Ambient Noise Measurements with Vector Acoustic Hydrophones

Vector hydrophones are compact acoustic sensors that provide intrinsic non-omnidirectional (e.g. dipole) beampatterns with a single sensor. Often the three orthogonal components of the particle velocity or particle acceleration field are measured using a three-axis neutrally buoyant motion sensor. This type of sensor can form dipole beams steered to any azimuth or elevation angle. If a conventional pressure-sensing hydrophone is integrated with the three-axis motion sensor, then different types of single-sensor responses can be formed including cardioids (with varying front-to-back ratios) and the acoustic intensity. An array of vector hydrophones can form beams using the different element-level responses or the weights for each sensor component can be set adaptively. A method for forming an "intensity beam" response which is not purely multiplicative is developed. A vertical line array of 5 vector hydrophones was used to measure ambient noise near Ketchikan, Alaska. Free-field voltage sensitivity calibrations are discussed. Frequency auto-and cross-spectra are presented for the pressure and particle acceleration components. The pressure, velocity, cardioid, and intensity single-sensor and beam responses are analyzed. Array gain results are compared to theoretical models using different vertical noise power directivities. Advantages and disadvantages of each type of beam response are discussed