Strutt|Criteria|Ratings|Sound Level|RNC inserts the Room Noise Criterion (RNC) rating of the current row of the worksheet into Column D of the active row.
The RNC method is recommended to be used in cases where the services noise includes fluctuations or 'surging' at low-frequencies. Refer Section 5.3.2 of ANSI S12.2 for objective tests for surging/fluctuations
Reference: ANSI/ASA Standard S12.2 (2008) Criteria for Evaluating Room Noise