NR Curve (Noise Rating)

NR curves were standardized by the International Organization for Standardization and are the predominant background noise rating method in Europe, the Middle East, and much of Asia. While NC curves start at 63 Hz, NR curves extend down to 31.5 Hz, providing better low-frequency assessment for modern buildings where HVAC low-frequency noise is increasingly problematic. The NR curve family defines octave-band SPL limits from NR-0 to NR-100. Like NC curves, they slope downward with frequency but with slightly different shape. At 1 kHz, an NR value approximately equals the SPL in dB; at low frequencies, the NR curve permits higher SPL, reflecting reduced human sensitivity. European building regulations frequently reference NR values. Typical specifications: NR-20 for concert halls and hospitals, NR-25 for offices and libraries, NR-30 for restaurants and retail, NR-35 for sports facilities and workshops. The UK Building Bulletin 93 specifies NR-25 for classrooms (equivalent to about NC-25 to NC-28). The key difference between NR and NC curves is their shape in the low-frequency region. NR curves are slightly more permissive below 125 Hz and slightly stricter above 2 kHz compared to NC curves. This means a noise spectrum might rate NC-30 but NR-32, or vice versa, depending on its spectral content. NR curves share the limitations of NC curves: they do not penalize tonal noise, they rate based on the worst single band (potentially over-penalizing an otherwise quiet spectrum because of one problematic band), and they do not distinguish between different types of noise (broadband hiss vs. tonal hum). More modern methods like RC (Room Criteria) and RNC (Room Noise Criteria) attempt to address these shortcomings. SonaVyx computes both NC and NR ratings simultaneously from octave-band measurements, enabling compliance checking against any regional standard.