How to read acoustic panel ratings: test conditions, mounting and what "equivalent" really means.

This guide explains the ratings you will meet on data sheets, why the same board can score very differently, and how to check whether one panel can stand in for another. It uses our own test results as the worked example throughout.

The three numbers you will meet.

αw (weighted sound absorption coefficient). The UK and European rating, calculated to ISO 11654. It runs from 0 (absorbs nothing) to 1 (absorbs everything). The lab measures absorption across the frequency range, then fits a standard reference curve to the results. αw is where that curve sits at 500Hz.

You will sometimes see letters after it, such as αw 0.30 (MH). The letters flag where the panel does noticeably better than the curve: L for low frequencies, M for mid and H for high. They are a quick hint at the panel's character.

Absorption class (A to E). A band based on αw, also from ISO 11654.

NRC (noise reduction coefficient). The US rating, from ASTM C423, though it is also often calculated from ISO 354 data. It is a simple average of absorption at four frequencies (250, 500, 1000 and 2000Hz), rounded to the nearest 0.05.

Why they don't convert. αw and NRC are built differently. NRC averages four points. αw fits a curve and penalises weak spots. The same panel will often carry a higher NRC than αw. You cannot turn one into the other with a formula. To compare them properly you need the frequency data underneath.

Mounting changes the number.

In a lab, absorption is measured in a reverberation room to ISO 354. How the sample is mounted in that room has a large effect on the result.

Type A. The sample is laid flat on the floor of the test room, with its edges sealed. There is no air behind it. This represents a panel fixed flat to a wall or ceiling.

Type E (for example E-200). The sample is held above a sealed air gap. The number is the depth of the gap in millimetres, so E-200 means 200mm of air behind the panel. This represents a suspended ceiling tile or a panel on battens.

Suspended objects. Baffles and hanging panels are often tested as free objects in the room, with both faces exposed. Results are then usually given as absorption per object rather than per square metre of face.

Air behind a porous panel lets it absorb lower frequencies it would miss when fixed flat. So the same board tested at E-200 will almost always score higher than at Type A. Neither result is wrong. They describe different installations.

Think of a car tested on the road and on a test track. Both figures are accurate. They differ because the conditions differ, and each is the better guide for its own use. A flat Type A result is the road test for a panel fixed to a wall. A result with an air gap, or with the panel hung free, is the guide for a panel held off the surface. Set a road figure against a track figure and you learn nothing about the panels.

The rule: only compare two panels tested on the same mounting, and make sure that mounting matches how the panel will actually be installed.

Thickness and frequency.

Throw a pebble into a pool of water and the ripples travel cleanly across it. Throw the same pebble into a pool of spaghetti and they go nowhere, because the energy has no clean path through the tangle. PET felt is that tangle: a mat of fine fibres with air trapped between them. Sound travels in, loses its energy working through the fibres and fades to a trace of heat instead of bouncing back as echo.

Deeper sound carries longer waves, so it needs a deeper tangle to lose itself in. That is why thickness, or air behind the panel, matters most at the low end.

Thin porous panels work best on mid and high frequencies: voices, clatter and the harsh ring of hard rooms. Lower, deeper sound needs more depth. That depth can come from thicker material or from an air gap behind it.

Our two board thicknesses show this clearly, because both were tested on the same Type A mounting.

How our boards were tested.

Both boards were tested flat, with no air gap behind them (Type A, ISO 354). These are figures for the material itself, fixed direct.

Practical absorption coefficient (αp) by octave band. 12mm values as reported by the lab. 24mm values calculated by us from the lab's one-third octave results, using the ISO 11654 method.

Three things stand out.

• Doubling the thickness more than doubles absorption at 500Hz, from 0.25 to 0.60. Thicker board reaches further down the frequency range.
• Both boards are strong at high frequencies. Above 2000Hz, both absorb most of the sound that reaches them.
• The same board carries two different-looking numbers. The 24mm board is αw 0.50 and NRC 0.65. That's not a contradiction, just two methods applied to one test. It is why a data sheet quoting only NRC can't be compared directly with one quoting only αw.

We have not published results with an air gap behind the board. If your design puts panels on battens or over a void, performance will be different from the figures above, and usually higher at lower frequencies. Your acoustician can advise.

Fire. Both thicknesses are classified Class B-s1,d0 to BS EN 13501-1:2018. Classification reports are available on request.

Fire classification, decoded.

UK projects use the Euroclass system in BS EN 13501-1. A rating such as B-s1,d0 has three parts.

• The letter is reaction to fire, from A1 (non-combustible) to F. B means a very limited contribution to fire.
• s1, s2 or s3 is smoke production. s1 is the lowest.
• d0, d1 or d2 is flaming droplets. d0 means none.

