These single-number ratings correlate in a general way with subjective impressions of sound transmission for speech, radio, television, and similar sources of noise in offices and buildings. This classification method is not appropriate for sound sources with spectra significantly different from those sources listed above. Such sources include machinery, industrial processes, bowling allies, power transformers, musical instruments, many music systems, and transportation noises such as motor vehicles, aircraft and trains. For these sources, accurate assessment of sound transmission requires a detailed analysis in frequency bands. A single-number sound transmission rating for building fagade elements is given in Classification E 1332.

The single-number ratings obtained can be used to compare the potential sound insulation of partitions or floors tested in laboratory conditions (STC) or the actual sound isolation between different suites in buildings (NNIC, NIC). The rating for a partition built and tested in a building may be lower than that obtained for a partition tested in a laboratory because of flanking transmission or construction errors.

Note 1-A similar rating procedure, described in ISO 717, provides single figure sound insulation ratings with a frequency range that extends from 100 to 3150 Hz with no maximum deficiency specified at individual frequencies. For most partitions, the two ratings differ by only one or two points.