Acoustic underlay for carpet: comfort with a clear specification

Carpet brings softness to a bedroom, but the underlay still deserves a precise specification. Thickness, support and compatibility affect the finished floor in different ways.

If the carpet is part of an apartment’s acoustic arrangement, keep a record of both layers. Replacing an underlay with an unnamed alternative makes it harder to understand what changed if noise becomes an issue later.

How much quieter is carpet than a hard floor?

On site, the gap is large. In Body Corporate for Admiralty Towers II v Johnson [2017] QCATA 29, the tribunal recorded the carpeted areas of a Brisbane tower at about LnT,w 28 and the tiled kitchens at about 61 to 65. In another apartment in the same tower, tests put carpet and underlay at 33, the bare 180 mm slab at 75, and floating bamboo and timber floors on underlay at 41 to 52. LnT,w is a field result, measured in the finished building, and lower is quieter. The tribunal also noted that a 10 dB difference is typically heard as halving or doubling the sound.

In the laboratory, the same pattern shows up in one maker’s tests on one slab. Damtec tested on a 140 mm concrete slab with no ceiling, to ISO 140 and ISO 717.2:

• DAMTEC Rubber, 2 mm, under loose-laid carpet: ΔLw 28 dB, Ln,w 45 (carpet brochure).
• DAMTEC Standard, 2 mm, under loose-laid carpet: ΔLw 25 dB, Ln,w 48.
• DAMTEC Standard, 2 mm, under 9 mm ceramic tile: ΔLw 18 dB, Ln,w 56 (tiling brochure).

The German maker’s data sheet gives ΔLw 29 dB “with carpet” and 18 dB “under 9mm tiles”. ΔLw is the laboratory improvement over a bare slab, so higher is better. These tests also show why the underlay matters: the same 2 mm thickness gave 28 dB in rubber and 25 dB in rubber and cork.

What is the difference between foam, felt and rubber carpet underlay?

The Carpet Institute of Australia lists the main types as re-bond foam, SBR latex, rubber and felt. Here is what Australian makers publish for each.

Sources: Dunlop Carpetmate, Dunlop Springtred, Dunlop commercial underlay and the Damtec carpet brochure.

Dunlop does print an “acoustic performance” figure in dB for each of its underlays. The page does not say whether it is ΔLw, Ln,w or another measure, or what floor it was tested on, so we have not used those numbers here. Ask for the test report if you need one for an application.

The foam and fibre underlays in the table are 7 to 11 mm thick and light. Rubber and rubber and cork underlays are thin and dense. Damtec says its rubber underlays “will not compress like most foam underlays” (Damtec carpet page); that is a maker’s claim, and we found no independent long-term test to confirm or reject it.

Does density or thickness matter more for carpet underlay?

For comfort and wear, Australian carpet underlay is classed under AS 4288, “Soft underlays for textile floor coverings”. The Carpet Institute lists five classes of intended use:

• LR: light residential use, not suitable for stairs
• GR: general residential use
• L: luxury use, domestic or commercial, where high energy absorption is required
• GC: general commercial, normal foot and wheeled traffic
• HC: heavy commercial, heavy foot and wheel traffic and castor chairs

Dunlop’s range shows how class, density and thickness line up. Its 10 mm Supergreen at 69 kg/m³ is classed General Residential, while its 10 mm Eureka at 80 kg/m³ and 10 mm Comfort Master at 100 kg/m³ are classed Luxury (Carpetmate). So “10 mm” alone tells you little. Dunlop says higher density gives the carpet a longer life; that is the maker’s view.

For noise, neither number is a rating. AS 4288 classes and density describe comfort and durability, not impact sound. The Australian Carpet Classification Scheme star labels grade the carpet’s durability and appearance retention, not the underlay or its acoustics. If your by-law sets a number, you need an impact test, not a class.

Do carpet tiles need an acoustic underlay?

Commercial carpet tiles usually come with a backing, and some backings include a cushion. The Carpet Institute lists “carpet tiles with integrated pad as part of the backing” as an installation option.

Be careful with the numbers you see. GH Commercial says its Enviro Bac Plus cushion backing has a noise reduction coefficient 66% greater on average than its standard backing. NRC measures sound absorbed in the room, not impact noise passing through the floor to the unit below. Tarkett quotes its DESSO SoundMaster backing at 8 dB better impact sound insulation than its standard backing, but that is a European page and gives no Australian test.

If you want a separate layer under carpet tiles, some acoustic underlays are made to have them bonded on top. REGUPOL lists carpet tiles among the finishes for its sonus multi 3 underlay. Check that the carpet tile maker approves the combination and the adhesive.

Why does a by-law that says “carpet” care about the underlay?

Most by-laws set an outcome, not a product. The standard floor coverings by-law for older NSW schemes, by-law 14 in Schedule 2 of the Strata Schemes Management Regulation 2016, says floor space must be “covered or otherwise treated to an extent sufficient to prevent the transmission from the floor space of noise likely to disturb the peaceful enjoyment” of another lot. It does not apply to kitchens, laundries, lavatories or bathrooms. Carpet laid straight on the slab, or on a worn-out underlay, may not meet that test.

Tribunals also talk about the two together. In 18 Kingsford Street [2016] QBCCMCmr 78, the Queensland adjudicator’s order for a noisy tiled floor named “carpet and underlay over the tiles” as one fix. And the as-built carpet that set the benchmark in the Admiralty Towers case was carpet and underlay.

The AAAC’s explanatory note adds a warning that applies here too: no single product, especially an underlay, can be guaranteed to reach a rating in your building, because the slab and ceiling change the result.

What to ask for before you buy

• The AS 4288 class of the underlay, and whether it suits stairs
• An impact test under carpet, with the slab thickness and whether a ceiling was present
• Whether the result is a lab ΔLw or Ln,w, or an on-site LnT,w
• The carpet maker's approval of the underlay type and thickness
• For carpet tiles, an impact test of the tile and backing, not only an NRC figure
• The committee's written answer on what the by-law expects

If you are still choosing between carpet and a hard floor, our guide to the best flooring for apartments compares the options, and soundproof flooring explains what a floor can and cannot fix.

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