DOPPLER-EFFECT FOR SOUND EMITTED BY A MOVING AIRBORNE SOURCE AND RECEIVED BY ACOUSTIC SENSORS LOCATED ABOVE AND BELOW THE SEA-SURFACE

The acoustic emissions from a propeller-driven aircraft are received b y a microphone mounted just above ground level and then by a hydrophon e located below the sea surface. The dominant feature in the output sp ectrum of each acoustic sensor is the spectral line corresponding to t he propeller blade rate. A frequency estimation technique is applied t o the acoustic data from each sensor so that the Doppler shift in the blade rate can be observed at short time intervals during the aircraft 's transit overhead. For each acoustic sensor, the observed variation with time of the Doppler-shifted blade rate is compared with the varia tion predicted by a simple ray-theory model that assumes the atmospher e and the sea are distinct isospeed sound propagation media separated by a plane boundary. The results of the comparison are shown for an ai rcraft flying with a speed of about 250 kn at altitudes of 500, 700, a nd 1000 ft.