Backscattering of sound by nonlinear interaction in gas media and its possible application to a remote sensing acoustical probe

We consider the non-linear interaction of non-collinear acoustical waves in gas media. Conditions for the resonant backscattering are formulated in an alogy with the four-wave mixing known in optics. The efficiency of the phas e-matched interaction is calculated in the resonant approximation for gas m edia. The possibility of using the resonant backscattering as a remote acou stical probe in the atmosphere is analyzed. It is found that for acoustical waves with wave number 10 m(-1), sources of size 1 m, and a distance to th e interaction region of 100 m the amplitude of the reflected wave can be la rger than 10(-10) times the atmospheric pressure.