Sound Fields depend not only on the sound power and directional characteristics of the sound source, but also on the properties of any medium it passes through, or is reflected, absorbed or diffused by.
Active Sound Fields occur when the particle velocity and the sound pressure are in phase. All the acoustic energy is transmitted; none is stored and the product of the sound pressure and the particle velocity = the net sound intensity.
A plane wave propagating in free field is an example of a purely active sound field.
Critical Distance, in acoustics, is the distance from the sound source where the direct sound level and the reverberant sound level are equal. So at the critical distance the combined sound pressure level is 3 dB higher than both the direct and reverberant sound levels, i.e. the sum of the two levels.
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 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 Field Sensitivity Definition 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 transducerDiffuse 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 Field, 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 Sound Field, a sound field region with no adjacent reflecting surfaces. In practice a free-field can be said to exist if the direct sound is 6 dB or preferably 10 dB greater than the reverberant or reflected sound.
Free Field Room, a room whose walls, ceiling, and floor are lined with sound-absorbing materials to minimise all sound reflections
Free Field Room Definition IEC 801-31-18, room whose boundaries absorb substantially all the sound incident thereon, thereby affording free-field conditions
Near Sound Field, that part of a sound field, usually within about two wavelengths of a noise source, where there is no simple relationship between sound level and distance, where the sound pressure does not obey the inverse square law and the particle velocity is not in phase with the sound pressure.
Reactive Sound Field, a sound field in which the particle velocity is 90° out of phase with the pressure, therefore the sound intensity is zero.
Reverberant Sound Field, the region in a 'room' where the reflected sound dominates, as opposed to the region close to the noise source where the direct sound dominates.
Sound Diffusion, the scattering or random reflection of sound waves. A diffuser changes the reflected sound so listeners may perceive the sound coming from all directions.