Starting from the wave equation for the electric field of the light, the theory of the diffraction of light by two superposed supersonic waves is developed for sound waves, the frequency ratio of which isn1:n2, wheren1 andn2 are simple incommensurable whole numbers, different from one. A system of difference-differential equations is derived, using the Fourier series method of Raman and Nath. The diffracted light waves make angles defined by $$\sin \theta _{rs} = - \left( {\frac{r}{{\lambda _1 ^* }} + \frac{s}{{\lambda _2 ^* }}} \right)\lambda $$ with the direction of the incident light and have frequency changes −(rν1*+sν2*), (r, s) being all the couples of integers satisfying the relationn=rn1+sn2, wheren is the number of the diffraction order. Neglecting the right-hand side of the difference-differential equation, approximation which corresponds with very intense ultrasonic waves having great wavelengths, the exact solution of the problem is obtained by using a complex integral method. In this special case the diffraction pattern is symmetric with respect to the zero order ifn2−n1 is even; forn2−n1 odd, the pattern is asymmetric, except whenn1 is odd and the phase angle of the sound wavesδ=(k+1/2)π. It has further been shown, that in the general case the intensity of the ordersn and −n are always different, excepting for some special values of the phase angles of the supersonic waves.