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

摘要

Simultaneous measurements of the acoustic pressure field across the air-waterrninterface are analyzed in terms of the Doppler signature of propeller driven aircraft.rnThe data are from a site in Puget Sound, Washington, and measurements in the 50-rn300 Hz range are considered for various aircraft over flights. The Doppler shiftrnrecorded in air by a microphone placed 2.5 m above the air-water interface isrnmodeled using standard Doppler theory. The hydrophone, however, verticallyrnseparated from the microphone and at a depth 3m below the air-water interface,rnreveals two Doppler shifts, one associated with the propagating transmitted acousticrnfield, the other associated with an evanescent field. A parametrized model for theserntwo observed Doppler shifts is presented, utilizing ray theory and Snell’s lawrnapplied to a plane wave decomposition of the source wave fronts with, kH 1,rnwhere H is the source height and k is the wave number. It will be shown thatrnanalysis of the underwater acoustic field can be used to determine the altitude andrnclosest point of approach (CPA) of a passing tonal source, provided the hydrophonernis located within the detection limits of the evanescent field.