Sources of sound

The properties of acoustic sources which are relevant in aerodynamic sound production namely: the monopole, dipole, and quadrupole respectively are presented. The simplest of these is the monopole, classically regarded as a small pulsating sphere. A more useful concept of the simple source, relevant to aerodynamic noise is a source of time varying mass injection. Two such simple sources, pulsating in antiphase, are combined to form a dipole. While no net mass flux exists in this situation a fluctuating force is present and represents the flow analog for dipole radiation. A combination of four appropriately phased simple sources to form a quadrupole is undertaken and shown to be equivalent to a fluctuating stress.