Converting Noise Levels from NC to NR

http://www.engineeringtoolbox.com/nc-noise-criterion-d_725.html

I have silencer selections with target levels in NC. But project specification requires data in NR. How to convert the available NC levels to equivalent NR .

In quantifying loudness, you may use NC (Noise Criteria) or NR(Noise rating) or PNC(Preferred Noise Criteria) curves. Both are measured against Octave Band Frequency(HZ) and Octave Band Sound Pressure Level(SPL)dB consisting a family of curves across the eight octave bands. If you have the targeted NC, it must be accompanied with SPL and frequency. Reading off from both family curves will give you equivalent number.