Active Field a sound field in which the
particle velocity and the
sound pressure are in
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.
Anechoic without echo - refers to the absence of sound reflections. It is almost impossible to create a truly anechoic environment, as there is no such thing as a perfect sound absorber see anechoic chamber.
Critical Distance the distance at which the sound pressure level of the direct sound and the reverberant sound are equal when dealing with a directional source.
Diffuse Field 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, the same as reverberant field.
In a diffuse field the sound pressure and the particle velocity phase varies randomly so the net sound intensity is zero.
Sound that is completely random in phase; sound that appears to have no single source.
Diffusion the scattering or random reflection of a sound wave from a surface. A diffuser changes the reflected sound so listeners may perceive the sound coming from all directions.
Direct 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 Field a region in free space, distant from a sound source, where the sound pressure level obeys the inverse square law (the sound pressure level decreases 6 dB with each doubling of distance from the source). Also, in this region the sound particle velocity is in phase with the sound pressure.
Free 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
Near 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.
Reverberant 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.