Near, far and free field are all sound fields which depend not only on the sound power level and directional characteristics of the sound (noise) source but also on the properties of the medium the sound waves encounter and whether they pass through it, get reflected or diffused from it, or become absorbed by it.
Near Field is the area of the sound field that is closest to the sound (noise) source. At this part of the sound field, there is no simple relationship between sound (noise) level and distance. The sound pressure level may vary significantly with a small change in position. At this region the sound pressure does not obey the inverse square law and the particle velocity is not in phase with the sound pressure.
Far Field is the area of the sound field that is distant from the sound (noise) source. In this region the sound pressure level obeys the inverse square law in which the sound pressure level decreases 6 dB with each doubling of distance from the source. In this region the sound particle velocity is in phase with the sound pressure. Within far field there are two additional fields: free field and reverberant field.
Free Field is a sound field region with no adjacent reflecting surfaces in which sound behaves as if in open air without any form of obstruction.
Reverberant Field is a sound field region in which the reflections from walls and other objects may be just as strong as the direct sound from the noise source.
dB Noise Reduction is an expert in the design and supply of noise control equipment including silencers and noise enclosures. We would be more than happy to discuss your specific application to try and help attenuate the noise for the health and safety of your workers.