A sound field is a region where there is sound. It is classified according to the manner and the environment in which the sound waves travel. Some examples will now be described and the relationship between pressure and intensity discussed. This relationship is precisely known only in the first two special cases described below.
The Free Field
This term describes sound propagation in idealized free space where there are no reflections. These conditions hold in the open air (sufficiently far enough away from the ground) or in an anechoic room where all the sound striking the walls is absorbed. Free field propagation is characterized by a 6 dB drop in sound pressure level and intensity level (in the direction of sound propagation) each time the distance from the source is doubled. This is simply a statement of the inverse square law. The relationship between sound pressure and sound intensity (magnitude only) is also known. It gives one way of finding sound power which is described in the International Standard ISO 3745.
The Diffuse Field
In a diffuse field, sound is reflected so many times that it travels in all directions with equal magnitude and probability. This field is approximated in a reverberant room. Although the net intensity is zero, there is a theoretical relationship which relates the pressure in the room to the onesided Intensity, Ix. This is the intensity in one direction, ignoring the equal and opposite component. One-sided intensity cannot be measured by a sound intensity analyzer but it is nevertheless a useful quantity: By measuring pressure we can use the relationship between pressure and one-sided intensity to find the sound power. This is described in ISO 3741.

Active and Reactive Sound Fields
Sound propagation involves energy flow but there can still be a sound pressure even when there is no propagation. An active field is one where there is energy flow. In a pure reactive field, there is no energy flow. At any instant energy may be travelling outward, but it will always be returned at a later instant. The energy is stored as if in a spring. Hence the net intensity is zero. In general a sound field will have both active and reactive components. Pressure measurements for sound power in fields which are not well-defined can be unreliable, since the reactive part is unrelated to the power radiated. We can, however, measure sound intensity. Since sound intensity describes energy flow, there will be no contribution from the reactive component of the field. Two examples of reactive fields follow.
Standing Waves in a Pipe
Consider a piston exciting the air at one end of a tube. At the other end there is a termination which causes the sound waves to be reflected. The combination of the forward-travelling and reflected waves produces patterns of pressure maxima and minima which occur at fixed distances along the tube. If the termination is completely rigid all the energy is reflected and the net intensity is zero. With an absorptive termination some intensity will be measured. Standing waves are also present in rooms at low frequencies.
The Near Field of a Source
Very close to a source, the air acts as a mass-spring system which stores the energy. The energy circulates without propagating and the region in which it circulates is called the near field. Only sound intensity measurements for sound power determination can be made here. And because it is possible to get close to the source, the signalto-noise ratio is improved. (from Bruel & Kjar)