Hard floors in apartments: acoustic ratings, underlay and testing explained

Swapping carpet for timber, hybrid or tiles is one of the most common apartment renovations, and one of the most common causes of disputes between neighbours. Most of those disputes are not about whether the owner was allowed to lay a hard floor. They are about whether the floor that went down actually performs. This article covers the technical side: what the acoustic ratings mean, why a product's rating is not your floor's rating, how testing works, and the details that decide whether the people downstairs can hear you walking.

For the approval side (what your by-laws and the strata legislation require before you start), see our companion post on hard floors in apartments: the approval rules.

Two kinds of sound

Sound gets between apartments in two ways, and floors are rated for both:

• Airborne sound is sound that travels through the air and then through the floor or wall: voices, television, music.
• Impact sound is sound created by something striking the floor itself. The ABCB's sound transmission handbook describes the impact noise the code is aimed at as noise from footsteps and moving furniture.

Carpet and underlay deal with impact sound very well, because the soft surface absorbs the strike before it reaches the slab. A hard floor does not. That is why the impact rating is the one that matters most when carpet comes up.

What the ratings mean, in plain English

You will see a handful of terms in by-laws, product brochures and acoustic reports. The definitions below follow the ABCB's Sound transmission and insulation in buildings handbook.

• Ln,w (weighted normalised impact sound pressure level) is a laboratory measure of how much impact sound gets through a floor. A standard tapping machine strikes the floor and the sound reaching the room below is measured. Lower is better, because it means less sound reached the room below.
• LnT,w (weighted standardised impact sound pressure level, often written L'nT,w in by-laws and reports) is the field equivalent, measured on site in a real building. Again, lower is better.
• Rw and Rw + Ctr describe airborne sound insulation in the laboratory. Higher is better. The Ctr adjustment accounts for low-frequency sound, such as bass from home theatre systems, which the basic Rw figure does not capture well.

For new apartment buildings, NCC 2022 Volume One sets the benchmark for floors between units. Under the Deemed-to-Satisfy provisions, a floor separating sole-occupancy units in a Class 2 or 3 building needs an Rw + Ctr of not less than 50 and an Ln,w of not more than 62. Where compliance is shown by testing on site instead, the performance requirement uses field measures, including an LnT,w of not more than 62. These figures are written for new construction. Your by-laws, and any conditions the owners corporation puts on its approval, decide what a floor replacement in an existing building has to achieve, and many by-laws set a stricter number than the code.

Why the underlay's rating is not your floor's rating

This is the single most misunderstood part of the topic. An underlay product may come with a laboratory test report showing an impressive impact rating. But that test was done on a particular floor build-up: a specific slab thickness, sometimes a specific ceiling below, and a specific floor finish on top. Change any of those and the result changes.

The performance of your floor depends on the whole system:

• The slab. Thickness and construction matter. A thicker, heavier slab transmits less impact sound than a thinner one.
• The ceiling below. A resilient suspended ceiling in the apartment below can make a large difference. A plasterboard ceiling fixed directly to the slab makes much less.
• The underlay. Its thickness, density and resilience, and whether it is designed for the finish going on top.
• The finish. Engineered timber, hybrid, laminate and tiles all behave differently, even on the same underlay.

The ABCB handbook is blunt about what happens when carpet is replaced with hard flooring in renovation: impact performance will invariably reduce, and a hard floor that meets the code can still transmit noticeably more sound than the carpet it replaced. It notes it is not unusual for the neighbour below to complain in that situation. That does not mean hard floors cannot work in apartments. It means the system has to be specified properly, not chosen from a sample board.

Lab results and field results are different things

A laboratory measures a floor under ideal conditions, in test chambers designed to stop sound travelling around the sample. A real apartment is not like that. The ABCB handbook lists the reasons field results can differ, including:

• Flanking, where sound bypasses the floor through walls, ceilings, penetrations and the structure itself;
• room sizes and furnishings that differ from the lab;
• higher background noise on site; and
• poor sealing at junctions, untreated penetrations and substandard installation.

