Using the velocity operator derived from the Heisenberg equation of motion, the authors have investigated the magneto-acoustic effect in n-InSb in the presence of a DC magnetic field B directed along the propagation of acoustic waves. Since they are interested in both high and low frequency regions, an electron relaxation time due to the scatterings in solids is taken into account. It is found that there exists a discontinuous point in the absorption coefficient or change in the sound velocity as a function of the sound frequency. When the sound frequency is below 1.3*10¹¹ rad s^-1, the absorption coefficient increases with the sound frequency and the sound velocity decreases with the sound frequency from the sound velocity nu _s=4*10⁵ cm s^-1. However, when the sound frequency is over 1.3*10¹¹ rad s^-1, the absorption coefficient decreases with the sound frequency and the sound velocity increases abruptly with the sound frequency from nu _s=4*10⁵ cm s^-1.