Acoustic underlay: the layer that deserves a closer look

Underlay disappears when the floor is finished. Its specification should be settled long before that happens.

Begin with the board or tile you intend to use, check what the maker permits beneath it, then compare evidence for that complete construction. A thicker roll or a higher star claim is not enough on its own.

What is acoustic underlay and what does it do?

Acoustic underlay is a resilient layer, usually foam, rubber, cork, felt or a blend, laid between the floor finish and the slab. When a heel hits a hard floor, the impact shakes the floor and the slab, and the slab radiates that vibration as sound into the room below. The underlay separates the finish from the slab so less vibration gets through.

The ABCB’s sound insulation handbook puts it plainly: structure-borne sound “cannot be attenuated by barriers or walls but requires the isolation of the vibration itself”, and isolation improves with “a resilient element such as rubber”. That is the underlay’s whole job.

Does acoustic underlay stop airborne noise?

Not much. Airborne noise, such as voices and television, travels through the air. The ABCB handbook says added mass “is particularly effective in reducing airborne noise”, and an underlay adds very little: REGUPOL’s 5 mm sonus eco 5 weighs 2.2 kg per square metre (data sheet), while the 150 mm slab in a CSIRO test weighed about 360 kg per square metre.

Underlay also works best on higher-pitched impact noise. In that CSIRO test of 14 mm engineered boards on 2 mm foam, the improvement was about 3 dB at 100 Hz but 19 dB at 1,000 Hz and 38 dB at 2,000 Hz. Heel clicks drop a lot; the low thud of footfall drops much less.

How is the improvement measured?

Underlay makers quote ΔLw (delta L w). A laboratory lays the floor and underlay on a bare, heavy concrete slab and runs a standard tapping machine on top, under ISO 10140-3. The sound in the room below is rated under ISO 717-2, which defines ΔLw as the reduction measured on the bare heavy reference floor, with and without the covering. Higher ΔLw is better. The same test gives Ln,w, the level through the tested floor, where lower is better.

Your by-law may ask for a different measurement: L’nT,w, measured on site between your unit and the one below. The ISO 10140-3 scope warns that laboratory results “are not applicable directly to the field situation” without accounting for flanking paths and other factors.

Two real documents show the gap. In the CSIRO lab, 2 mm foam under engineered boards on a bare 150 mm slab gave Ln,w 61 dB. In a Sydney apartment with a 200 mm slab and a plasterboard ceiling, a 2 mm foam from another brand under similar floors measured L’nT,w 41 to 43, but that bare slab and ceiling already measured 55 (Koikas Acoustics). The building did much of the work. Our acoustic ratings guide explains every number in full.

Acoustic underlay thickness: 2 mm to 12 mm

Sources: REGUPOL test reports, DAMTEC brochure, Damtec Australia, Amorim T66, AngelStep GOLD8 and Uniroll.

Is thicker acoustic underlay always quieter?

No. Damtec Australia lists DAMTEC Color at ΔLw 18 dB for 3 mm and 19 dB for 5 mm: two more millimetres, one more decibel. Under 8 mm laminate on 150 mm lab slabs, REGUPOL’s 4.5 mm and two 5 mm products all scored ΔLw 19 dB. Under floating engineered boards, a 2 mm foam (18 dB) and a 5 mm rubber and cork mat (17 dB) finished within a decibel, though in different tests. Material, stiffness and the floor on top matter as much as thickness. Floating floor makers also cap thickness: Preference and Signature both warn that extra or soft underlay can compromise the click joints.

What do 5 star and 6 star acoustic underlay claims mean?

The stars come from the AAAC Guideline for Apartment and Townhouse Acoustic Rating. For impact noise through floors, 5 star is an on-site L’nT,w below 45 and 6 star is below 40, and the values “relate to the actual performance achieved in a completed apartment”.

