To balance all of these factors while also taking into account the size and cost of the materials means the design of walls is crucial. The acoustic properties of a wall are measured as a weighted sound reduction index (Rw), where the higher the figure, the better the insulating properties of the wall. This refers to the overall properties of the entire wall, including irregularities such as gaps, windows or pipes, not just the raw materials. Rw is measured by looking at frequencies that are most commonly found in ambient noise, around 100 Hz to 4,000 Hz frequencies.
The Building Code of Australia (BCA) does not currently set a standard Rw for external walls. Individual councils may rule on specific structures differently, but an Rw of 50 is usually a good guideline for internal separating walls. The minimum Rw required for internal separating walls varies depending on the location within the building, but is usually around 50 Rw. For example, when a habitable room such as a bedroom or living room is adjacent to the kitchen or laundry of a separate unit, the walls must have an Rw of at least 50.
Due to their porous nature, bricks have fairly poor acoustic properties on their own because sound can easily travel through any gaps filled with air. A brick wall sealed with render or a cement-based paint will have a much better Rw than an unsealed wall, according to Cathy Inglis, technical advisor at Think Brick Australia. “Rendering seals all the little air gaps in a brick wall as well as adding mass. Plasterboard can potentially reduce the acoustic performance of a solid brick wall — if a gap is left between the plasterboard and the wall, sound can reverberate in that gap.”
Because they are more solid, a single brick wall is also a good conductor of structural sound, such as that arising from plumbing or from appliances such as washers and driers. A double brick or double stud wall effectively handles this problem. Having a gap filled with insulation breaks up the conduction of sound and a double stud wall filled with insulation such as polyester or fibreglass can achieve a better Rw than a brick wall.
The higher density of a brick wall means it is much more effective at filtering out lower frequency sounds. The strong bass frequencies produced by a home entertainment system can more easily pass through a wall made of lighter materials such as stud. When it comes to acoustic performance, the more mass the better, says Inglis. “The acoustic performance in a lightweight framed wall is not as good. The higher density of a brick wall will help take out the lower frequency noises.” Inglis says the best overall acoustic performance will be achieved with a double leaf brick cavity wall which is then rendered.
Ross Baxter, NSW general manager of Boral Bricks, agrees that the more dense the wall, the more sound will be filtered out. “Boral now produce a ‘party wall’ brick which is 150 mm rather than 110 mm, and due to its higher mass, is more effective at filtering out sound.” He adds that when looking at the acoustic performance of a wall, it is important to look at the properties of the finished wall, not just the bricks. “No matter how good your materials are, if you don’t seal the joins at the ceiling or if you drill a hole through for a powerpoint, sound will still be able to travel through.”
Ventilation can also be a big issue in larger buildings. Ducted air takes up a lot of space, as well as being expensive. Recycling air can also lead to sick building syndrome, but opening up a window can often mean letting in too much sound. Silencair is a new product that presents a cost effective solution to the problem of how to circulate fresh air from outside without allowing in all the ambient sound.
Silencair was developed as a response to the issue of noise in buildings by Dr Chris Field through his doctoral research at the University of Sydney. The company, Silencair International, was established to take over development and marketing of the product. Field describes the product as “a high-tech solution to one of the biggest problems of city living”.
It is effectively a wall vent that allows air to circulate while keeping the sound out. Field says the vent works by allowing air to pass across a series of acoustic attenuators which are tuned to particular frequencies. “So when sound enters this box, which has a ventilation opening in it, the sound interacts with the tubes and is reflected back away from the inside of a building.” Air circulates through the vent passively, so any air movement across the vent will result in air transfer. A gentle breeze is enough to transfer 25m3 of air per hour.
The vents are designed to keep out the elements, but also have an adjustable cover that can be closed in extreme conditions. It has a screen to keep out bugs and since it has no moving parts, it is easy to clean and maintain. The materials are resistant to UV light and environmental decay, so the units should last indefinitely. The vents are around the size of two house bricks and are made of 100 per cent recyclable plastic.
The Silencair 240 is 240 mm thick and will fit into the external wall of most buildings, although there is the option of the Silencair 140 for thinner walls. As far as the BCA is concerned, the product is classified as a window and comes under the same restrictions — for example, it cannot be installed within 900 mm of a neighbour’s window or property boundary.
Chris Matthews, managing director of Silencair International, says: “the key to the product is that it can provide ventilation in acoustically compromised buildings. It can also be used in conjunction with other systems to save on ducting. This can reduce energy consumption and maintenance costs.”
In a recent apartment complex developed in Mascot, installing Silencair wall vents saved developers $3,000 on each of the 110 apartments by not having to install ducted air in each unit. Matthews says the incentives to developers are more than just financial too. “Silencair is environmentally friendly. It is made of 100 per cent recyclable plastic as well as saving on the amount of energy used in a building.”
