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Impact Isolation Class (IIC)

Impact Isolation Class (IIC) is, in basic terms, a measure of sound heard in one room from an impact made on a floor in the space directly above. This issue is commonly raised when installing floor coverings in multi-storey residential apartments. Both the Strata Titles Act and the Building Code of Australia now raise this issue, setting requirements that need to be followed when installing floor coverings in multi-storey apartments.

A Complex Issue

The Impact Isolation Class of a floor is tested using highly technical tapping machines (used in the testing room) and listening devices (used in the receiving room below). This work is carried out by specialised acoustic engineers. Results of testing are then entered into complex mathematical equations to determine an Impact Isolation Class or the new acoustic results of Ln,w + C1.

How the Testing is Carried Out

By measuring the sound transferred by the tapping machine through a floor across a wide range of frequencies, a series of data is collected. This data is then inserted into complicated mathematical formulas to determine the Impact Isolation Class (IIC)and the Ln,w+C1. While an Impact Isolation Class can only really be determined in a laboratory, field tests are carried out on various installations in service. These tests are recorded as Field Impact Isolation Class (FIIC) tests.

Variables that Influence Test Results

Many variables can influence the IIC or acoustic performance of a floor or floorcovering. Subfloor type (timber or concrete), subfloor thickness, subfloor density, subfloor construction method, frequency of subfloor supports or beams, the presence of a suspended ceiling, the floorcovering itself and underlay used in the testing room and background noises can all have a bearing on the acoustic rating achieved. Whether a receiving room is furnished can also have a bearing on the result. In most cases however, three main factors are looked at to draw broad comparisons. The thickness of the subfloor, the floorcovering used and the underlay used seem to have the most significant bearing on results achieved. To this end, Readyflor has been tested using different underlays on concrete subfloors of varying thicknesses.

Complying with Standards

The Strata Titles Act does not ask that a specific Impact Isolation Class be achieved. It simply states that “peaceful enjoyment” be afforded to the occupant below. This subjective terminology has created the need for acoustic engineers across Australia to apportion an IIC that affords “peaceful enjoyment”. In terms of Readyflor, we tend to follow the requirements of the largest apartment developers in Australia, whose acoustic engineers require a product to meet an FIIC55. Early in 2004, the Building Code of Australia introduced acoustic requirements stating that multi-storey apartments achieve a rating no greater than Ln,w+C1 62. It is important to understand that with Field Impact Isolation Class (FIIC) results, the higher the figure, the better the result. Conversely, with Ln,w+C1 testing, the lower the result, the better the acoustic rating.

How ReadyFlor performs

There has been a massive growth in the number of acoustic underlay products on the market for floating floors. However it can be shown that in general, underlay selection will have only an incremental influence over the completed test result.

Of far greater significance in the test result is the thickness and density of the concrete construction and associated ceiling treatments.

This is why most underlay suppliers do not provide comparative results from a given site, because even thick expensive and impressive rubber underlayments may only prove negligible improvement.

To illustrate this, here is a table of results yielded from blind testing with an impartial acoustic engineer on a given Multiplex building site. The results are given as IIC (Impact Insulation Class) where the higher the number the better the result.


Premium Combi-Lay 2mm  –  55

Granulated rubber 3mm  –  55

Granulated rubber 3mm  –  55

Granulated rubber 5mm  –  53

Granulated rubber 10mm  – 57

Cork 6mm – 56


It should be stated that the variation measured here does not constitute an audible variation, that is, the differences can be measured, but not differentiated with the human ear.

As you can see, in this site test the 5mm rubber has yielded a poorer test result than the 3mm rubber, while the 10mm rubber has tested similarly to the 2mm Premium Combi-Lay even though the rubber is a much more expensive product, it is also 5 times thicker.