Radiation mass
Transition between piston velocity field (normal piston velocity) and sound field in free space is illustrated below:
Figure on the left represents a sine of a phase shift
between pressure and velocity. Pressure is in quadrature (
) in the immediate vicinity of the membrane (radiation mass), and then tends to be in phase (
) at greater distance (propagation).
Figure on the right represents a ratio between local acoustic velocity and that corresponding to a "pure" propagation situation. Additional (tangential) component dominates at the edges, as the fluid particles are "chased" outward rather than compressed (radiation mass);
