Flooring acoustic ratings explained: read the whole result

An acoustic number only becomes useful when you know what it measures. A laboratory floor result, an underlay improvement and a test between two apartments can look similar on a brochure while answering different questions.

Start by matching the letters beside the number. Then check the test construction and the rule you need to satisfy. Keep those distinctions together rather than treating every figure as a score for your own home.

What does a flooring acoustic rating actually measure?

Most floor ratings measure impact sound: footsteps, dropped objects and chairs moving on the floor above. The ABCB sound handbook calls this structure-borne sound and says it is reduced by isolating the vibration with a resilient element. An acoustic underlay is that kind of resilient layer.

To rate impact sound, a tester puts a standard tapping machine on the floor and measures the sound in the room below. The machine has five hammers that drop 40 mm, again and again, as described in a 2014 Queensland field study. The result is weighted into one number under AS ISO 717.2:2024, the Australian adoption of ISO 717-2:2020. For impact ratings, lower is quieter.

Airborne sound is voices, TV and music. It is rated as Rw (laboratory) or DnT,w (on site), often with a Ctr term added for bass. For airborne ratings, higher is better.

Each rating also comes in two versions:

• Laboratory. The floor is built in a test facility where sound paths around the floor are suppressed, under ISO 10140-3:2021. ISO says these results are not directly applicable to a real building without allowing for those other paths.
• Field (on site). The floor is tested in a real building, under ISO 16283-2:2020. The result includes everything: the slab, the ceiling below, the walls and the quality of the installation.

What does the NCC require for floors between apartments?

The rule is in the National Construction Code, Volume One, Part F7. Clause F7D5 of NCC 2022 says a floor in a Class 2 or 3 building “must have an Rw + Ctr (airborne) not less than 50 and an Ln,w (impact) not more than 62” if it separates sole-occupancy units, or separates a unit from certain other spaces. Clause F7D4 says the Ln,w must be worked out “using results from laboratory measurements”, or the floor must match a construction listed in Specification 28.

The same Part also sets a performance requirement in on-site terms. Clause F7P1 asks for a DnT,w + Ctr of not less than 45 and an L’nT,w (the NCC writes LnT,w) of not more than 62, and clause F7V1 verifies this by measuring the floor in situ. A new building can show compliance through the deemed-to-satisfy figures or by meeting the performance requirement, and clause F7D1 says meeting the deemed-to-satisfy provisions satisfies it.

So “62” appears twice, but as two different measurements: a laboratory Ln,w and an on-site L’nT,w.

In NCC 2019 the same floor rules were clauses F5.4 (floors), F5.3 (how the impact rating is determined) and FV5.1 (on-site verification), as shown in the NCC 2019 Part F5. NCC 2025 keeps the same clause numbers and the same figures. The ABCB adoption table shows Victoria, Western Australia and the ACT adopted NCC 2025 on 1 May 2026, and NSW and Queensland adopt it on 1 May 2027. The Northern Territory has not adopted NCC 2025, and its own variation of Part F7 in NCC 2022 asks floors between units for Rw 45 with no impact figure.

What if there is a car park, shop or plant room below?

The same F7D5 figures apply when the floor separates a unit from “a plant room, lift shaft, stairway, public corridor, public lobby or the like, or parts of a different classification”. Under NCC Part A6, a car park is Class 7a, a shop or cafe is Class 6 and an office is Class 5, so each is a different classification from the Class 2 apartments. The NCC’s own example is “a floor separating ground floor shops from an apartment level above”. A ground-floor unit on a slab with nothing below it has no floor that separates it from another space, so on a plain reading F7D5 does not apply to that slab.

Where does “Ln,w + CI of 62” come from?

You will still see “Ln,w + CI not more than 62” on older spec sheets and some websites. That was the wording in NCC 2015 (clause F5.4). NCC 2016 changed it to Ln,w alone, and the ABCB handbook (Appendix D.1) says the CI term “was removed from the BCA in 2016 at the request of industry”.

CI is a spectrum adaptation term that adds the character of walking noise. On a bare concrete slab it is usually negative, so Ln,w + CI gave a lower, easier number. In the Inter-noise 2014 case study, a bare 190 mm slab measured L’nT,w 69 with a CI of minus 12. When you compare two reports, check that both leave CI out or both include it.

Does the NCC set a rule for acoustic underlay?

No. The NCC rates the whole floor, not the underlay. The only underlay in the deemed constructions of Specification 28 is carpet underlay: clause S28C8(3) says a “200 mm thick concrete slab with carpet on underlay” meets Rw + Ctr 50 and Ln,w 62.

That is why hard floors cause trouble. The ABCB handbook (section 5.1.1) warns that replacing carpet with a hard floor “can result in as much as 25 to 30 dB extra sound transmission”. The NCC figure is also only a minimum. A Queensland appeal tribunal said so in McLeod v Body Corporate for Stradbroke Tower and Villas at [29]: the BCA “is a minimum construction standard” and “does not set levels for the purposes of good neighbourly relations”. For a floor change in an existing apartment, read your by-laws and ask a building certifier whether the work needs approval.

What do AAAC star ratings mean?

The Association of Australasian Acoustical Consultants publishes the Guideline for Apartment and Townhouse Acoustic Rating, Version 1.0, dated June 2017. The AAAC says its guidelines are advisory and “not statutory documents”. Its star ratings measure “the actual installed performance” in a completed apartment, so every figure is on site. Only an AAAC member firm can issue a star rating certificate. There are five levels, from 2 star to 6 star, and no half stars.

