Abstract
An inhomogeneous wave equation, exact to second order in the field variables, is derived for the sum and difference frequency pressure generated by two plane waves, of angular frequencies ω1 and ω2, which intersect at an angle θ in a lossless fluid. The coefficient of nonlinearity pertaining to the sum or difference frequency wave (ω± = ω1 ± ω2) is shown to be β± (θ) = B/2A + cosθ ± 4(ω1/ω±2)sin4(θ/2), where B/A is the parameter of nonlinearity. The same result may be deduced from the work of Zverev and Kalachev [Soy. Phys. Acoust. 15, 322 (1970)]. The first term of β± is due to the isentropic equation of state, the second term represents convection, and the third term comes from the momentum equation. Alternative formulations of the inhomogeneous wave equation are presented, and comparisons are made with the analyses of others. An experiment designed to measure the angular dependence of β± was conducted with noncollinear waves in an airfilled waveguide. Results are reported. [Work supported by ONR.]
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