So a star rating belongs to a floor in a building, not to a roll of underlay. The AAAC’s explanatory note says “it cannot be claimed that any one building product in isolation (especially an acoustic underlay), is guaranteed to achieve a certain acoustic rating or AAAC Star Rating”. It asks makers to back claims with test results and “the complete floor/ceiling system build-up clearly listed”, and says the finished floor should be tested, with a certificate from an AAAC member.

When you see “6 star” on a product, ask for that report. Some makers do publish on-site results, such as AngelStep’s L’nT,w 38 with engineered timber on plywood in one building. Those are genuine results for that building and build-up. Yours may differ.

Brands you will see

These documents describe different systems rather than a brand ranking. Sources: Dunlop brochure, AngelStep GOLD8, Quiet-Step brochure, Karndean data sheet, Uniroll, Altro and the REGUPOL test reports. Note that Dunlop’s lab tests include a ceiling and REGUPOL’s do not, so their Ln,w figures cannot be compared directly. ΔLw on the same floor finish is the fairer comparison.

Can you buy acoustic underlay at Bunnings?

Yes. As at September 2026, Bunnings lists acoustic underlay in its flooring underlay range, including a QEP 2 mm IXPE Acoustic Pro underlay for floating floors and an Ultimate Flooring rubber acoustic underlay. Stock varies by store. Wherever you buy, the same checks apply: ask for the test report, and check your flooring maker’s rules before you put anything under a plank that already has an attached pad.

How to lay acoustic underlay: the basics

• Moisture first. Most floating floor makers want a moisture test and a 200 µm polyethylene film on a slab, with taped joins. Preference Floors notes its attached pad “does not contain a PE moisture barrier film”.
• Loose laid or glued. Foam rolls are usually laid loose with taped joins. Rubber and cork mats may be laid loose or glued, as the maker specifies.
• Keep off the walls. The ATFA information sheet warns that a floor hard against a wall lets noise travel through the wall into the next unit.
• No fixings through it. The ABCB handbook warns that nails and screws through a resilient underlay into the slab “diminish its performance”.
• Do not double up. Adding underlay under a plank with an attached pad can void the warranty (Signature Floors; Preference Floors).

Acoustic underlay in strata and apartments

In an apartment, the underlay is usually how a hard floor gets past the by-law. The stakes are real: the ABCB handbook says replacing carpet with hard flooring can mean “as much as 25 to 30 dB extra sound transmission”. Get the required figure in writing before you buy, find out whether it is a laboratory or an on-site number, and ask whether a test after installation is required. The AAAC’s explanatory note also cautions committees against setting by-law figures from its star table without specialist advice. Start with our strata flooring rules and the strata flooring checker.

What to check on the data sheet

• ΔLw and Ln,w with the test standard (ISO 10140-3 and ISO 717-2, or older ISO 140-8) and the lab named
• Laboratory or on-site result, and which one your by-law asks for
• The full build-up: floor finish and thickness, underlay, slab thickness, any ceiling
• Floating or glued, and the test sample size
• Density or weight per square metre, and compression and recovery if given
• Compatible floor types and approved adhesives
• Moisture film included, or a separate barrier needed
• What the flooring maker's warranty says about this underlay

Choose acoustic underlay by floor type

• Laminate. Match the underlay to the laminate maker’s requirements and assess acoustic evidence for the full construction.
• Hybrid and vinyl plank. Many hybrid planks have an attached pad, and several makers void the warranty if you add another layer. Read the makers’ rules and the IXPE test figures.
• Timber. Engineered, solid and bamboo floors each use a different system, from floating on foam to plywood glued over rubber. Height at doors is the common trap.
• Tiles. Tiles are usually bonded through a thin mat. In REGUPOL’s lab test, 10 mm porcelain tiles bonded to a 4.5 mm sonus multi mat gave ΔLw 17 dB.
• Carpet. Carpet on underlay is the quiet baseline that hard floors are measured against, and the underlay choice still matters.
• Rubber. Recycled tyre rubber and rubber and cork mats, from 3 mm to 10 mm, used under tiles, timber and some floating floors.
• Regupol vs Damtec. Two rubber underlay makers, compared on their published figures.