The shape of the acoustic ray surface of cubic crystals is investigated with the object of providing a framework within which the results of phonon imaging and other ballistic phonon experiments can be interpreted. This surface is shown to display considerable variability in shape, particularly with regard to the way in which it is folded. The correspondence between these folds and contours of zero Gaussian curvature on the slowness surface is explored, and the bearing this has on the presence of caustics in the anistropic flux of phonons emanating from a localized hear source is discussed. Several of the elementary catastrophes as well as some remarkable types of structural instability are shown to occur in these caustics. Conditions on the elastic constants are established for the existence of various systems of folds in the ray surface.

• Received 11 March 1981

DOI:https://doi.org/10.1103/PhysRevB.24.3456