Definition
NRC (Noise Reduction Coefficient) is a single-number rating (0.00 to 1.00) that represents how much sound a material absorbs, calculated as the arithmetic average of its sound absorption coefficients at the four mid-frequency octave bands: 250 Hz, 500 Hz, 1000 Hz, and 2000 Hz, rounded to the nearest 0.05.
An NRC of 0 means the material reflects essentially all incident sound. An NRC of 1.00 means the material absorbs essentially all incident sound across those four bands. Because it is a simple average over a limited frequency range, NRC is a convenient summary — but never a complete description of a material's acoustic performance.
What Does It Measure?
NRC measures sound absorption: the fraction of sound energy that strikes a material and is not reflected back into the room. Absorbed energy is converted into heat (a small amount) or passes into the material.
NRC describes how well a material reduces echo and reverberation inside a room. It does not measure how well a material blocks sound from passing through a wall or floor — that is a different quantity (STC or Rw).
To be precise, NRC is one of several related but distinct quantities:
- Sound Absorption Coefficient (α) — the frequency-dependent ratio of absorbed to incident sound energy, from 0.00 to 1.00 (values can slightly exceed 1.00 in laboratory tests due to edge and diffraction effects).
- NRC — the single-number average of α at 250, 500, 1000 and 2000 Hz.
- SAA (Sound Absorption Average) — the average of α over the twelve one-third-octave bands from 200 to 2500 Hz (a newer, wider-band single number defined in ASTM C423).
- αw (weighted sound absorption coefficient) — the single-number rating defined in ISO 11654, derived over 125–4000 Hz and expressed with an absorption class (A–E).
Units
NRC is dimensionless. It is written as a decimal between 0.00 and 1.00 (for example, 0.75, 0.90, 1.00). It is not a percentage, even though it is often read that way informally.
How It Is Determined
NRC is derived from laboratory measurement of the sound absorption coefficient in a reverberation room — a large, hard-surfaced room designed to reflect sound for a long time.
Absorption is measured at each standard octave band (and one-third-octave bands), and NRC is then calculated as:
NRC = (α250 + α500 + α1000 + α2000) / 4 (rounded to nearest 0.05)
Relevant Standards
- ASTM C423 — Standard Test Method for Sound Absorption and Sound Absorption Coefficients by the Reverberation Room Method (the reference for NRC and SAA).
- ISO 354 — Acoustics — Measurement of sound absorption in a reverberation room (the international equivalent test method).
- ISO 11654 / EN ISO 11654 — Sound absorbers for use in buildings — Rating of sound absorption (defines αw and the A–E absorption classes).
These standards are the only references used here for absorption testing. No other standards apply to NRC directly.
Test Method (Simplified)
- Test environment: a reverberation room — a large enclosed space (typically 200 m³ or more) with hard, sound-reflective surfaces.
- Test specimen/system: a sample of the material of a specified area (commonly around 10 m²), installed in a defined mounting configuration.
- Measurement principle: the sound decay rate (reverberation time) of the room is measured with and without the specimen. The faster the sound decays, the more energy the specimen absorbed.
- Main output: the sound absorption coefficient (α) per frequency band, from which NRC (or SAA) is calculated.
This summary is for general understanding only — it does not replace the standard itself.
What Affects the Result?
The measured absorption — and therefore the NRC — depends strongly on how the material is installed and tested:
- Frequency — most absorbers work better at mid and high frequencies than at low frequencies.
- Thickness — thicker material generally absorbs more low-frequency sound.
- Density — affects which frequencies are absorbed.
- Mounting — direct-to-wall versus suspended, and the specific mounting type used in the test (ASTM C423 defines several mounting configurations).
- Air gap — leaving an air space behind a panel can improve low-frequency absorption.
- Surface area — edge effects mean the tested sample area and shape influence the measured coefficient.
This is why a single NRC number must always be read together with its test configuration.
What It Does NOT Tell You
- NRC does not equal soundproofing. A material with a high NRC can have almost no ability to block sound through a wall (low STC/Rw).
- NRC does not describe low-frequency performance (below 250 Hz) or the highest frequencies (above 2000 Hz).
- "NRC 0.90" does not mean "90% of all sound is absorbed." It is an average over four specific bands.
- NRC does not predict reverberation time (RT60) by itself — that also depends on room volume and total absorbing area.
Formula
The only formula used is the averaging formula shown above. There is no additional physical formula needed to understand NRC.
Practical Example
A panel with NRC 0.75 means that, on average across 250–2000 Hz, it absorbs 75% of the sound energy that strikes it and reflects 25%. For a room treatment calculation, total absorption is approximated by summing (NRC × surface area) over all treated surfaces — the result is expressed in Sabins. For example, 10 m² of a material with NRC 0.75 contributes about 7.5 Sabins of absorption.
Common Misunderstandings
- "NRC 0.90 means 90% of all sound is absorbed" — incorrect; it is an average over four mid-frequency bands, and says nothing about low frequencies.
- "High NRC means good soundproofing" — incorrect; absorption and insulation are different properties.
- "NRC is the percentage of noise removed from the room" — incorrect; NRC describes absorption at a surface, not a room-wide noise reduction.
- "NRC and STC measure the same thing" — incorrect; STC measures sound transmission between spaces.
Limitations
- NRC averages only four mid-frequency bands; it can hide poor low-frequency or high-frequency performance.
- Laboratory NRC values are measured under specific mounting and air-gap conditions that may not match a real installation.
- NRC can slightly exceed 1.00 in laboratory tests because of edge and diffraction effects — an apparent value above 1.00 is not a measurement error, but it is not directly physical.
- NRC alone is insufficient for choosing treatment for low-frequency or speech-privacy problems.
Related Yamamah Solutions
The following product families are designed for sound absorption (their NRC performance is stated in each product's technical documentation and test reports, not as a general value here):
For help selecting the right absorption material, contact us for a free acoustic consultation.
See also the Sound Absorption vs Sound Insulation hub.
Read more in the four-part acoustic fundamentals series: What Is RT60? · What Is Rw? · What Is STC? — and see Best Sound Absorption Materials.
Verified Sources
- ASTM C423 — Standard Test Method for Sound Absorption and Sound Absorption Coefficients by the Reverberation Room Method.
- ISO 354 — Acoustics — Measurement of sound absorption in a reverberation room.
- ISO 11654 / EN ISO 11654 — Rating of sound absorption (αw, absorption classes).
Frequently Asked Questions
What is a good NRC rating? For most interior spaces, an NRC of 0.60–0.75 is considered good for general echo control; 0.85 and above is high-performance absorption. The right target depends on the room's use.
Is a higher NRC always better? Not necessarily. Over-absorbing a room can make it feel "dead." The correct amount of absorption depends on the space and its function.
Does NRC block sound between rooms? No. NRC describes absorption inside a room. Blocking sound between rooms is described by STC or Rw.
Why do some materials show NRC above 1.00? Laboratory values can slightly exceed 1.00 due to edge and diffraction effects. This does not mean the material absorbs more than 100% of energy in practice.