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Monopoles and dipoles are elementary models for acoustic radiation. Their usefulness depends on source size, wavelength and the surrounding boundaries. The animations illustrate wavefront geometry; they do not by themselves establish the spreading law of a three-dimensional source.
Monopole
A compact monopole represents an oscillating volume flow, such as a small pulsating sphere. In an unbounded, homogeneous three-dimensional medium, it radiates equally in every direction and its far-field pressure amplitude decreases as \(1/r\).
The compact-source approximation requires \(ka\ll1\), where \(a\) is a characteristic source radius and \(k=2\pi/\lambda\) is the acoustic wavenumber. A two-dimensional line-source model instead has cylindrical spreading.
Dipole
A compact dipole can be represented by two nearby monopoles of equal strength and opposite phase. An oscillating force on the fluid provides another realization. The far-field pressure varies as \(\cos\theta\): two lobes have opposite phase, with a nodal plane perpendicular to the dipole axis.
An unbaffled, open-backed diaphragm can approximate a dipole because its front and rear faces radiate in opposite phase. A small enclosed loudspeaker can behave more like a monopole at low frequencies; the enclosure and baffle matter.
