Strutt|Criteria|Ratings|Absorption|NRC calculates the Noise Reduction Coefficient (NRC) of the current row of the worksheet
NRC is calculated as:
`NRC=(alpha_(250)+alpha_(500)+alpha_(1k)+alpha_(2k))/4`
where:
`alpha_(250,500,1k,2k)` is the absorption coefficient in the 250 Hz, 500 Hz, 1 kHz and 2kHz octave bands, respectively.