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

Abstract

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