In practice, a floor rarely performs on site as well as the lab figure suggests. That is why many by-laws require a field test by an acoustic consultant after installation, measured in the apartment below, rather than accepting a product data sheet. If your by-law requires one, the floor is only compliant once it passes that test.

Where hard floors commonly fail

Most acoustic failures we see come down to installation details, not the product.

• The floor touches the walls. A floating floor must be isolated from the walls, skirtings, door frames and kitchen joinery. If the boards or the skirting are pressed hard against the wall, footstep vibration goes straight into the structure, bypassing the underlay. An expansion gap at the perimeter, filled with a flexible acoustic sealant rather than rigid filler, and skirtings fixed so they do not clamp the floor, are what keep the isolation intact. Our post on choosing silicone sealant explains why the sealant type matters.
• The underlay is bridged. The ABCB handbook warns that nails and screws fixing a floor through a resilient underlay into the slab short-circuit it. Every fixing that passes through becomes a path for sound.
• The wrong underlay for the finish. An underlay designed for floating timber is not automatically suitable for tiles, and vice versa.
• The build-up does not match the test report. The product was tested on a 200 mm slab with a suspended ceiling, and the apartment has neither.
• Penetrations and services. Gaps around pipes and ducts passing through the floor can let sound through regardless of how good the floor is.

Tiles and wet areas

Tiled floors need particular care. A tile is rigid and brittle, so a soft underlay that suits floating timber can let tiles flex and crack. Acoustic systems for tiles typically use a thinner, firmer resilient layer bonded within the build-up, and the tile and adhesive system needs to be compatible with it.

In bathrooms and laundries, the acoustic layer also has to work with the waterproofing. The membrane, the falls to the drain and the acoustic layer all have to be detailed together, in the right order, so neither compromises the other. Waterproofing work in a strata building usually needs a higher level of approval than a simple floor change, which our post on renovating a strata apartment covers.

Questions to ask your supplier and installer

1. What impact rating does our by-law, or the owners corporation's approval, require, and is it a lab rating or a field test?
2. Do you have a test report for the complete system that matches our slab, our ceiling below and the exact finish we are installing?
3. How will the floor be isolated from walls, skirtings, door frames and joinery?
4. Will any fixings pass through the underlay into the slab?
5. For tiles: is the acoustic layer designed for tiles, and how does it work with the waterproofing in wet areas?
6. If a field test is required, who arranges it, and what happens if the floor does not pass?

How we approach it

• We confirm the required rating first. Before anything is ordered, we check the by-law and any approval conditions for the rating and whether a field test is needed.
• We specify a tested system, not a product. We match the test report to the actual slab, ceiling and finish, and bring in an acoustic consultant where the build-up is unusual.
• We detail the isolation. Perimeter gaps, acoustic sealant, skirting fixing and joinery junctions are part of the scope, not left to the day.
• We inspect before it is covered. The underlay, perimeter isolation and any wet-area membrane are checked and photographed before the finish goes down.
• We arrange the field test where it is required, and hand the result to you and the owners corporation with the rest of the job records.

If you are planning to replace carpet with a hard floor in an apartment, get in touch and we can talk through what your building requires and what will actually work.

Sources

• Sound transmission and insulation in buildings handbook, Australian Building Codes Board (definitions of Ln,w, LnT,w, Rw and Ctr; F7D5 floor requirements; Appendix D on laboratory and field results; sections 4.4 and 5.1 on underlay bridging and carpet-to-hard-floor changes)
• NCC 2022 Volume One, Part F7 Sound transmission and insulation, Australian Building Codes Board

This article is general information, not acoustic or legal advice for your apartment. Requirements depend on your building, your by-laws and the approval given by your owners corporation, so confirm them with your strata manager and, where needed, an acoustic consultant. Full website disclaimer