ISO 532 B loudness charts span five maximal band levels and two sound-field types

Evidence

This loudness calculation procedure is part of an international standard (ISO 532 B) . Its use may be illustrated in detail by a noise produced in a wood factory. The unweighted sound level of the noise produced by a machine is 73 dB, the A-weighted sound pressure level is measured to 68 dB(A). All the thin lines plotted in Fig. 8.24 belong to the normalized patterns of the loudness calculation procedure that are available for five different maximal levels of third-octave-bands (35, 50, 70, 90, and 110 dB) and for two kinds of sound field (free and diffuse). The optimal choice for using such a graph is to select that which just contains the largest third-octave-band level measured in a spectrum. In factory halls, a diffuse field may be a better approximation than a free field. Very close to the sound source, however, or in an area with very little reffection, a free-field condition may be assumed. — Professor Dr.-Ing. Hugo Fastl, Professor Dr.-Ing. Eberhard, Psychoacoustics- Facts and Models, 1321