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ACOUSTIC PANEL REFERENCE

Acoustic Panel NRC Rating Chart: What the Values Mean

NRC is the average of four octave bands — 250, 500, 1,000 and 2,000 Hz — measured under ASTM C423 and rounded to the nearest 0.05. It leaves out 125 Hz, it changes with the mounting, and it says nothing about blocking sound between rooms.

The same panel can carry more than one valid NRC depending on how it is fixed — and the single number hides the range where most complaints begin. This page sets out what the chart actually averages, how much mounting moves it, and how NRC relates to the European absorption classes.

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NRC REFERENCE

What the Acoustic Panel NRC Rating Chart Actually Measures

The NRC (Noise Reduction Coefficient) is a single-number rating that averages how much sound a surface absorbs at four octave bands: 250 Hz, 500 Hz, 1,000 Hz, and 2,000 Hz. It is measured in a reverberation room under ASTM C423 and rounded to the nearest 0.05.

01

Covers the Speech Range, Not the Whole Spectrum

NRC uses four of the six bands a datasheet normally prints. Speech sits mostly between 500 Hz and 4,000 Hz, which is why NRC describes speech problems well — and music, machinery, and traffic noise poorly.

02

Reported in 0.05 Steps

Because of the rounding, a figure published as “NRC 0.83” or “NRC 0.87” is not an NRC. It is usually the Sound Absorption Average, a different metric reported to 0.01. That single detail tells you which number a supplier is quoting.

03

Can Exceed 1.00

Edge diffraction makes a specimen behave as though it has more absorbing area than its face, so values above 1.00 appear legitimately. ISO 354 acknowledges this, and design guidance is to round down to no more than 0.99.

What the number is — and is not

NRC is an average of four readings, not a description of the material. Two panels with the same NRC can absorb very differently in a specific room, and the rounded value hides which bands are doing the work. Where a project has a real acoustic target, ask for the full per-band absorption data alongside the single number.

WORKED EXAMPLE

How a Panel Reaches NRC 0.85: A Worked Example

The clearest way to see what the average hides is to watch one being calculated. The row below is a published datasheet for a fabric-wrapped absorptive panel, converted to metric thickness.

PUBLISHED DATASHEET

Two nominal thicknesses of the same panel are shown, with their absorption coefficient at each of the six printed bands and the single NRC value the datasheet reports. Only the four central bands enter the NRC average.

ABSORPTION DATA

Datasheet Absorption by Octave Band

Fabric-wrapped panel
Nominal thickness 125 Hz 250 Hz 500 Hz 1,000 Hz 2,000 Hz 4,000 Hz NRC
25 mm 0.14 0.27 0.80 1.11 1.14 1.14 0.85
50 mm 0.40 0.74 1.24 1.25 1.22 1.14 1.10
01

NRC Uses Four of Six Bands

Of the six bands a datasheet normally prints, only 250, 500, 1,000 and 2,000 Hz enter the NRC average. The 125 Hz and 4,000 Hz figures are shown for context — they never count.

02

0.14 at 125 Hz Never Enters

The same panel absorbs 0.14 at 125 Hz and 1.14 at 2,000 Hz, yet the chart reports one figure. The 125 Hz number is entirely absent from the calculation.

THE CALCULATION

Average the Four Bands That Count

Take the four bands NRC uses and average them. The result is then rounded to the nearest 0.05.

Band Value
250 Hz 0.27
500 Hz 0.80
1,000 Hz 1.11
2,000 Hz 1.14
Sum of four 3.32
Divide by 4 0.83
Rounds to NRC 0.85
WHICH BANDS COUNT
0.14
125
0.27
250
0.80
500
1.11
1k
1.14
2k
1.14
4k
Counts in NRC
Excluded from NRC

For speech, the 125 Hz figure hardly matters. For HVAC rumble, road noise, or musical bass, it is the number that decides whether the room works. That is the range NRC was never designed to describe, and it is why a single-number chart is not enough for a project with a low-frequency problem.

MOUNTING MATTERS

The Mounting Decides Your NRC, Not Just the Panel

This is the part of the chart most suppliers leave out. Under ASTM practice, a test specimen is fixed in one of several standard ways, and the mounting moves the result more than most finish decisions do.

AIR SPACE BEHIND THE PANEL

Type A · B · C · D · E-400

ASTM mounting types Bigger gap → higher NRC
Type A

Laid on the Floor

Specimen laid directly on the chamber floor.

Lowest NRC
Type B

Bonded to 25 mm Board

Specimen bonded to a 25 mm rigid board, no air space.

Low NRC
Type C

Bonded to 12.5 mm Board

Specimen bonded to a 12.5 mm rigid board, no air space.

Low NRC
Type D

Grid Supported

Specimen supported on a grid, with a 400 mm air space.

Higher NRC
Type E-400

Grid Suspended

Specimen laid in a grid and suspended, with a 400 mm air space.

Highest NRC
THREE PUBLISHED EXAMPLES

The Swing Between Mounting Conditions

All three examples come from third-party product data. Same panel, different mounting, different result.

