Bruce K. Newhall
Johns Hopkins Univ., Appl. Phys. Lab., Laurel, MD 20723
Walter S. Allensworth
Appl. Hydro-Acoustics Res., Inc., Rockville, MD 20850
Iman W. Schurman
Johns Hopkins Univ., Laurel, MD 20723
Single horizontal line arrays are often utilized to resolve the azimuthal character of the low-frequency noise dominated by long-range shipping sources. However, such arrays are subject to an azimuthal ambiguity on conjugate bearings. Coherent beamforming of an array composed of two parallel horizontal lines was recently employed to eliminate this left/right ambiguity using a minimum variance adaptive algorithm with single-point main response constraints under an assumed horizontally isotropic noise field. The ability of this array and beamformer to resolve the directional characteristics of the acoustic noise field was demonstrated in the Atlantic and the Mediterranean. Median left/right rejection of 20 dB or more was observed over a wide frequency and azimuthal band. The measured beam noise exhibits temporal and spatial variability that could not have been quantified with a single-line system. Median noise gain improvements of up to 7 dB above that of a single line were measured. Distant low-frequency reverberation ambiguity resolution was also measured using the reverberation from signals underwater sound (SUS) explosive charges deployed in the Atlantic. Reverberation left/right rejection of 20 dB was achieved despite imperfect, nonparallel line tow conditions.