It is shown that the attenuation coefficient will decrease obviously with the increase of Mach number, and the nonlinear acoustic resistance and acoustic mass will reduce slightly.

So, the echoes from a tree are going to a mass of chaotic acoustic reflections, not like the echo from a moth.

The monitoring technique of rock mass acoustic emission is of wide use in the stope stability. A grey model GM (1.1) for scientific forecasting was presented in this paper.