5. Near and far field networks

To describe the directional characteristics of arrays, we generally assume that the n sources induce identical pressures at the point of observation, which leads us to speak of a far field for distances that are large compared to the dimensions of the array, regardless of frequency [r >> 2(n – 1)d ]. The far field of gratings is therefore defined solely by a geometric approach.

On the other hand, the far field of the point source is defined for the range where pressure and velocity are in phase (kr >> 1): the limit of this range no longer depends solely on distance, but also on frequency.

The aim of this paragraph is to describe the evolution of a network's radiation with distance, and to define the boundary between near-field and far-field.