What is the meaning of noise reduction coefficient 0.8

It belongs to the medium sound absorption frequency. Noise reduction coefficient is the average value of the sound absorption coefficient (NRC) of the four frequencies of 250, 500, 1000, and 2000 (Hz) of the sound absorbing material. With 0.05 as the smallest multiple. Indicates a single value for the sound absorption characteristics of the material. Noise reduction coefficient NRC is equal to (α250 plus α500 plus α1000 plus α2000) divided by 4, where α250, α500, α1000, α2000 are the sound absorption coefficients of the material 250, 500, 1000, 2000 (Hz).