This section of this International Standard specifies a method for calculating the loudness experienced by a typical (median) listener under the following conditions : 2.1 Diffuse sound field The sound is assumed to reach the listener's ears from all directions at the same intensity. This condition is approximated in an ordinary room. 2.2 Kind of spectrum The method is designed specifically for the types of broad band spectra most commonly encountered. Errors may arise if it is applied to certain types of line spectra, or to spectra having two or more sharp maxima that are separated by more than an octave. The magnitude of the error that may result from an unusual spectrum can only be determined experimentally. 2.3 Steady state The method is designed for noises that are steady rather than intermittent. Nevertheless, in a diffuse field, many noises of impulsive origin behave effectively as steady noises, because reverberation serves to diminish temporal discontinuities. NOTES 1 The error that may result with intermittent noises is not necessarily attributable to the calculation procedure. It may also be affected by the averaging characteristics of the meter used to determine the sound pressure levels. 2 Since the loudness of a sound depends upon the nature of the enclosure in which it is heard, it is important that comparative evaluations of different noise sources should be based on measurements made in essentially similar enclosures. 3 The band levels in the diffuse field should be measured by means of an omni-directional microphone located in the unobstructed sound field at the position of the listener's head.