The results show that the wave velocity decreases sharply and linearly below the temperature of 400
C, before deceasing nonlinearly as the temperature increases to 800
C.

翻译：结果表明：低于400度时，波速线性的急速下降；在400度之后，800度之前波速非线性的下降。

The results showed a sharp linear decrease of the wave velocity when the temperature was below 400 C and its non-linear decrease when the temperature was between 400 C and 800 C.