Far field and near field

In acoustics the far field is the minimum distance from where a sound source can be seen as a source that is producing a plane wave with the sound pressure and sound velocity in phase.

In the near field this is not true and the directivity depends on the distance as well. Other definitions of far field may use the directivity as a definition, namely as the minimum distance where the directivity is independent of the distance. The distance boundary between the far field (Fraunhofer zone) and near field (Fresnel zone) is frequency-dependent.