Isolation of a negative pressure pulse by means of diffraction

A unipolar negative pressure pulse would be a useful tool in research in biomedical ultrasound and cavitation. To the extent to which an underwater spark generates a unipolar positive pulse (the accompanying negative tail is often not very noticeable), diffraction from a circular aperture may be used to produce the desired negative pulse. If the spark and hydrophone are located on the axis of the aperture, the signal received by the hydrophone is the direct (positive) pulse followed by the edge or diffracted wave, which is an inverted replica of the direct pulse. The direct wave may be blocked out by inserting an irregularly shaped occlusion in the center of the aperture (irregularity causes the diffracted wave from the occlusion to be incoherent). The edge wave is then isolated as the desired (nearly) unipolar negative pulse. Experiments to demonstrate this effect have been done with an underwater spark that generates a 1.5-μs positive pulse, a corprene aperture 50 mm in diameter, a corprene occlusion, and a piezoceramic needle hydrophone. Theoretical work to assess sensitivity of the desired pulse to misalignment is in progress. [Work supported by ONR and NIH.]