Acoustic slatted panels: reading absorption ratings

At a glance

Clause references are to ISO 11654:1997, read in full in a publisher's free preview; Annexes B and C were not in it. NRC, SAA and Rw rows are pointers: no figures from those standards appear on this sheet.

Absorption is not insulation

A slatted or perforated timber panel is often chosen to make a room less echoey. That is absorption: it reduces reverberation inside the room by soaking up sound that reaches the surface. It is not insulation, which is how much sound passes through a wall or door (the Rw rating on our acoustic doors sheet). A panel that absorbs well can be a poor insulator, and the other way about.

Absorption is measured by putting a sample of the assembly in a reverberation room and timing how the sound decays. BS EN ISO 354:2003 is the measurement standard, which a trade listing summarises as a method for the sound absorption coefficient of materials used as wall or ceiling treatments; it is listed by BSI as current, under review. The result is a coefficient between 0 (all reflected) and 1 (all absorbed) in each frequency band. A test report belongs to the assembly as it was mounted: slats, gap, felt, backing and air space together. It does not belong to the timber.

How αw is found and what it hides

The standard rates absorption with one number so that a specification can set a target. The steps are worth knowing because they show what the number hides.

1. The test gives a coefficient at each one-third-octave band. Each three are averaged into an octave band, rounded to the nearest 0.05, and capped at 1.00. These are the practical coefficients, αp (clause 4.1).
2. A fixed reference curve is laid over the bands from 250 Hz to 4,000 Hz. Its unshifted values in the standard’s Figure 1 are 0.80 at 250 Hz, 1.00 at 500, 1,000 and 2,000 Hz, and 0.90 at 4,000 Hz.
3. The curve is moved down in steps of 0.05 until the sum of the amounts by which the product falls below the curve is 0.10 or less. Amounts above the curve do not count (clause 4.2).
4. αw is the value of the shifted curve at 500 Hz.
5. If any band is 0.25 or more above the shifted curve, a letter is added in brackets: L if it is at 250 Hz, M at 500 or 1,000 Hz, H at 2,000 or 4,000 Hz (clause 4.3). The standard says to add a sentence recommending the full curve be used with the number.

Worked example from the standard. Annex A gives an absorber with practical coefficients of 0.20, 0.35, 0.70, 0.65, 0.60 and 0.55 in the 125 to 4,000 Hz bands. Shifting the curve down by 0.40 puts it at 0.40, 0.60, 0.60, 0.60 and 0.50 for 250 to 4,000 Hz; the only shortfall is 0.05 at 250 Hz, so αw = 0.60. At a shift of 0.35 the shortfalls total 0.15, which is too many. A second absorber, identical except for a coefficient of 1.00 at 500 Hz, gets the same αw of 0.60, but with an M added, because its 500 Hz value is 0.40 above the curve. Two products with the same number can behave very differently, which is why the standard says to use the number with the full curve. The drawing shows the second case.

What the standard says it is for. The single number can be used for products in routine applications in normal offices, corridors, classrooms and hospitals. It is not appropriate in qualified environments needing careful acoustic design, where only complete absorption data are satisfactory. It does not apply unless the application covers the whole range of the reference curve, it is not appropriate below 250 Hz, and it is often not suitable for single items such as chairs and baffles (clause 1.2).

What changes the number: face, backing and air space

Three things set how much a slatted wall absorbs, and a test report names all of them. These are general acoustic principles, not figures from the standard.

• The face. The slats and the gaps between them let sound through to what is behind. A wider gap or a higher open fraction lets more through; a closed face reflects it. The slat width, gap and thickness are part of the product.
• What sits behind. Sound that passes the face is only absorbed if there is something to absorb it: a felt, fibre or foam layer, or a porous backing. Without one, the slats behave much like a solid surface.
• The air space. Depth between the absorbing layer and the hard wall behind it changes which frequencies are absorbed; more depth generally helps lower frequencies. ISO 11654 requires the depth of construction to be given for any product tested with an air space behind it, and notes that in Europe at least 200 mm is recommended (clause 5.4). The same panel fixed direct to a wall and fixed on battens is two different test specimens.

