Sound reduction index (SRI)
The sound reduction index (SRI, or sometimes just R) is a measure of the reduction in the intensity of sound when it passes through part of a building; in other words, the level of sound insulation provided. It is the difference between the sound intensity that hits one side of an object (such as a wall, door, window, partition and so on) and the sound intensity measured on the other side, expressed in decibels (dB).
When sound hits the surface of a material, some will be reflected, some will be absorbed by the material and some will be transmitted through it. By determining the SRI, the effectiveness of the element as an insulator (the amount of sound that it has prevented from passing from one side to another) can be assessed.
SRI is defined in the series of international standards ISO 16283 (parts 1-3). Standard testing methods, both in laboratory and field environments, have been established for measuring SRI.
In the USA, the sound transmission class rating is typically measured as the Noise Reduction Coeffiecient (NRC). This measurement is in the process of transitioning to the Sound Absorption Average (SAA).
NB Approved document E: Resistance to the passage of sound, 2003 edition incorporating 2004, 2010, 2013 and 2015 amendments, defines sound reduction index (R) as: ‘A quantity, measured in a laboratory, which characterises the sound insulating properties of a material or building element in a stated frequency band. See BS EN ISO 140-3:1995.’
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