An analysis of the Doppler shifted aircraft noise signature as measured by a hydrophone located within an evanescent field and implication for localization.

Abstract

Simultaneous measurements of the acoustic pressure field across the air‐water interface, at site in Puget Sound, WA, are analyzed in terms of the Doppler signature of propeller driven aircraft. Measurements in the 50–300‐Hz range are considered for various aircraft over flights. The Doppler shift recorded in air by a microphone placed 3 m above the air‐water interface is modeled using standard Doppler theory. The hydrophone, placed at a depth 2.5 m below the air‐water interface, reveals two Doppler shifts; one associated with the propagating transmitted acoustic field and the other associated with an evanescent field. A parametrized model for these two observed Doppler shifts is presented, utilizing ray theory and Snell’s law applied to a plane wave decomposition of the source wave fronts with, kH ≫ 1, where H is the source height and k is the wave number. It will be shown that analysis of the underwater acoustic field can be used to determine the altitude and closest point of approach (CPA) of a passing tonal source, with support from experimental data recorded by the hydrophone lying within the detection limits of the evanescent field. [Work supported by Washington Sea Grant and ONR.]