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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.