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&oarr;₀) and the gradient of the acoustic pressure, ∇p(t, r&oarr;₀), at some measurement point r&oarr;₀. Thus, a three-dimensional (3D) 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 (2D) 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 for bearing to a low-frequency target from a single point in space. Other researchers have used the outputs of acoustic vector sensor to estimate intensity vector (watts/m²). In the literature, these intensity-related vector sensors have been referred to as intensity probes or "acoustic watt-meters". In this paper, we consider 3D acoustic vector sensors that uses three orthogonal underwater acoustic accelerometers with a scalar acoustic pressure sensor of hydrophone packaged in a single housing. The sensors will be referred to as low-frequency vector sensors since they were optimized for the 50 Hz to 2 kHz 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 s in the spatial response or beam pattern, which can be used to out interfering noise sources. Experimental results, recorded at the US Navy Seneca Lake Test Facility, Dresden, NY using two different designs of the aforementioned 3D vector sensor, show how vector sensors perform in the presence of multiple low-frequency sources and interferers.