Problem of reversal of an acoustic wavefront and amplification of the reversed wave

A parametric phase-reversing amplifier of sound is described. Its operating principles and estimates of the parameters are given. An actual amplifier is described: its active medium is a liquid containing gas bubbles and it is pumped by an alternating electric field. The phenomenon of two-pulse electroacoustic echo is analyzed from the point of view of parametric reversal of acoustic wavefronts.