Active Sound Fields occur when the sound pressure and the particle velocity are in phase and all the acoustic energy is transmitted, none is stored and the sound intensity = sound pressure x particle velocity.
Anechoic Room under free-field room
Critical Distance see diffuse field distance
Diffuse Field Distance Definition (IEC 801-31-17) that distance from the acoustic centre of a sound source at which the mean-square sound pressure of the direct sound, in a specified direction, is equal to the mean-square sound pressure of the reverberant sound in the room containing the source
Diffuse Sound Field Definition (IEC 801-23-31) sound field which in a given region has statistically uniform energy density, for which the directions of propagation at any point are randomly distributed.
Direct Sound is the region in which the sound measured can be attributed to the source alone without reflections. Early reflections that reach the listener within 50 ms integrate with the direct sound and can improve speech clarity. However later reflections may have a negative effect on the clarity
Far Sound Field is the region, some distance from the sound source, where the sound level conforms with the inverse square law i.e. the sound level decreases by 6 dB for each doubling of the distance from the sound source.
Free-field Room Definition (IEC 801-31-18) room whose boundaries absorb substantially all the sound incident thereon, thereby affording free-field conditions
See also anechoic chamber
Free Field Sound Level is the sound pressure level, well away from any reflecting surfaces
Measurements made 1.2 m to 1.5 m above the ground and at least 3.5 m away from other reflecting surfaces are usually regarded as free-field
Free Sound Fields occur where there are no nearby reflecting surfaces. In practice a free sound field can be said to exist if the direct sound level is 6 dB higher than the reverberant or reflected sound level.