Classical Electrodynamics and Acoustics: Sound Radiation by Moving Multipoles

Abstract

In classical electrodynamics (CED) P. Dirac used the average of retarded and advanced fields to represent the bound field and their difference to represent the free field in his derivation of the (Lorentz-Dirac) equation of motion for an electron. The latter skew-symmetric combination filtered out the radiation part of the field. It can also be used to derive many properties of the power radiated by acoustic sources, such as angular and frequency distributions. As in CED there is radiation due to source acceleration and radiation patterns exhibit the “headlight effect.” Power radiation patterns are obtained by this approach for point multipoles undergoing various motions. Applications to sound radiation problems from rotating machinery are shown. Numerous computed plots illustrate all cases.