Tom Wright of George Barnsdale explains the difference between acoustic glass varieties, and provides examples of where increased investment is worth it.
Increasingly, window manufacturers are being asked by developers and specifiers about the acoustic performance of windows and doors. All glass has some acoustic ability – even 4 mm single glazed windows reduce noise by about 25 decibels. For optimal performance, specialist acoustic glass is the preferred choice, but housebuilders and developers must decide if it is really necessary, and whether the increased cost is worth it.
Efficiency & necessity
Mass is the best way to absorb sound, which means that the thicker the glass, the more sound that is absorbed. Windows and doors generally perform less well than walls and roofs, and sound travels through any place where there is air leakage, which means that a window that performs well in an air test also performs well in an acoustic test. Having said this, the same window, poorly fitted, will not perform as well if there is space between the window and the wall.
When it comes to the glass, double and triple glazing provide effective sound- proofing, and argon is preferable to air in the cavity because it is a denser gas. The optimal gas is krypton, but the difference in acoustic performance is marginal.
Very often, the standard annealed glass is sufficient for most purposes, but the best performance is offered by acoustic laminated glass. However, the way in which different frequencies perform is very complicated. An acoustic sound engineer is often employed by housebuilders and developers to advise on this. Acoustic laminated glass takes two layers of annealed glass and laminates them with a special acoustically absorbent plastic (PVB, polyvinyl Butyral) layer in between to give the appearance of a single pane of glass. Thicknesses of acoustic glass can range from 6.8 mm to 16.8 mm with a dB rating of between 35 and 42. Bearing in mind that a reduction of 10 decibels (dB) will halve the perceived sound intensity, the thickest acoustic glass is highly effective, but also only really necessary in the most extreme cases such as when building near an airport runway.
When selecting a window product, it is important to ensure that the whole window has been tested and not just the glass unit. Pricewise, thicker glass is more expensive, as is triple glazing and acoustic laminated glass. Developers should also be wary of suppliers over-engineering products, which can be a fairly common problem for those specifiers that lack the in-depth knowledge and data required for an informed recommendation.
Levels & location
World Health Organisation guidelines state that a maximum of 40 dB is recommended at night, which corresponds to the sound from a quiet street in a residential area. People exposed to higher levels (55 dB) – similar to the noise from a busy street – over the long term can trigger elevated blood pressure and heart attacks. Acceptable levels in reception rooms and other general living areas are below 50 dB.
Even in a fairly noisy environment in a domestic setting, it may not be necessary to incorporate acoustic glass into every room. The bedrooms which need to be quieter than the kitchen and living areas may be the only rooms that really need it.
Developers do need to consider the location of the development and the specification they are aiming to achieve, however. Low cost housing developments will naturally specify the lowest level of acoustic performance possible to satisfy planning and Part E Building Regulations. A luxury development in the centre of a city or town however might want to provide a higher performance solution.
If there is a measured noise outside of 70 dB and a bedroom inside with a required level of 30 dB, the reduction would be 40 dB. It would require a very high performing window to reduce it by that much. Conversely, if there is a measured sound outside of 55 dB and a living room inside with a required level of 50 dB the reduction of 5 dB could easily be achieved by a single glazed window.
In addition to the sound reduction, acoustic glass has other benefits including impact safety, creating a safe and secure barrier, and UV protection, helping to reduce fading and the ageing effects of the sun.
DBA, RW & RW+CTR
dBa is an’A-weighted decibel,’which means it is an expression of the relative loudness of sounds in air as perceived by the human ear. In the A-weighted system, the decibel values of sounds at low frequencies are reduced compared with unweighted decibels as the ear is less sensitive to low audio frequencies.
Rw is the weighted sound reduction index in decibels, and describes the airborne insulating power of a building element. It can apply to ceilings, walls, doors and windows. The higher the Rw rating the better the sound insulation. Increasing the Rw by one translates to a reduction of about 1 dB in noise level.
Rw+CTR is an adjustment factor which is used to account for low frequency noise, typically the biggest problem with sound insulation. Rw+Ctr is always a negative number so the Rw+Ctr will in practical terms always be less than the Rw value. For windows in an urban area where there is traffic noise this is the most realistic measure.
Don’t forget looks
It is also important to consider the aesthetic impact of using some of the thickest laminated glass solutions, especially where there is a conservation requirement. Conservation windows and acoustics can be a challenging match!
Tom Wright is managing director at George Barnsdale