far field

5.04  far field. The acoustic field sufficiently distant from a distributed source that the sound pressure decreases linearly with increasing distance (neglecting reflections, refraction, and absorption).

Annotation 1          For a source of extent L, the “Fresnel range” can be used to define the minimum distance for the far field: L²/λ, where λ is the wavelength of the sound.

Annotation 2          The beam pattern of the sound in the far field does not depend on the distance from the source.

Annotation 3          Arrays of sound sources are deliberately extended in space and a receiver must be relatively far away to be in the far field.

Annotation 4          In the acoustic far field, sound pressure and particle velocity are in phase