Acoustic intensity methods and their applications to vector sensor use and design

by Naluai, Nathan Kahikina.

Applications of acoustic intensity processing methods to vector sensor output signals are investigated for three specific cases: acoustic intensity scattering, spatial correlations of intensities, and conceptual design of a high frequency inertial vector sensor with a novel suspension. An overview of intensity processing is presented and the concept of a complex intensity is illustrated. Measurement techniques for determining the complex intensity spectra from the signals received by a standard acoustic vector sensor are demonstrated. Acoustic intensity processing of signals from SSQ-53D sonobuoys is used to enhance the detection of submerged bodies in bi-static sonar applications. Deep water experiments conducted at Lake Pend Oreille in northern Idaho are described. A submerged body is located between a source and a number of SSQ-53D sonobuoy receivers. Scalar pressure measurements change by less than 0.5 dB when the scattering body is inserted in the field. The phase of the orthogonal intensity component shows repeatable and strong variations of nearly 55o. The classical solution for the spatial correlation of the pressure field is presented. The derivation techniques are expanded to derive previously unsolved analytic forms for the spatial correlations of separated intensity field components iii based on combinations of the solutions for various pressure and velocity components. Experimental validation of these correlation solutions are performed computationally and in an underwater environment. The computational experiments are designed to test highly controlled variations to the idealized case (e.g. sound field content, transducer phasing issues, additive output noise, etc.). Additional verification is provided from physical tests measuring the correlations between a pair of acoustic vector sensors in a large reverberant tank which is excited acoustically with broadband noise. The results successfully corroborate the derivation methods for correlations of individual vector sensor components and for intensity processed vector sensors. A conceptual design for an improved neutrally buoyant underwater acoustic vector sensor is proposed. The design incorporates a novel suspension design that can be rigidly mounted to an existing support structure without affecting the sensor’s performance. The sensor response is shown to be locally frequency independent across a frequency range from 1.0 to 30.0 kHz, with a total phase variation of less than 0.3 degrees. Fundamental limits of signal detection are determined to be inherent primarily in the velocity sensing component. These limits are shown to be roughly equivalent to sea-state zero at 1 kHz and increasing to sea-state 1 or higher for frequencies greater than 10 kHz. iv