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.
Diffraction : phenomenon by which a sound wave is changed in direction by an obstacle or other heterogeneity in the medium.
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
Sound that is completely random in phase; sound that appears to have no single source.
Diffuse Field Distance : distance from the sound source at which the sound pressure of the direct sound is equal to the sound pressure of the reverberant sound in the room containing the 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
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.
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