Development of a standing-wave apparatus for calibrating acoustic vector sensors
Underwater acoustic pressure transducers measure pressure fluctuations, a scalar parameter of the acoustic field. Acoustic vector sensors contain an omnidirectional pressure transducer (omni) and also bi- or tri-axial sensing elements that respond to either the particle velocity or pressure gradient of the acoustic field; which are vector quantities. The amplitude of the signal output of each directional channel of a vector sensor is proportional to the orientation relative to the direction of acoustic pressure propagation. The ratio of the signal amplitudes between two directional channels and the cross-spectra between the vector sensor omni and directional channels enable one to estimate the bearing to the source from a single point measurement. In order to accurately estimate the bearing across the usable frequency band of the vector sensor, the complex sensitivities of the omni and directional channels must be known. Since there is no standard directional reference transducer for a comparative calibration, the calibration must be performed in an acoustic field with a known relationship between the acoustic pressure and the acoustic particle velocity. Free-field calibrations are advantageous because this relationship is known for both planar and spherical wave fronts. However, reflections from waveguide boundaries present a practical limitation for free-field calibrations, especially at low frequencies. An alternative approach is to perform calibration measurements in a standing-wave field, where the relationship between pressure and particle velocity is also known. The calibration facility described in this thesis is composed of a laboratory-based, vertically-oriented, water-filled, elastic-walled waveguide with a piston velocity source at the bottom end and a pressure release boundary condition at the air/water interface at the top end. Some of the challenges of calibrating vector sensors in such an apparatus are discussed, including designing the waveguide to mitigate dispersion, mechanically isolating the apparatus from floor vibrations, understanding the impact of waveguide structural resonances on the acoustic field, and developing the calibration algorithms. Data from waveguide characterization experiments and calibration measurements are presented along with engineering drawings and calibration software.