Acoustic Performance
Why are there Acoustic Performance Measurements?
The Building Code of Australia (BCA) requires that building elements have certain levels of insulation from airborne noise and impact sound. This means that certain sound performance levels for different building situations are specified. For example, the BCA requires that the walls dividing apartments have a certain amount of sound resistance.
How is Acoustic Performance Measured?
Sound or acoustic performance is measured by the weighted sound reduction index (Rw). The Rw value will increase as the acoustic performance of a material or construction improves.
Correction factors are also used to take into account common noise sources. There are two types of correction factors, Ctr and C. Lower frequency noise (such as urban road traffic or jet aircraft at long distances) is accounted for by Ctr, whilst C is for higher frequency noises (such as highway road traffic and aircraft at short distances).
The BCA often specifies in terms of Rw + Ctr, where Ctr is a correction factor for low to medium frequency noise. Therefore, the correction factor is used to show how effective a particular wall construction is against those types of noise. The correction factors are negative numbers, so the smaller the number the better. A small number (eg -1) shows that the construction does not have a large performance drop in that sound frequency range.
How is a Sound Result Interpreted?
An acoustic performance result is expressed in terms of Rw and the correction factors. For example, a wall might have Rw(C,Ctr) of 55(-1,-5), which means that Rw is 55, C is -1 and Ctr is -5. The BCA often specifies the Rw + Ctr. For this wall the Rw + Ctr will be 55 + (-5) or 55 - 5. So the Rw + Ctr is 50.
How is Rw any Different to STC?
Often people still refer to the sound transmission class (STC). The STC is no longer used to measure sound performance; the Rw is used in its place. This means that STC result can no longer be used, as the requirements of the BCA have changed. The STC was based on different criteria and did not include any correction factors.
Building Code of Australia Requirements
Building Classes
The Building Code of Australia (BCA) classifications for acoustic performance are categorized based on the building type. The BCA should be referred to for more detail on these and other classifications.
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Class 1
Buildings include single dwellings that do not have another dwelling above or below it, such as a stand-alone house or a row of townhouses. |
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Class 2
Buildings include buildings that contain two or more sole-occupancy units, such as an apartment unit. |
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Class 3
Buildings include residential buildings that contain a number of unrelated persons, such as a guest house or the residential part of a school, hotel, etc.
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The acoustic requirements discussed below apply only to NSW, other states may have different requirements. The descriptions are an aid only and the BCA should be referred to for the full requirements.
Class 1 BCA Requirements for NSW
Walls separating a habitable room in one dwelling from another habitable room in a second dwelling in Class 1 buildings are required to have an Rw + Ctr of not less than 50. A habitable room is a living room, dining room, bedroom, study or the like.
Walls separating a habitable room in one dwelling from a wet room in a second dwelling in Class 1 buildings are required to have an Rw + Ctr of not less than 50 and to have a discontinuous construction. A wet room is a kitchen, bathroom, sanitary compartment or laundry.
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CONSTRUCTION: Habitable to Habitable
REQUIREMENTS: Rw + Ctr > 50
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CONSTRUCTION: Habitable to Wet
REQUIREMENTS: Rw + Ctr > 50
With discontinuous construction. |
Class 2 & 3 BCA Requirements for NSW
Walls separating a habitable room in one dwelling from another habitable room in a second dwelling in Class 2 or 3 buildings are required to have an Rw + Ctr of not less than 50.
Walls separating a habitable room in one dwelling from a wet room in a second dwelling in Class 2 or 3 buildings are required to have an Rw + Ctr of not less than 50 and to have a discontinuous construction.
These requirements are the same so far as for Class 1 buildings. However, Class 2 or 3 buildings have an additional stipulation. The BCA requires that walls in Class 2 or 3 buildings that separate a sole-occupancy unit from a public area (plant room, lift shaft, stairway, public corridor, public lobby or the like) must have an Rw of not less than 50. In addition, if the wall separates the sole-occupancy unit from a plant room or a lift shaft, the construction must be discontinuous.
What is Discontinuous Construction?
Discontinuous construction requires:
- A minimum 20mm cavity between two separate leaves
- Resilient wall ties such as those provided by Matrix Industries, if the wall is masonry, and
- No mechanical linkage if the leaves are not masonry.
Achieving the Required Acoustic Performance
The sound performance requirements of the BCA can be satisfied by:
- Building a deemed-to-satisfy wall as specified in the BCA
- Demonstrating compliance by lab testing of an exact construction, or demonstrating compliance by field testing.
The deemed-to-satisfy walls are the lowest bound results. Walls built of specific clay bricks may have better performance, as indicated by the manufacturer from individual tests. Austral Bricks conducts extensive testing of different wall constructions and a large number of acoustic solutions are available to suit your construction. It is important to recognize that bricks from different manufacturers and manufactured in different plants give different results.
The NSW Clay Commons Ratings brochure has been produced showing some of the tested acoustic performance of different wall constructions. Alternatively, comprehensive and up-to-date Sound Solutions are available on our website.