For floors between tenancies, the guideline asks for an impact L’nT,w less than 65 (2 star), 55 (3 star), 50 (4 star), 45 (5 star) or 40 (6 star). The airborne figure is a DnT,w + Ctr greater than 35, 40, 45, 50 or 55. The NCC’s on-site impact limit of 62 sits between the AAAC’s 2 star and 3 star figures. The AAAC says a 5 star floor is “close to the maximum achievable in normal building construction” and that 6 star is “in some aspects” very hard to achieve.

Is there such a thing as a 6 star acoustic hybrid floor?

Not as a product. The AAAC explanatory note (May 2023) 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”. A “6 star” or “5 star” claim on hybrid flooring needs supporting evidence for a named complete construction. Do not assume the label establishes that a test exists or that it applies to your building. Ask for that report and compare its build-up with yours. The note also says the guideline should not be used by committees to set by-law limits arbitrarily, without specialist advice for the building.

Why is the number on a spec sheet different from the number in a by-law?

A spec sheet usually shows a laboratory figure: an Ln,w for one tested build-up, or a ΔLw improvement figure for an underlay. A by-law usually asks for an on-site L’nT,w (or, in some Queensland schemes, an FIIC) measured in your building after installation. The ABCB handbook (Appendix D.1) explains why the field result can be worse: sound travels around the floor through walls, ceilings and penetrations, the rooms are not ideal, and workmanship varies.

Four things move the number in your building:

1. The slab. In South-East Queensland field data, bare 180 mm slabs with no ceiling measured L’nT,w 68 to 76, and 200 mm slabs measured 65 to 69, according to the Inter-noise 2014 study.
2. The ceiling below. In the same study, one layer of 13 mm plasterboard ceiling improved a bare 190 mm slab by 14 points.
3. The installation. The study found perimeter contact to be a common problem: a floor that touches the wall or skirting becomes a path for sound.
4. The underlay’s improvement is not fixed. The same five underlays improved the slab by 4 to 17 points with no ceiling, but by 4 to 11 points with a ceiling. So you cannot subtract a data sheet’s ΔLw from your building’s figure.

Here is where published field figures and the rules sit on one scale.

What do published tests show?

These are on-site results from published sources. Each one is a single building or a small set of buildings, so treat them as examples, not promises.

The tower results come from Body Corporate for Admiralty Towers II v Johnson [2017] QCATA 29 at [12]. The slab and tile results come from the Inter-noise 2014 study. These examples illustrate why the construction belongs beside the result; they do not predict another floor’s performance. For backing and installation questions, read acoustic hybrid flooring and acoustic underlay.

How do IIC and FIIC relate to Ln,w and L’nT,w?

IIC (Impact Insulation Class) is the American laboratory rating, measured under ASTM E492 and calculated under ASTM E989. FIIC (Field Impact Insulation Class) was the on-site version in older editions of ASTM E1007. The current ASTM E1007-25 reports AIIC, ISR and NISR instead. For all of these, higher is better, which is the opposite of Ln,w and L’nT,w.

The AAAC guideline (section 6.0) gives a rule of thumb: subtract either number from 110 “to approximately indicate the other”. So L’nT,w 50 is roughly FIIC 60. There is no exact conversion, because the ISO and ASTM methods fit different reference curves. In the Inter-noise 2014 case study, L’nT,w plus FIIC added up to between 104 and 108 for floors with underlay, and 100 to 102 for the bare slab.

Some Queensland by-laws still use FIIC. In McLeod v Body Corporate for Stradbroke Tower and Villas [2015] QCATA 146, the by-law allowed conditions of FIIC 77 in the lounge and dining rooms, the adjudicator found that FIIC 77 was not achievable with hard floors and ordered FIIC 65 instead, and the appeal tribunal dismissed the owner’s appeal. See body corporate flooring rules in Queensland for how these by-laws work.

How do you read an acoustic test certificate?

A test report for flooring or underlay should let you answer every question below. The list follows the reporting items in the ABCB handbook (Appendix D.3.3) and the statement of results in ISO 717-2.

• Is it a laboratory result (Ln,w or ΔLw, ISO 10140-3) or an on-site result (L'nT,w, ISO 16283-2 or the older ISO 140-7)?
• Is the rating calculated under ISO 717-2 or AS ISO 717.2, and does it include + CI or not?
• Is the full build-up listed: floor finish, underlay and its thickness, slab type and thickness, and any ceiling below?
• Which rooms were tested, how big were they, and where were the tapping machine and microphones?
• Who tested it, who paid for the test, and on what date?
• Does the report carry a NATA endorsement, and is the test inside the lab's scope of accreditation?
• Does it note anything that affected the result, such as background noise or flanking?
• Does it include the frequency graph or table, not only the single number?

On NATA: NATA says accredited organisations may put its endorsement on reports only for testing inside their scope of accreditation. You can check a lab’s scope with NATA’s Find Organisation search.

What “tested on a 150 mm slab with no ceiling” means for you. A laboratory result tells you how that exact build-up performed on that slab. If your building has a thicker slab or a ceiling below, your result may be better. If the floor is installed with no gap at the walls, or your building has more flanking paths, it may be worse. The AAAC explanatory note recommends laying a sample of the system in your building and testing it on site, and says the finished floor should be tested for certainty.

Flooring acoustic ratings explained side by side

What should you ask for before you choose a floor?

Start with the rule, then the product:

• Ask the committee, in writing, which rating the by-law uses (L’nT,w or FIIC), the number, and which rooms it applies to. Our strata flooring checker lists what to request.
• Ask the supplier for the full test report, not a brochure figure, and check the build-up against your building.
• If the margin is small, ask an acoustic consultant about a sample test on site before you buy the whole floor.

For the practical sequence, including access to the room below and what to do with the result, see the post-installation acoustic test. Use the test report reader alongside the original report.