Sound Field Definitions (see ISO 12001)

Free field

The free field is a region in space where sound may propagate free from any form of obstruction.

Near field

The near field of a source is the region close to a source where the sound pressure and acoustic particle velocity are not in phase. In this region the sound field does not decrease by 6 dB each time the distance from the source is increased (as it does in the far field). The near field is limited to a distance from the source equal to about a wavelength of sound or equal to three times the
largest dimension of the sound source (whichever is the larger).

Far field

The far field of a source begins where the near field ends and extends to infinity. Note that the transition from near to far field is gradual in the transition region. In the far field, the direct field radiated by most machinery sources will decay at the rate of 6 dB each time the distance from the source is doubled. For line sources such as traffic noise, the decay rate varies between 3 and 4dB.

Direct field

The direct field of a sound source is defined as that part of the sound field which has not suffered any reflection from any room surfaces or obstacles.

Reverberant field

The reverberant field of a source is defined as that part of the sound field radiated by a sourcewhich has experienced at least one reflection from a boundary of the room or enclosure containing the source.

PHYSICS OF SOUND

Noise can be defined as "disagreeable or undesired sound" or other disturbance. From the acoustics point of view, sound and noise constitute the same phenomenon of atmospheric pressure fluctuations about the mean atmospheric pressure; the differentiation is greatly subjective. What is sound to one person can very well be noise to somebody else. The recognition of noise as a serious health hazard is a development of modern times. With modern industry the multitude of sources has accelerated noise-induced hearing loss; amplified music also takes its toll. While amplified music may be considered as sound (not noise) and to give
pleasure to many, the excessive noise of much of modern industry probably gives pleasure to very few, or none at all. Sound (or noise) is the result of pressure variations, or oscillations, in an elastic medium(e.g., air, water, solids), generated by a vibrating surface, or turbulent fluid flow.
Soundpropagates in the form of longitudinal (as opposed to transverse) waves, involving a succession of compressions and rarefactions in the elastic medium, as illustrated by Figure 1.1(a). When a sound wave propagates in air (which is the medium considered in this document), the oscillations in pressure are above and below the ambient atmospheric pressure.