Window and facade acoustics
By Wojciech Brozyna, MD of Aluprof UK.
When specifiers design facades for new or refurbished properties in our towns and cities, the and acoustics are becoming crucial – both in workplace and residential properties.
Cities are becoming noisier, and while there are street designs that help reduce airborne noise, eventually it will try to penetrate the building structure through the building facade.
Windows or areas of glazing are the most susceptible parts of the facade as they are generally the ‘lightest’ of materials used. Sound attenuation is best achieved by using unbroken ‘mass’ of material, such as a brick wall. The greater the mass, the greater the sound attenuation.
Depending on the time of day, noise levels of road traffic can vary between 45-70+ decibels and this is just over the sound level that is seen to be healthy.
Old, draughty single glazed windows with poor or non-existent weather seals are the poorest performing windows when it comes to sound attenuation; the more airtight the window, the greater the sound reduction. Typically, a well-sealed single glazed window will offer a -30dB reduction in sound transmission, or what is known as ‘attenuation’.
For every 10dB reduction in the noise level achieved, the human ear perceives this as halving the volume.
Glazing plays the biggest part in sound attenuation. Double glazing helps, especially if the glass used is of differing thickness. Most materials at a given thickness will resonate at certain frequencies and allow some of those frequencies through. Using a different thickness ensures what passes through one will not pass through the other.
A well-designed triple glazed unit with differing air spaces and glass thickness will perform even better. Acoustic laminates used in laminated glass are specially designed to minimise sound transmission and a well-designed unit can achieve up to -45dB sound attenuation.
The use of triple seals further improve sound reduction, and high thermal performance systems with wide high performance centre seals will further attenuate sound around a high performance glazed unit.
Building Regulations – Part E, Resistance to the passage of sound – should be consulted, as well as a number of standards that are currently in place. There are also specific requirements for schools, hospitals and offices, etc. Some company chains, such as Premier Inn, have their own acoustic requirements. When specifying BREEAM-rated projects, there are points than can also be awarded for compliance in certain circumstances.
Sound attenuation is also a little more complex in application, and specifiers need to be aware that a change in window size can have an impact on expected attenuation levels.
Furthermore, attenuation is not linear. Transmission of frequencies may mean that at one level of frequency, in a given window specification, sound is well attenuated while at another frequency, sound attenuation will be less. Knowing what frequencies to specify to is crucial in getting the installation best specified for the client.
Glazed-in or in-frame acoustic ventilators, which consist of baffles and acoustic absorbing materials, allow ventilation with attenuated noise transmission. Acoustic ventilators may not offer the level of attenuation required and this is where an alternative mechanical ventilation provision should be considered. Depending of the configuration and specification of the design of windows or curtain wall required, our Aluprof design team can assist in the best options to be specified.
Where prime windows need to remain in place, for example in listed facades, then the option to install a secondary window system will offer a good solution. Using spacings of 100mm to 150mm between these windows, including the inside reveal lined with acoustic absorbing material, will offer very high levels of sound attenuation.
Flanking sound transmission is quite a talking point at the moment in the industry. On almost every acoustic assessment, when there is a continuous curtain wall, sound can travel through the facade to other floors, the reason for this is that the box will allow sound to travel unless it is broken per floor. Provisions can be incorporated such as an acoustic insert, and multiple transoms can also be used to allow the sound to dissipate. Careful design is required to allow movement of the curtain wall system while ensuring that sound does not travel between compartments or floors of the building. Curtain wall also has the ability to transmit impact sound across compartments as well as airborne sound and selecting suitable methods of construction will improve attenuation.
While there is no one fix for all conditions, our design team in London and at our head office here in Altrincham can offer advice on how to tackle sound attenuation in both new and existing buildings.