If you work on projects in Europe, the Middle East, Africa, or Australia, you will encounter NR instead of NC as the standard for specifying background noise. NR — Noise Rating — is functionally similar to the American NC system but uses different curve shapes, extends to lower frequencies, and is referenced in ISO and European standards. Understanding NR is essential for any acoustic consultant working on international projects.
TLDR
NR (Noise Rating) is a single-number rating system for steady-state background noise in buildings, defined in ISO 717 and widely used across Europe, the Middle East, Africa, and Australasia. Like NC, NR uses a family of octave-band curves, but the NR curves span 31.5 Hz to 8 kHz (versus NC's 63 Hz to 8 kHz) and have a slightly different shape. A room's NR rating is the highest NR curve tangent to or exceeded by any measured octave band. NR-25 is typical for concert halls. NR-30 to NR-35 for classrooms and offices. NR-40 to NR-45 for commercial and retail spaces. NR values are typically 2 to 5 points higher than the equivalent NC rating for the same noise spectrum, because the NR curves are slightly more lenient in certain octave bands. The system was developed by Kosten and van Os in 1962 and remains the predominant European noise specification method.
Real-World Analogy
NR and NC are like Celsius and Fahrenheit for noise. They measure the same thing — background noise — using different scales with different curve shapes. Just as 20°C and 68°F describe the same temperature, NR-35 and NC-33 might describe approximately the same noise level. You need to know which scale the specification uses, just as you need to know whether a temperature is in Celsius or Fahrenheit, because the numbers are not interchangeable.
Technical Definition
The NR Curve Family
NR curves are defined across octave bands from 31.5 Hz to 8 kHz. Each curve is characterised by a number (NR-15, NR-20, NR-25, etc.) that corresponds to the sound pressure level at 1000 Hz. The curves slope downward from low to high frequencies, reflecting the ear's reduced sensitivity to low-frequency noise.
Selected NR curve values (sound pressure level in dB re 20 µPa):
| NR | 31.5 Hz | 63 Hz | 125 Hz | 250 Hz | 500 Hz | 1 kHz | 2 kHz | 4 kHz | 8 kHz |
|---|---|---|---|---|---|---|---|---|---|
| 15 | 55 | 44 | 35 | 29 | 24 | 20 | 17 | 14 | 13 |
| 25 | 62 | 51 | 44 | 38 | 34 | 30 | 27 | 25 | 23 |
| 35 | 68 | 59 | 52 | 47 | 43 | 40 | 37 | 35 | 33 |
| 45 | 75 | 66 | 60 | 56 | 53 | 50 | 47 | 45 | 43 |
Key Differences from NC
Frequency range: NR includes the 31.5 Hz octave band, which NC does not. This is relevant for very low-frequency noise from large HVAC equipment, industrial machinery, and traffic.
Curve shape: NR curves are slightly steeper than NC curves in the high-frequency octave bands. This means NR is marginally more permissive at high frequencies and stricter at low frequencies compared to NC at the same number.
Approximate conversion: For broadband noise, NR is approximately equal to NC + 2 to NC + 5, depending on the spectral shape. A precise conversion requires plotting the spectrum against both curve families.
How to Determine the NR Rating
The method is identical to NC:
- Measure background noise in octave bands (31.5 Hz to 8 kHz) using a calibrated sound level meter.
- Plot measured levels against the NR curve family.
- The NR rating equals the highest NR curve tangent to or exceeded by any octave band measurement.
- The controlling octave band identifies the dominant noise source.
NR Targets by Room Type
| Room Type | NR Target | Approximate NC Equivalent |
|---|---|---|
| Concert hall / recording studio | NR-15 to NR-20 | NC-13 to NC-18 |
| Classroom / lecture hall | NR-25 to NR-30 | NC-23 to NC-28 |
| Private office / hospital ward | NR-30 to NR-35 | NC-28 to NC-33 |
| Open-plan office | NR-35 to NR-40 | NC-33 to NC-38 |
| Restaurant / retail | NR-40 to NR-45 | NC-38 to NC-43 |
| Workshop / kitchen | NR-45 to NR-55 | NC-43 to NC-53 |
These targets appear in European standards, BS 8233:2014 (UK), DIN 4109:2018 (Germany), and building services guides from CIBSE (UK) and VDI (Germany).
Why It Matters for Design
On international projects, the noise specification will typically reference NR in Europe and NC in North America. Specifying the wrong system creates confusion and potential compliance failures. An NC-35 target is slightly quieter than an NR-35 target — designing to the wrong one could mean the building fails its commissioning noise test.
The 31.5 Hz octave band in NR is significant for buildings near industrial facilities, railways, or concert venues where very low-frequency noise (infrasound border) can be problematic. NC's absence of the 31.5 Hz band means these issues go uncaptured in the specification, which is a recognised limitation.
For buildings serving international clients — hotels, airports, embassies — acoustic specifications should explicitly state whether NR or NC is used, include the target value, and specify measurement positions and conditions. Ambiguity between the two systems has caused disputes on real projects.
How AcousPlan Uses This
AcousPlan supports both NR and NC curve analysis. When you input background noise levels, the platform calculates both the NC and NR ratings and displays the controlling bands for each. The results dashboard plots your measured or estimated levels against whichever curve family your project specification requires. For international projects, you can switch between NC and NR display to verify compliance under both systems.
Related Concepts
- What is an NC Rating? — The American equivalent
- What is an RC Rating? — Enhanced spectral quality criteria
- What is a Background Noise Survey? — How NR is measured
- Noise Criteria (NR/NC/RC) Explained — Side-by-side comparison
- What is STI? — How background noise affects intelligibility
Calculate Now
Plot your background noise against NR curves in AcousPlan. The platform shows the NR rating, controlling band, and compliance status for your room type.