Noise reduction coefficient

The noise reduction coefficient (commonly abbreviated NRC) is a single-number rating intended to describe the average sound absorption performance of a material, derived from reverberation-room measurements. In common usage it is reported on a scale from 0.0 (very low absorption) to 1.0 (very high absorption), though values greater than 1.0 can occur in reverberation-room testing due to measurement effects such as edge diffraction and non-ideal diffuse-field conditions rather than “more than 100%” absorption.

Source: Wikipedia — Noise reduction coefficient (CC BY-SA 4.0)