vertical arrival angles. Combined with measurements made on several different towed array headings, an estimate at the 3-D directionality at the noise field can be obtained. An algorithm that uses such a technique on single line towed array data to generate an estimate at the 3-D directionality at the noise field is described herein. Same results using measured data from a recent noise measurement exercise are presented and discussed.

A technique to estimate the three dimensional (3-D) character of the undersea ambient noise filed with the use of a horizontal line array as the measurement tool has been developed. Horizontal line array beam noise data, on several array headings, and knowledge of the array's 3-D beam response patterns were used as an input into the 3-D noise filed estimation technique. The final product indicates that the resulting 3-D field estimate can be used to determine both the horizontal and vertical directionality of the noise field. Furthermore, having an estimate of the 3-D noise field facilitates other investigations of the noise field that are normally not possible with either a vertical or a horizontal line array, such as the vertical directionality of the noise in a small azimuthal sector or the horizontal directionality at a particular elevation angle. It also appears that the use of horizontal line arrays and this 3-D technique could provide necessary measurement inputs to support design of volumetric array systems without the expense and/or technical difficulty of deploying a volumetric array.

The ambient noise field is a complex and highly variable phenomena with many components, including contributions due to shipping. Spatial characteristics (e.g., the horizontal directionality) of the ambient noise field have been measured with towed line arrays in many of the major water masses of the world. This paper addresses the effects of bathymetry on the propagation and measurement of ambient noise can be fully understood only when the bathymetry along the propagation paths has been considered. Examples of measured data are presented illustrating the effects of bathymetric stripping and bathymetric shielding on the spatial character of the ambient noise. Other examples of measured data will be shown demonstrating that bathymetric influences are not limited to a narrow frequency range. In addition, modelled data is used to illustrate the effects of the surrounding bathymetry on noise and signal propagation.

Data bases are important in underwater acoustics. They are the means whereby past experience and measured or calculated data can be utilized to predict or improve the performance of existing and planned sonar systems. The success by which this can be done depends, to a large degree, on the format of the data base and the nature of the parameters stored therein. Selecting the parameters or statistics to accumulate and determining the best format for storage is not a simple straightforward task. Examples are given that demonstrate the utility of data bases for which parameters and formats have been chosen to optimize the storage, retrieval, and utilization of the information stored. Utilization by and application to the operational Fleet as well as the R&D community are discussed.

Several algorithms and techniques have been developed to estimate the horizontal directionality of ambient noise from horizontal line arrays. In most cases, the two-dimensional (2-D) representation of the array beam pattern is adequate to estimate the ambient noise horizontal directionality. However, when significant amounts of the noise are distributed out of the horizontal plan, a 2-D representation of the noise field can be misleading. An algorithm is described herein that utilizes the three-dimensional conical nature of the beam response patterns of the towed array and an iterative technique to estimate the 3-D ambient noise directionality. The algorithm is described and results form actual at-sea measurements are presented. This work was supported by funding from the Antisubmarine Warfare Environmental Acoustic Support (AEAS) Program of the Office of Naval Research.