Brian G. Ferguson
Maritime Operations Div., Defence Sci. and Technol. Organisation, P.O. Box 44, Pyrmont 2009, Australia
Robert C. Trider
Defence Res. Establishment Atlantic, P.O. Box 1012, Dartmouth, NS B2Y 3Z7, Canada
James L. Hanlon
Iotek, Inc., 1127 Barrington St., Halifax, NS B3H 2P8, Canada
A turbo-prop aircraft flew at a constant altitude and speed over and along the axis of a line array consisting of 15 microphones spaced 0.9 m apart. The acoustic data from the microphones were processed by a frequency-domain beamformer, which led to the acoustical Doppler effect being observed on the various output displays of the processor. Color displays showing the variation of the beamformer's output power as a function of frequency and angle of arrival are presented here for a conventional beamformer, an adaptive beamformer, and an adaptive beamformer with a weight norm constraint. Adaptive beamforming maximizes the array gain resulting in the spatial leakage being lower and the widths of the beams being narrower when compared with conventional beamforming. Imposing an upper bound on the norm of the adaptive weight vector reduces the signal suppression that occurs in adaptive beamforming as a result of system errors. The observed variation of the Doppler shift in the propeller blade rate with the angle of arrival is compared with the variation predicted using an isospeed sound propagation model.