A theoretical and experimental investigation of low-frequency acoustic vector sensors

An acoustic vector sensor is a compact device which simultaneously measures the scalar acoustic pressure p(t, (r) over right arrow (0)) and the gradient of the acoustic pressure, delp(t, (r) over right arrow (0)), at some measurement point (r) over right arrow (0). Thus, a three-dimensional (3-D) acoustic vector sensor produces four time series outputs, which can be processed to provide some degree of spatial filtering and a direction of arrival (DoA) estimate to an acoustic target. Historically, two-dimensional (2-D) acoustic vector sensors have been used in several Navy systems, like the DIFAR sonobuoy (AN/SQQ-53 series) and the AN/WLR-9 acoustic intercept receiver (which uses the multimode hydrophone). These devices have made important contributions to the Navy sonar community, where it is desirable to obtain an accurate, azimuthal DoA estimate or bearing to a low-frequency target from a sinple point in space. Other researchers have used the outputs of acoustic vector sensors to estimate the acoustic intensity vector (Watts/m(2)). In the literature, these intensity-related vector sensors have been referred to as intensity probes or "acoustic watt-meters." In this paper, we consider 3-D acoustic vector sensors that use three orthogonal underwater acoustic accelerometers with a scalar acoustic pressure sensor or hydrophone packaged in a single housing. These sensors will be referred to as low-frequency vector sensors since they were optimized for the 50 Hz to 2,000 Hz band. We will show that these vector sensors can produce a frequency-independent spatial response over the aforementioned band. Specifically, a 3-dB beamwidth of 105 degrees across the stated frequency band and an array gain against isotropic noise of 6 dB can be achieved. We will also show that the four time series vector sensor outputs (x, y and z accelerations and 1 pressure) can be processed to produce multiple nulls in the spatial response or beam pattern, which can be used to null out interfering noise sources. Experimental results, recorded at the U. S. Navy's Seneca Lake Test Facility, Dresden, NY using two different designs of the aforementioned 3-D vector sensor, show how vector sensors perform in the presence of multiple low-frequency sources and interferers.