Abstract
Using a closed circuit television camera (CCTV), a method is developed to obtain quantitative sound pressure measurements directly from the sound field imaged in a schlieren system. By using the exact time format of the composite video signal and a series of time delay circuits, it is possible to take vertical intensity scans of a schlieren image at any position on the TV raster. The method uses the camera's horizontal blanking pulse to trigger a broadband, high-speed sample-hold amplifier. This amplifier is triggered to sample each horizontal scan line at a selected time in the horizontal scan sweep and to feed this value into a transient recorder. The signal is digitized, and a continuous analog voltage signal corresponding to the black-white information from a schlieren image is displayed on a scope. The same analog signal is also plotted on an X-Y recorder. The schlieren system can be operated using only a single calibration curve at each frequency. The curves plotted applied transducer voltage against the video output level. Thus, by comparing the video output levels for several vertical image intensity scans on a schlieren image, relative sound pressure measurements are routinely obtained.
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