Acoustic Underlay Ratings Explained: L'nT,w, IIC and Star Ratings for Apartment Floors

The short answer

If your body corporate by-laws ask for an acoustic rating, they are asking how much impact noise (footsteps, dropped objects, chairs being moved) is allowed to reach the apartment underneath once your new floor is down. The underlay does most of that work under a hybrid, vinyl plank or timber floor, which is why it is called an acoustic underlay.

The confusing part is that there are several rating systems, and they do not all run in the same direction. With L'nT,w and Ln,w, a lower number is better. With IIC and with star ratings, a higher number is better. Once you know which measure your building uses and which direction it runs, comparing underlays becomes much simpler.

This guide explains each rating in plain terms. Our guide to apartment flooring and body corporate acoustic rules covers the approval process itself.

Impact sound and airborne sound are different things

Buildings deal with two kinds of noise. Airborne sound is voices, music and television travelling through the air and then through walls and floors. Impact sound is created when something strikes the floor itself, and the structure carries it into the room below.

A concrete slab is heavy, so it blocks airborne sound well on its own. Impact sound is the weak point, because a hard floor covering fixed straight onto concrete passes each footstep into the slab. Flooring by-laws are almost always about impact sound for that reason, and it is the only part of the building's acoustic performance that changing your floor covering really affects.

The ratings you will see on by-laws and data sheets

These are the terms that turn up most often in Gold Coast by-laws, acoustic reports and underlay data sheets:

• L'nT,w: the weighted standardised impact sound pressure level, measured in a finished building. A tapping machine is run on the floor upstairs and the sound level is measured in the room below. Lower is better. This is the measure most Australian by-laws and acoustic consultants use.
• Ln,w: the same idea measured in a laboratory, with a test floor built between two purpose-built rooms. Lower is better.
• IIC and FIIC: Impact Insulation Class, a North American rating. IIC is the laboratory figure and FIIC is the field figure. Higher is better. It appears on many imported hybrid and underlay data sheets.
• Delta Lw: the improvement an underlay and floor covering give compared with the same bare concrete test slab. Higher is better. It is useful for comparing underlays with each other, but it is not the result you will get in your building.
• AAAC star rating: a rating from 2 to 6 stars set out by the Association of Australasian Acoustical Consultants. For impact sound through floors, each star level corresponds to an L'nT,w value. More stars is better.

Lower or higher: reading the number the right way

L'nT,w and Ln,w measure how loud the impact noise is in the room below, so a floor rated 45 is quieter than a floor rated 55. IIC and FIIC measure how well the floor resists impact noise, so a floor rated 60 is quieter than one rated 50. Mixing these up is the most common mistake owners make when comparing products.

The numbers are in decibels, which do not behave like ordinary numbers. As a rough guide, a change of about 10 is heard as roughly twice or half as loud, and a change of 1 or 2 is hard to notice. A product that tests 2 points better than another is not meaningfully quieter, while 8 or 10 points is a clear difference.

There is no exact conversion between IIC and Ln,w, because they come from different standards and are calculated differently. If your by-laws specify L'nT,w, ask for a test result expressed in L'nT,w or Ln,w rather than relying on a converted IIC figure.

AAAC star ratings in plain terms

Many newer Gold Coast by-laws use the AAAC star system because it is easier to read than a raw number. For impact sound through a floor between two apartments, the AAAC guideline sets these levels:

• 2 stars: L'nT,w of 65 or less
• 3 stars: L'nT,w of 55 or less
• 4 stars: L'nT,w of 50 or less
• 5 stars: L'nT,w of 45 or less
• 6 stars: L'nT,w of 40 or less
• Carpet over a good underlay usually sits comfortably at the quiet end of this scale. Hard floors need the right acoustic underlay to reach the higher star levels, and the result depends on the building as well as the product.

What the building code requires, and why by-laws often ask for more

The National Construction Code sets the minimum that a new apartment building must be designed and built to. For impact sound through floors between apartments, the current code uses a rating of not more than 62, measured as Ln,w in a laboratory or L'nT,w on site. On the star scale above, that sits a little better than 2 stars.

That is a minimum for construction, and many people find a floor that only just meets it noticeably noisy to live under. Bodies corporate are free to set a stricter standard in their by-laws for owners who change their floor coverings, and many do. Requirements of 4 or 5 stars, or an L'nT,w of 50 or 45, are common in newer and premium buildings. Some older buildings simply require the new floor to perform no worse than the carpet and underlay it replaces.

For a renovation, the by-laws are the standard you have to meet. In Queensland these sit in the building's community management statement. In New South Wales strata schemes, such as those in Tweed Heads, the scheme's by-laws commonly require floors to be covered or treated so noise does not disturb other residents, and many schemes add a specific rating.

Why the same underlay gives different results in different buildings

An underlay does not have one rating of its own. The result depends on everything it was tested with, which is why a figure from a data sheet cannot simply be carried over to your apartment.

• Slab thickness: a thicker, heavier slab passes less impact sound. Older towers sometimes have thinner slabs than new ones, so the same floor system tests worse in them.
• The ceiling below: a suspended plasterboard ceiling with insulation above it improves the result compared with a ceiling fixed directly to the slab or a bare concrete soffit.
• The floor covering: a floating hybrid, a glue-down vinyl plank and an engineered timber board each behave differently over the same underlay.
• Room size and layout: field results vary a little from room to room, even in the same apartment.
• Flanking paths: sound that travels around the underlay, through walls, skirting or filled perimeter gaps, lowers the real result no matter how good the underlay is.

