Development of solid state nonlinearity

The nonlinear acoustics of solids developed from the nonlinear acoustics of fluids. Originally, the problem was to undo correctly the Hooke’s Law assumption without losing nonlinear terms. At present, one problem is to find a use for nonlinear acoustics of solids. The first problem has been solved; the second one is an ongoing one. It is the reason for special sessions. To develop geological exploration based on nonlinear acoustics, at first it was thought that nonlinear acoustics of crystalline solids would do. Now it is known that other nonlinearities must be considered. The purpose of the present paper is to show how solid state nonlinear acoustics developed, how nonlinear acoustics of crystals requires only one nonlinear term, and to give evidence of the fact that the subject needs to be expanded.