12 mm PET Panel

Direct-mounted vs 20 mm air gap

Direct 0.45
20 mm gap 0.85
25 mm Polyester Panel

Absorption at 500 Hz by air gap

Direct 0.45
200 mm gap 0.90
Datasheet Quoted Panel

NRC with a 200 mm air gap vs direct

Direct 0.55
200 mm gap 0.95
THE MECHANISM

An Air Space Behaves as a Quarter-Wavelength Resonator

Absorption peaks when the gap behind the panel is about a quarter of the wavelength. That is why a gap lifts low-frequency performance most, and why a panel glued flat to a wall loses exactly the bands the NRC already ignores.

Ask for the mounting with the number. If your installation is a direct fix onto plasterboard, a datasheet figure measured at E-400 does not describe your wall.

Request the Fixing Detail
FOUR RATINGS · NO CONVERSION

NRC, SAA, αw, and STC: Four Numbers That Do Not Convert

An absorption or acoustic statement can be written in at least four different measurements, and they cannot be derived from one another.

READ THE SCALE BEFORE YOU COMPARE

NRC, SAA and αw all describe absorption, but each averages different bands on different steps. STC does not describe absorption at all. A figure carried across scales is a guess, not a conversion.

COMPARISON

Rating, Standard, Scale and Coverage

Four Systems, Four Scales
Rating Standard What It Averages or Converts Scale Covers 125 Hz?
NRC ASTM C423 Four octave bands: 250, 500, 1,000, 2,000 Hz 0.00 – 1.00 · 0.05 steps No
SAA ASTM C423 Twelve one-third-octave bands, 200 to 2,500 Hz 0.00 – 1.00 · 0.01 steps No
αw ISO 11654,
from ISO 354 data 		Octave-band data fitted to a reference curve Class A – E No
STC ASTM E90,
ASTM E413 		Transmission loss across a partition Decibels N/A
EUROPEAN RATING

αw Is the European Rating, and It Is Not NRC

NRC is a United States metric. Much of the world specifies absorption as αw, a weighted rating under ISO 11654 derived from ISO 354 laboratory data. It is expressed as a class, not a decimal.

A 0.90 and above
B 0.80 – 0.85
C 0.60 – 0.75
D 0.30 – 0.55
E 0.15 – 0.25
— 0.10 and below
  • An NRC at or above 0.75 usually lands around classes A to B, so the two are broadly comparable in practice. They do not formally convert.
  • The rating curve’s lower limit is the 250 Hz octave band, so αw says nothing below that frequency.
  • ISO 11654 states it is not appropriate for qualified environments requiring careful acoustic design — where only the complete frequency-dependent data is satisfactory.
DIFFERENT PROBLEM

STC Measures Blocking, Not Absorption

NRC changes how sound behaves inside a room. STC governs how much of it passes into the next one, and it depends on mass, structural decoupling, and airtightness rather than on surface absorption.

SEPARATE PROBLEMS

A Panel With NRC 0.85 Will Not Stop a Neighbour Hearing a Conversation

The two ratings describe different physical behaviour. A product cannot solve one with the other — treat absorption and transmission as independent questions, each with its own measurement and its own specification route.

SPACE REFERENCE

What NRC Your Space Needs

An NRC with no target attached is half an answer. Published design guidance gives these bands — pair the rating with a reverberation target, and the space becomes specifiable.

HOW TO USE THIS TABLE

Each row gives an NRC target and a reverberation target (RT60) for the same space. The two are specified together — a rating without a coverage and volume check is only half a specification.

SPACE BY TARGET

NRC and RT60 Target by Space Type

Published Design Guidance
Space Target NRC Reverberation Target (RT60)
Open-plan office 0.60 – 0.80 minimum 0.80 or above preferred 0.4 – 0.6 seconds furnished and occupied
Meeting room 0.80 or above 0.4 – 0.6 seconds
Restaurant or cafeteria 0.70 – 1.10 by the atmosphere intended 0.5 – 0.8 seconds
Classroom 0.70 or above Per the local education acoustic requirement
Corridor or lobby 0.60 – 0.80 Not usually specified
01

Coverage Matters as Much as the Rating

A high-NRC panel over a small area will not reach the reverberation target. Open-plan ceiling treatments are commonly quoted at roughly 50 to 80 percent coverage — the rating is only useful once the coverage is settled.

02

Absorption and Volume Work Together

Run the panel area and the room volume through the acoustic panel RT60 calculator before the specification is issued. Same panel, different room volume, different result.

NRC RATING FAQ

Frequently Asked Questions

Common questions about what NRC measures, how it relates to soundproofing, how mounting changes it, and what value a space actually needs.

It means the four octave bands NRC uses, 250 Hz through 2,000 Hz, average 0.85 after rounding to the nearest 0.05 under ASTM C423.

It is an average of four readings, not a measurement of the panel. The individual bands behind it can range from under 0.20 to over 1.10.

No, because NRC does not measure blocking. It measures how much sound is absorbed inside the room, so a high NRC reduces echo and reverberation.

Soundproofing depends on mass, decoupling, and airtightness instead — rated in STC or Rw, and unrelated to the NRC.

Yes, substantially. A 12 mm PET panel is published at NRC 0.45 direct-mounted and NRC 0.85 on a 20 mm air gap. The air space acts as a resonator and lifts absorption, especially at low frequencies.

Always ask for the mounting condition alongside the figure.

Open-plan offices are commonly specified to NRC 0.60 to 0.80, with 0.80 or above preferred, alongside a reverberation target of 0.4 to 0.6 seconds in the furnished, occupied room. Meeting rooms usually need 0.80 or above.

The rating alone is not enough — coverage matters as much.