NR Curve

A Noise Rating (NR) curve is a standardized octave-band spectral contour used to rate and specify background noise levels in buildings. Defined in ISO 1996 and widely used in European and international practice, NR curves span octave bands from 31.5 Hz to 8000 Hz. The NR rating of a measured noise spectrum is determined by the highest NR curve that the spectrum touches or exceeds in any octave band. NR curves decrease with frequency, reflecting the higher sensitivity of human hearing to mid and high-frequency noise. Typical design targets include NR 25 for classrooms, NR 30–35 for open offices, NR 20–25 for concert halls, and NR 35–40 for industrial offices. The NR system gives more weight to low-frequency content than A-weighted levels, making it better suited for evaluating HVAC and mechanical noise. NR is the default criteria system in the UK (BS 8233:2014) and many European and Asian building codes.