Acoustic Environment 973.12 Classifying Sound Fields .........................................................................................
3.12.1 Free Fields
A sound fi eld is said to be a free fi eld if it is uniform, free of boundaries, and is
undisturbed by other sources of sound. In practice, it is a fi eld in which the effects of the
boundaries are negligible over the region of interest. The fl ow of sound energy is in one
direction only. Anechoic chambers and well-above-the-ground outdoors are free fi elds.
The direct sound level from a sound source in a free fi eld is labeled LD.
3.12.2 Diffuse (Reverberant) Fields
A diffuse or reverberant sound fi eld is one in which the time average of the mean square
sound pressure is the same everywhere and the fl ow of energy in all directions is equally
probable. This requires an enclosed space with essentially no acoustic absorption. The
reverberant sound level is labeled LR.
3.12.3 Semireverberant Fields
A semireverberant fi eld is one in which sound energy is both refl ected and absorbed.
The fl ow of energy is in more than one direction. Much of the energy is truly from a
diffused fi eld; however, there are components of the fi eld that have a defi nable direction
of propagation from the noise source. The semireverberant fi eld is the one encountered in
the majority of architectural acoustic environments. The early refl ections, that is, under
50 ms after LD , are labeled LRE.
3.12.4 Pressure Fields
A pressure fi eld is one in which the instantaneous pressure is uniform everywhere. There
is no direction of propagation. The pressure fi eld exists primarily in cavities, commonly
called couplers, where the maximum dimension of the cavity is less than one-sixth of
the wavelength of the sound. Because of ease of repeatability, this type of measurement
is used by the National Bureau of Standards when they calibrate microphones. At low
frequencies the pressure fi eld can be large, that is, big enough for a listener to sit in.
3.12.5 Ambient Noise Field
The ambient noise fi eld is composed of those sound sources not contributing to the
desiredLD (i.e., active sources). The ambient noise level is labeled LN.