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Experiments on two kinds of threshold for the acoustic pressure gradient of a schlieren system

Two kinds of threshold for the acoustic pressure gradient of a schlieren system for the observation of acoustic waves in transparent media are studied experimentally in this paper. The first kind of threshold is related to a short-duration ultrasonic pulse. In this case the acoustic pressure gradient must be large enough to make the deflected light pass the diaphragm (optical filter) and reach the screen. The second kind of threshold is related to the physical limitations of diffraction theory (where the continuous acoustic field is considered as an optical phase grating) and the diffracted light of every non-zero order has a related pressure gradient threshold. The existence of the two kinds of threshold is examined using well-designed experiments and absolute measurements of the acoustic pressure.