Sound Reduction Index (SRI)
In countries outside the USA, SRI (Sound Reduction Index) is a more common measurement. Typically, companies offering soundproofing products will suggest they can achieve an improvement of so many decibels (dB) sound reduction or SRI. Everyday materials have widely differing SRIs. A thin plane of glass would achieve about 20–25 dB, light concrete slabs would be about 40 dB, while two brick walls separated by a large air cavity would cut noise by 60–75 dB. Like STC, SRI measurements are highly dependent on sound frequencies: a material that gives a considerable improvement in sound insulation for human speech (cutting out conversation from your neighbors upstairs) is likely to be much less effective at cutting lower sound frequencies (so you may still hear the deep bass of their stereo).