(without echo), refers to the absence of sound reflections. It is almost impossible to create a truly anechoic environment, as the perfect sound absorber does not exist. However the anechoic chamber can come close.
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
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
sound field which in a given region has statistically uniform energy density, for which the directions of propagation at any point are randomly distributed
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.
, 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
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.