Laboratory tests and on-site tests

Most figures on product data sheets come from laboratory tests. A laboratory removes the variables, which makes it a fair way to compare one underlay with another, but it also means the building is not part of the result. A laboratory figure tells you what the system can do under ideal conditions.

An on-site test, sometimes called a field test, measures your actual floor in your actual building. Some by-laws require one after installation, carried out by an independent acoustic consultant, and some ask for one only if a neighbour complains. Field results are usually a few points worse than laboratory results for the same system, so it is sensible to choose a system with some margin above the requirement rather than one that only just meets it on paper.

We supply and install the floor and underlay, and we provide the manufacturer's product documents and acoustic test information for your body corporate. Acoustic testing itself is done by an acoustic consultant, not by the flooring installer.

How to read an underlay test report

When a supplier says an underlay "achieves 5 stars" or "IIC 70", ask for the test report behind the claim and check these details:

• Which rating is reported: L'nT,w, Ln,w, IIC or Delta Lw, and whether it was a laboratory or field test
• The slab used in the test, including its thickness. A result on a 200 mm slab will not be repeated on a 150 mm slab
• Whether there was a ceiling under the slab in the test, and what kind
• The floor covering tested on top: the type of floor, its thickness, and whether it was floated or glued
• Who carried out the test and when, and whether the laboratory or consultant is independent of the manufacturer
• Whether the product named in the report is the exact underlay and thickness being quoted for your job

Types of acoustic underlay

Acoustic underlays are made from a few main materials, and each suits certain floors:

• Rubber underlays, often made from recycled rubber granules, are dense and resilient. They are widely used under floating hybrid and engineered timber, and some are designed to be glued down under direct-stuck timber or vinyl plank.
• Foam underlays made from cross-linked polyethylene, EVA or polyurethane are lighter and are common under hybrid and other floating floors. Performance varies widely between a basic foam and a purpose-made acoustic foam.
• Cork and rubber-cork blends are firm and stable, which suits glue-down floors where the underlay must not compress under the adhesive and the floor.
• Pre-attached backing on hybrid planks is a thin layer of foam or cork bonded to each plank. It makes the floor quieter to walk on, but on its own it often does not reach the rating a body corporate requires.
• Carpet underlay is a different product again. Carpet with a quality underlay absorbs impact so well that it meets most by-laws without special treatment, which is why carpet remains a simple choice for apartment bedrooms.

Matching the underlay to the floor

The floor and the underlay have to be chosen as a pair. Each flooring manufacturer sets out which underlays can be used under its product, and using the wrong one can affect both the acoustic result and the product warranty.

Hybrid flooring is floated, so the underlay sits loose between the planks and the slab. The underlay needs to be firm enough to support the click joints. If the planks already have an attached backing, check the installation instructions before adding a second underlay, because some hybrid floors allow an approved acoustic underlay beneath the attached backing and some do not.

Glue-down vinyl plank is thin and flexible, so it needs a firm acoustic sheet that is bonded to the slab first, with the planks then bonded to the sheet. Engineered timber can be floated over an acoustic underlay or direct-stuck over an acoustic mat with a flexible adhesive. Direct-stuck floors feel more solid underfoot, and floated floors are usually faster to install.

Our guide to hybrid flooring problems explains what happens when a floating floor is laid over an underlay that is too soft or a slab that is not flat.

Thicker is not always better

It is tempting to assume the thickest, softest underlay will be the quietest. Up to a point a softer layer does absorb more impact, but a floating floor also needs support. If the underlay compresses too much under foot traffic or furniture, the click joints flex, and over time they can creak, open or break. Flooring manufacturers set a maximum underlay thickness and a minimum firmness for this reason.

Height is the other limit. An acoustic underlay adds to the finished floor height, which affects door clearances, the step at the front door and balcony sliders, kitchen kickboards and the transition to tiled bathrooms. We measure these at the free in-home measure so the build-up is known before you choose, and trims and transitions are included in the fixed written quote.

Installation details that change the result

A tested system only performs as tested when it is installed the same way. The details that matter most:

• The slab is prepared flat first, so the underlay and floor sit evenly without hollow spots
• The underlay is continuous, with joins butted and taped as the manufacturer directs and no gaps or overlaps
• The floor is kept clear of walls, door frames, pipes and kitchen cabinets, so nothing hard bridges the gap between the floor and the building
• Perimeter gaps are covered by skirting or trims but not packed with rigid filler, grout or adhesive
• Skirting and trims are fixed to the wall, not through the floor into the slab
• The system installed is exactly the one that was approved, with no substitutions

Getting the right result for your building

Start by getting the flooring by-law in writing from your building manager or body corporate, and note the exact measure and number it uses. At the measure we look at the existing floor, the slab at doorways and edges, and the door and joinery heights, then suggest a floor and underlay combination with a manufacturer's test result expressed in the same measure as your by-laws.

You submit that specification to the committee if approval is required, and once it is approved we install that system. If your by-laws require an on-site test afterwards, your body corporate or an acoustic consultant can tell you how that is arranged. Keep the approval, the product documents and the invoice together, because a future buyer or the committee may ask for them.

An acoustic underlay costs more per square metre than a standard underlay, and the amount depends on the product your building's requirement calls for. It is listed in your fixed written quote along with removal of the old floor, subfloor preparation, stairs and trims, so you can see the whole cost before you commit.