On the basis of Huygens' principle this paper considers the diffraction and interference of light traversing side-by-side sound fields at relatively low frequencies by which every order in the diffraction spectrum of light cannot be split but overlapped spatially with one another over the field to be observed. The sound fields greatly modify the spatial coherence state of light, so that the diffraction intensity distribution in the receiving plane depends on the electronically controllable sound pressure. From the analysis of the two-beam interference of light the complex degree of coherence is found to vary from the initial state at the entrance face of the sound fields to the completely incoherent state as a function of the sound pressure.