Behind a wood-slat acoustic panel the visible slats are largely reflective — the absorptive backing does the acoustic work, so the choice of backer matters more than the timber. Acoustic felt made from recycled PET is decorative, self-supporting and gives moderate broadband absorption, while mineral wool typically absorbs more, especially with thickness or an air gap behind it, which is why higher absorption classes usually rely on it. The two also behave differently in fire: mineral wool is an inorganic, generally non-combustible fibre, whereas PET felt is a thermoplastic whose reaction-to-fire class depends on the product and any treatment. Whichever you use, an absorption figure only means something when it is published against an ISO 354 test report for that exact build-up and mounting.
Which backing absorbs more, felt or mineral wool?
In most build-ups mineral wool absorbs more sound than acoustic felt, particularly across the low and mid frequencies, and particularly when it has real thickness or an air gap behind it. That extra depth is what tends to push a panel into the higher sound absorption classes. Recycled-PET felt gives useful, fairly even broadband absorption for its thinness, but it has less mass and depth to work with, so on its own it usually sits lower on the class scale.
Neither statement is a fixed rule, because absorption is a property of the whole construction — material, thickness, facing and the void behind it — not of the fibre alone. A thick felt with a deep cavity can outperform a thin wool board fixed hard to a wall. The only honest way to compare the two is against the same ISO 354 test at the mounting you will actually install.
The backing is the acoustic part, not the wood
On a wood-slat acoustic panel the timber slats are largely reflective; the sound energy is absorbed in the backer and in the gaps between the slats. That is why the backing material — felt or mineral wool — decides how the panel performs acoustically, while the wood mainly decides how it looks and wears. Change the backer and you change the acoustics, even with identical slats on the front. The mechanics are set out in how acoustic panels work.
It also explains why a slat panel and a plain fabric-faced absorber can behave very differently despite similar-sounding claims — the comparison in slat wall vs acoustic panels turns almost entirely on what sits behind the face. So read the build-up, not the surface.
Acoustic felt and recycled PET: decorative and self-supporting
Acoustic felt is typically a pressed board of recycled PET (polyester) fibre. Its appeal is practical: it is rigid enough to be self-supporting, comes in a range of colours, and presents a clean, hard-wearing face — so it doubles as a finished surface as well as an absorber. For lighter echo control in offices, studios and retail it often does enough on its own, and its recycled content suits sustainability-led specifications.
The honest caveat is depth and fire. Being relatively thin, PET felt has a ceiling on how much low-frequency energy it can absorb; and, as a thermoplastic, its reaction to fire depends on the specific product and any fire-retardant treatment, so the Euroclass must be read from the test report rather than assumed. See Euroclass reaction to fire explained for what those letters, smoke and droplet ratings actually mean.
Mineral wool: more absorption, different fire behaviour
Mineral wool — stone or glass fibre — brings more mass and thickness, so it generally absorbs across a broader range and reaches further into the low and mid frequencies, especially when set forward with an air gap. That is why panels aiming for a high absorption class so often rely on a mineral-wool backer rather than felt alone. Because the fibre is loose and open, it needs a facing tissue or the slat panel in front of it to hold its shape and stay clean.
Its fire behaviour is also different in kind: mineral wool is an inorganic, mineral-based fibre that is generally non-combustible, which is often decisive in commercial and public buildings. Even so, the performance that counts is the Euroclass of the whole assembled panel — treated timber slats included — evidenced by a test, not the backer considered in isolation.
So which acoustic backing should you choose?
It follows from two questions: what absorption class does the room genuinely need, and what reaction-to-fire class does the setting demand? If the target is light-to-moderate echo control with a decorative, low-maintenance finish and no onerous fire requirement, recycled-PET felt often does the job on its own. If the room needs a lot of absorption per square metre, or must reach a higher class, a mineral-wool backer — with depth or an air gap — is usually the more efficient route, and its non-combustible core helps where fire matters.
In every case the deciding evidence is the same: a figure only counts when it is published against a test report for that exact build-up and mounting. Specify the backer against the room's real absorption and fire targets, then confirm both numbers on the report before you commit — proven, not promised.
Frequently asked questions
Is felt or mineral wool better for acoustic panels?
Neither is simply better; they suit different targets. Mineral wool generally absorbs more, especially with thickness or an air gap, and reaches higher absorption classes, while recycled-PET felt is thinner but self-supporting, decorative and easy to specify for lighter echo control. Choose by the room's target absorption class and any fire requirement, and confirm the figure against the panel's ISO 354 test report.
Does the wood or the backing do the absorbing?
The backing. On a wood-slat panel the timber slats are largely reflective; the absorption happens in the backer and the gaps between the slats. That means the choice between felt and mineral wool, plus the thickness and any air gap behind it, decides the acoustic performance — the wood mainly sets the appearance and durability.
Is recycled PET felt fire-rated?
Not automatically. PET felt is a thermoplastic, so its reaction-to-fire class depends on the specific product and any fire-retardant treatment. Never assume a Euroclass; read it from the test report for the finished panel. In commercial or public buildings, check the rating of the whole assembly, slats included, against the fire requirement for that space.
Does an air gap behind the backing improve absorption?
Usually, yes. Setting an absorptive backer forward on battens with an air gap behind it improves absorption, especially at low and mid frequencies, for both felt and mineral wool. But the gain only applies to the exact build-up tested, so any quoted figure must come from an ISO 354 report at the same mounting you intend to use.