In Part XI we derived the radiation impedance for a rigid sphere that oscillates axially. For the results, refer to
Eq. (4.137) and
Fig. 4.27.
It is seen from
Fig. 4.27 that for
kR
<
1, that is, when the diameter is less than one-third the wavelength, the impedance load on the surface of the sphere is that of a mass reactance because the resistive component is negligible compared with the reactive component.
At all frequencies, the loading shown in
Fig. 4.27 may be represented by the equivalent circuits of
Fig. 4.34. The element sizes for the mechanical and acoustic admittances and impedances are given with the circuits.