A new correlation technique enables the determination of local acoustic source strength on the surface of flat plates and simple aerofoil shapes. The method is based on Curle's equation for surface-generated noise and involves cross-correlation between local surface pressure and the farfield acoustic pressure. Flat plate airfoils of circular planform were positioned in a “quiet” open jet airflow. The distribution of dipole strength was obtained by moving the point of surface-pressure measurement around on the surface. Distinctively different distributions were obtained for cases of incident turbulence (airfoil in turbulent mixing layer), separated flow (airfoil at 16° angle of attack in core of jet), and vortex shedding (airfoil at 0° angle of attack in core of the jet). The popular notion that noise comes predominantly from the edges of the plate was not strongly supported. In all cases, however, definite localized source regions were identified. The technique also enabled estimates of the correlation area for the surface-pressure pattern and of the spectrum radiated from each unit of surface area to be made. The correlation areas were generally a small fraction of the plate area (1%–10%).
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T. E. Siddon (1973) studied this question.