The distribution of sound energy at a very much greater distance from a sources than the linear dimensions of the source and in which the sound waves can be considered to be plane waves. That part of the sound field in which sound pressure decreases inversely with distance from the source. This corresponds to a reduction of approximately 6 dB in level for each doubling distance.
Also, in this region the sound particle velocity is in phase with the sound pressure. Closer to the source where these two conditions do not hold constitutes the "near field" region.
The electromagnetic field which exists at a distance of several wavelengths from the source of radiation.