The near field is the region close to a sound source usually defined as ¼ of the longest wave-length of the source.  Near field noise levels are characterized by drastic fluctuations in levels as much as 10 dBA for small changes in distance from the source.  Near field references can pertain to both indoor and outdoor environments.

The far field describes a sound field beyond the near field limits described above where the sound pressure level (SPL) drops off at the theoretical rate of 6 dB fore very doubling of distance from the source.  This rule of thumb is called the Inverse Square Law.  Please note that if the far field does not meet the criteria for a free field as described below, then less than the theoretical drop rate will pertain.  In such case doubling the distance from the source may yield a drop rate of 3-4 dB.

 
To be considered free field there can be no obstructing surfaces in the sound path of spherical wave propagation.  Free field conditions are characterized by SPL loss rates following the Inverse Square Law.  Free field references pertain to large open outdoor spaces or in rooms where walls and other surfaces are almost completely absorptive.  Anechoic (without echoes) acoustical test chambers simulate free field conditions where omnidirectional sound wave propagation exists.

The direct sound field is also used to describe far field conditions that follow the Inverse Square Law SPL loss rate of 6dB for every doubling of the distance. The actual formula used to make calculations at various distances in the far/direct field is as follows: SPL1  [20x log (d2/d1)] = SPL2 where SPL1  is the noise level at the location closer to the source at a distance of d1 from the source and SPL2 is the noise level at a location farther from the source at a distance of d2.

 
In a diffuse field there are so many reflections contributing to the total sound field that sound levels measured virtually anywhere in the sound field are the same. Diffuse fields usually pertain to indoor environments. Rooms that are categorized as “live” have larger diffuse fields than free fields. “Dead” rooms have much larger free fields than diffuse fields.

The reverberant field is essentially the same as the diffuse field. For indoor sound field discussions it is used to contrast direct fields. Reverberation test chambers have all room surfaces almost completely reflective so that total sound energy remains constant throughout the environment and sound levels can be measured independent of location and distance.