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 140 (parts 1-14), or the regional or national variants on these standards. In the United States, the sound transmission class rating is generally ...
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http://en.wikipedia.org/wiki/Sound_reduction_index
of a partition, for a specified frequency band, difference in decibels between the average sound pressure levels in the reverberant source and receiving rooms, plus ten times the logarithm to the base ten of the ratio of the area of the common partition to the total Sabine absorption in the receiving room
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http://www.electropedia.org/iev/iev.nsf/display?openform&ievref=801-31-39
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