If we include consideration of wave propagation in an ideal gas, we can trace the origin of theoretical nonlinear acoustics, at least as far back as Poisson’s work in 1808 [1]. The first experiments in air were done 1.27 centuries later [2]. With liquids Fox and Wallace [3] tried to explain experimental results on sound attenuation with a correct, but somewhat inadequate nonlinear theory. Keck and Beyer [4] used the equations of hydrodynamics and showed that nonlinear considerations lead to the prediction of nonlinear distortion, something that had been observed in fluids by optical techniques [5] and showed that the nonlinear equation for wave propagation in fluids has the same form as that for an ideal gas. Different thermodynamical quantities appeared in the nonlinear equations, however. It is significant that Keck and Beyer were able to perceive the inherent similarity of the two descriptions without becoming confused by the dissimilarities of certain details of equations describing ideal gases compared with those describing liquids.