Sound Reduction Index (SRI) is a measure of the reduction in the intensity of sound when it crosses an interface.

The sound reduction index is generally in use to measure the level of sound insulation provided by a structure such as a wall, window, door, etc. It is used to rate the effectiveness of a system as a noise insulator. It is also termed as sound transmission loss.

When sound is incident upon the surface of a material, some of it will be reflected and some will be transmitted through the material. The fraction of incident energy transmitted is called the transmission coefficient. The sound reduction index is in turn defined in terms of the transmission coefficient. It equals to 10 times the common logarithm of the reciprocal of the sound transmission coefficient of the surface.

Units and Measures

The sound reduction index of an element is measured in a laboratory by placing the element in an opening between two adjacent reverberant rooms. Noise is introduced into the source room where part of the sound energy is transmitted through the test element into the receiving room. The resulting mean space-average sound pressure levels between 100 Hz to 5000 Hz in one-third-octave band in the source and receiving rooms are measured to determine the sound reduction index of the element.

It is measured in decibel (dB)


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