When a body is large compared to the wavelength of the sound waves that it generates, interference of wave fronts from different parts of the body produce a complicated sound pattern. This is especially important in the region near the body or in the near field. When the body is small relative to the wavelength scale of the sound it generates or the sound is at high frequency, the phase differences between different source points is small and the body radiates like a point source. In acoustics this relative size of the body at a given frequency is called its compactness. Therefore a compact source radiates like a point source, whereas noncompact bodies must be treated in more detail.
In rotorcraft acoustics it is often sufficient to consider unsteady lift producing acoustic sources on the rotor as a series of compact sources. However, some contributions to rotor noise, such as thickness noise and HSI noise, arise because of phase differences between the different times of arrival of sound waves from the leading and trailing edges of the airfoil sections. In these cases, compactness cannot be assumed and the problem is considerably more complicated; see Brentner & Farassat (2003) for a summary.