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.
Critical Distance in acoustics, is the distance from the sound source where the direct sound level and the reverberant sound level are equal, also known as the direct sound distance.
Diffuse Sound Field is the region in a room where the sound pressure level is uniform i.e. the reflected sound dominates, as opposed to the region close to a noise source where the direct sound dominates.
Diffuse Field Distance 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 Field Sensitivity IEC 801-25-65, of an electroacoustic transducer for sound reception, at a specified location, for a specified frequency, quotient of the root mean square open circuit output voltage due to sound waves arriving more or less simultaneously with equal probability from all directions, by the root mean square sound pressure at that location due to the same sound waves but in the absence of the electroacoustic transducer
Direct Sound Field 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. Later reflections may have a negative effect on speech clarity
Far Sound Field is the region some distance from the sound source, where the sound level obeys the inverse square law i.e. the sound level decreases by 6 dB for each doubling of the distance from the source. Also, in this region the sound particle velocity is in phase with the sound pressure.
Free Field Level is the sound pressure level, away from 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 Field Room IEC 801-31-18, room whose boundaries absorb substantially all the sound incident thereon, thereby affording free-field conditions
See also anechoic chamber