B-s1,d0 is the top of the range for PET felt. A1 and A2 are reserved for non-combustible materials such as mineral products, so no PET panel can reach them. If a project needs A1 or A2, PET is not the answer at any price.

Watch for American ratings too. ASTM E84 "Class A" is a different system, and it does not mean Euroclass A.

A fire report covers the product that was tested. When comparing products, check that the classification covers the thickness you are buying.

Reading across between two products.

When one product was tested differently from another, work through these checks before comparing numbers.

1. Same standard? ISO 354 against ISO 354 is fair. An ASTM C423 result needs care.
2. Same mounting? Type A against Type A. A result at E-200 or with an air gap can't be set against a flat-fixed result.
3. Same thickness? Compare like for like, or accept that thickness is part of the difference.
4. Same rating? αw against αw, or NRC against NRC. Never one against the other.
5. Full curve if it matters. Two panels with the same headline number can behave very differently at 250Hz. Ask for octave-band data where low frequencies matter.
6. Fire class for the exact product. The classification should cover the thickness and product you are proposing.

If the conditions don't match and no matching test exists, the honest answer is that the two products can't yet be compared on paper.

Reading across to hung panels.

Hung rafts and baffles are often rated in sabins per panel rather than as a 0 to 1 coefficient. One sabin is one square metre of surface that absorbs all the sound reaching it. One rule links the two.

Sabins = absorption coefficient × area exposed

A 2400 x 1200mm panel has a face area of 2.88m². Our 24mm board, flat with no air gap, is NRC 0.65. So one panel fixed flat gives about 0.65 × 2.88 = 1.9 sabins. Divide a per-panel sabin figure by the face area to go the other way.

A hung panel can work out at more than 1.0 per square metre of face. Sound reaches both faces and the edges, so one panel absorbs more than its face area suggests. That is the mounting at work, not a better material. Any board gains the same way when hung.

Our figures are flat, one-face figures. We don't double them to guess a hung result. Compare sabins per panel with sabins per panel at the same hanging height.

Checking an "or equivalent" swap.

Specifications often name a product "or equivalent". When a contractor proposes a substitute, the job is to show it meets everything the named product was chosen for.

The evidence pack usually includes:

• the data sheet for the proposed product;
• the ISO 354 test report, stating the mounting;
• the fire classification report to BS EN 13501-1;
• recycled content or environmental information if the specification asks for it;
• physical samples for colour and finish;
• lead time and delivered price.

The most common reasons a swap fails are:

• a test mounting that doesn't match the installed detail;
• NRC set against αw;
• a fire report that doesn't cover the product supplied;
• a colour or finish the architect rejects.

What matters in a room at home.

Absorption panels change how a room sounds, not what gets through its walls. They soften echo and harshness inside the room. If sound is coming through from next door, that is a soundproofing job, which needs mass and isolation. See Shell, our soundproofing wall system, and the science of soundproofing and acoustics for why the two are different jobs.

For a room at home, three things matter more than the headline number:

• Coverage. More treated surface does more than a slightly better rating.
• Placement. Treat the hard surfaces that face each other, and the walls near where people sit and talk.
• Thickness. If voices boom rather than ring, go thicker.

Our wall panels and wall tiles are a decorative finish first. The absorption comes with them. For ceilings, see our ceiling rafts, ceiling baffles and the guide to the three types of acoustic ceiling.

Questions we get asked.

Is NRC 0.8 good?

It is strong at mid and high frequencies. Check how it was mounted. An NRC of 0.8 measured with an air gap is not the same as 0.8 measured flat.

What does Class C mean?

An αw between 0.60 and 0.75. It is a good all-round absorber for most rooms.

What is a Class A acoustic panel?

One with an αw of 0.90 or more. Most Class A results come from thick panels or panels tested with an air gap behind them.

Can I compare NRC with αw?

No. They are calculated differently. Compare like with like, or ask for the frequency data.

What does E-200 mean?

The panel was tested with a 200mm air gap behind it, which is typical of suspended ceiling tests. It will score higher than the same panel fixed flat.

What is a sabin?

One square metre of surface that absorbs all the sound reaching it. Multiply an absorption coefficient by the area exposed to get sabins. Hung panels are often rated in sabins per panel, which counts both faces and the edges, so a per-panel figure can't be set directly against a flat, per square metre one.

Is B-s1,d0 good enough for commercial fit-out?

It is the highest classification PET felt can reach and is widely used in commercial interiors. Where a project calls for A1 or A2, a non-combustible product is needed. Check the fire strategy for the building.

Can PET felt reach A2?

No. A1 and A2 are for non-combustible materials.

What do I need to submit for an "or equivalent" swap?

The data sheet, the ISO 354 test report with its mounting, the fire classification report, samples, and any environmental information the specification asks for.