So a number from a test on a build-up with a deep cavity does not carry over to the same panel glued to plasterboard. The drawing shows two build-ups of the same layers with and without an air space. Our fluted, reeded and slatted profiles sheet covers slat and gap set-out, and our wall panelling styles sheet shows the slatted build-up.

Reading a test report

When you are shown a figure for a slatted panel, check it against this table before you use it.

If the partner workshop has no report for the exact build-up, there are three honest answers: a report for a closely similar build-up with the differences stated (and the designer or acoustician accepting them), a new test for this assembly, or a design that the acoustician calculates from absorber data for the felt and the cavity. A bare absorption figure for a product in another build-up is not evidence for this one.

Other requirements and the limits of this sheet

The absorption figure is one requirement among several. A slatted panel on a wall also has to meet the fire requirements for a wall lining, and the felt and the glue have their own emissions; see our fire performance and formaldehyde and VOC emissions sheets. Approved Document E and BS 8233 are named in our research file as the references for sound in buildings; neither was read for this sheet, and reverberation targets for a project are set by the project acoustician, not read from this sheet. We give no typical absorption figures for any build-up: a figure is only worth having when it comes from a report for the assembly in your specification.

Acoustic panelling ref [AP-01]: slats [18 x 45] mm at [24] mm gap on [6] mm felt on [12] mm board, [on 50 x 25 battens at 400 centres, forming a [50] mm air space / fixed direct], as detail [D-22]. The assembly is to have a reverberation-room test report to BS EN ISO 354 and a weighted coefficient to BS EN ISO 11654 of not less than αw [0.60], with practical coefficients for each octave band 250 to 4,000 Hz and any shape indicator stated. The report is for this build-up including the depth of construction, or is submitted with a written statement of every difference, accepted by [the acoustician] before ordering.

With the sample, the partner workshop submits the test report (laboratory, date, specimen description, mounting and depth, test area, αp table, αw with shape indicators) and the fire classification of the same build-up. Absorption figures for a different build-up are not accepted as a substitute.

Slat width, gap, thickness, felt, board, fixing and cavity depth are as tested. A change to any of them is a variation requiring [the acoustician]'s written approval and, where the report no longer applies, a new test.

Standards and sources

• BS EN ISO 11654:1997 (ISO 11654:1997): BSI Knowledge: Current, Under Review; published 15 Nov 1997. Full text of clauses 1 to 5 and Annex A read in a publisher's preview on 7 Nov 2026; Annexes B and C were not in it.
• BS EN ISO 354:2003: BSI Knowledge: Current, Under Review; published 9 Jun 2003. Listing only; the standard was not read.

Questions we are asked

What is a good αw for an acoustic panel?

There is no single answer and we give none. The target depends on the room, the other surfaces and the acoustician's brief. The standard also says the single number suits routine rooms such as offices, corridors and classrooms, and that a room needing careful acoustic design needs the full absorption data.

Is NRC the same as αw?

No. NRC is a United States single number and αw is the European rating from BS EN ISO 11654; they are calculated differently and the two should not be swapped in a specification. We have not read the US method, so we do not give its definition here.

What does the letter after αw mean?

A shape indicator. L, M or H after the number means the product absorbs at least 0.25 more than the shifted reference curve at low, mid or high frequencies, so the single number understates it in that range. Look at the full curve.

Why does the test report mention depth of construction?

Because the air space behind the panel changes the result, and ISO 11654 requires it to be stated whenever a product is tested with one. A panel tested on a deep cavity does not have the same rating fixed direct.

Can I take an absorption figure from a catalogue for my slatted panel?

Only if the catalogue's build-up matches yours: slat width, gap, felt, board, fixing and cavity. Otherwise get a report for your build-up or a written statement of the differences from your acoustician.

Does absorbing sound mean the wall blocks sound?

No. Absorption is about echo inside the room; insulation is about sound passing through a wall or door. See our acoustic doors sheet for insulation (Rw).