Sound reduction index
The sound reduction index is used to measure the level of sound insulation provided by a structure such as a wall, window, door, or ventilator. It is defined in the series of international standards ISO 16283 (parts 1-3) and the older ISO 140 (parts 1-14), or the regional or national variants on these standards. In the United States, the sound transmission class rating is generally used instead. The basic method for both the actual measurements and the mathematical calculations behind both standards is similar, however they diverge to a significant degree in the detail, and in the numerical results produced.
Standardized methods exist for measuring the sound insulation produced by various structures in both laboratory and field (actual functional buildings and building sites) environments. A number of indexes are defined which each offer various benefits for different situations.
The most basic index is the Weighted Difference level Dw. This index is defined by measuring in decibels (dB), the noise level produced on each side of a building element under test (e.g. a wall) when noise is produced in a room on one side (or outdoors) and measured both in the room where the noise is produced and in the room on the other side of the element under test.
This measurement may be carried out by measuring the levels in octave bands, or in 1/3 octave bands. (the latter is normally used for most applications). The minimum requirements of the standards require for the frequency range from 100 Hz to 3.15 kHz to be measured (16 1⁄3 octave bands). In some situations measurements may be carried out in the bands down to 50 Hz and/or up to 10 kHz.
The measured levels in each 1/3 octave band (or octave band) from the source room (or area) (S) are then compared to the measured levels in the receiving room (R), and the difference is taken (S-R). this produces a measured difference level 'D' for each frequency band in the measured spectrum.
To produce a single integer number the measured spectrum is plotted on a graph, and compared against a reference curve (defined in ISO 717-1 for airborne sound insulation, and 717-2 for impact sound insulation). The reference curve is moved in 1 dB steps until the total of the unfavorable deviations (measured points on the graph below the reference graph) is as close to 32 as possible but not greater than 32.
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