Abstract
In this paper, the fundamental equation of the acoustic radiation pressure is discussed. The equation, given by Kotani and King, relating to the sound pressure variation valid to the second order is examined. It is shown that, if the solution of the first order wave equation is adopted in Kotani's expression for the sound pressure the time average of this expression on the surface of the object fixed in the acoustic field, expresses the radiation pressure in the Langevinian sense. Also the expression of the radiation pressure is derived for the movable object, placed in the field freely to move by the inertia effect, by using the moving coordinates.
