Acoustic sources
Monopole
This image is the pressure field of a puntual acoustic source. If the excitation of the media is exherted by a puntual variation of pressure, a monopole source is obtained. This behaviour is obtained in the case of a pulsating sphere.
In real situations, this behaiour is observed in loudspeakers in the low frequency limit, i.e. when the product \(\kappa ·a << 1\), where \(a\) is the source radius and \(\kappa\) is the wave number. Thus, a omnidirectional radiation pattern is observed, and the pressure distance decay with \(\frac{1}{r}\)
Dipole
On the other hand, if the excitation of the field is exherted by a puntual variation of the particle velocity a dipole acoustic source is obtained. The phase is inverted on one side of the source, so in the perpendicular axis the pressure is zero.
In this case, the directivity of the source is a bidirectional pattern. In real situations, this behaiour can be observed in a movig surface or diafragm.