High-Intensity Sound and Shock Waves

Abstract

High-intensity sound, or macrosonics, differs from ordinary sound in its manner of propagation. All parts of an ordinary sound wave travel with the same speed (the sound speed). However, in high-intensity sound the pressure maxima travel faster than the pressure minima. Consequently, high-intensity sound waves become distorted as they travel. This distortion may become so extreme that a discontinuity or shock wave develops when a maximum tries to overtake a minimum. Such a shock wave will start out very weak but will grow in size and then, perhaps, decay again. All of these effects are illustrated by the example of a piston moving periodically at one end of a semi-infinite tube. The free vibration of the gas in a closed tube is also examined and found to be modified by the foregoing effects. Similarly the sound waves produced by a piston at one end of a closed tube are found to be affected, in some cases shocks developing and in other cases the wave forms merely being distorted. Similar phenomena occur in three dimensions, as can be seen from a new theory of the propagation of high-intensity sound in three dimensions. This theory includes a new phenomenon, the possibility of refraction caused by intensity variations on a wave front. It also leads to shock formation in certain cases, and describes the way in which the shocks propagate, grow, and decay. The theory is an extension of a previous theory of weak shocks which was called geometrical acoustics. It too is a geometrical theory.