Acoustics: Sound Fields, Transducers and Vibration, 2nd Edition

4.15. Radiation from an oscillating sphere

We saw that the monopole point source is effectively a pulsating sphere with an infinitesimally small radius. Similarly, the dipole point source can be considered as a rigid sphere with an infinitesimally small radius oscillating back and forth along its axis. This provides us with some useful insight into the operation of a loudspeaker without any baffle or enclosure whatsoever, but unlike the more accurate circular piston in free space (see Section 13.10), it yields simple closed-form solutions for the pressure field and radiation impedance. As in the case of the pulsating sphere, the impedance can be represented by a simple equivalent circuit. We shall now consider a sphere of radius R that oscillates ...