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The Timber Industry Yearbook 2011 Timber Research And Development Association www.trada.co.uk
Acoustics
Panel stiffness Timber frame performs well because of a number of factors. Part of the performance is affected by panel stiffness. Within timber frame, the first part of the construction is a combination of the plasterboard and plank which is dense and strong but flexible enough not to pass all of the sound energy into the core structure of the wall.
Transmission and damping Another factor is the damping of sound. It is linked closely with panel stiffness. Damping of sound is where the sound that is transmitted cannot continue through the structure, much in the way the suspension on a vehicle works, but on a much smaller scale.
The following is a list of preparations that need to take place before the acoustic testing can be carried out and will ultimately affect the performance of the wall: • Windows are fitted without any gaps around the windows and trickle vents are fitted. • Internal and external doors are fitted – internal doors will allow an increase in sound pressure and give a more accurate result; external doors will stop sound travelling out of the building and into the plot being measured. • Socket covers are fitted and skirting boards are in place. • Quiet conditions – the sound level meter used is only a machine and cannot differentiate between site noise and noise that is being measured as part of the sound test.
Void depth and mass One of the most important factors is a combination of void depth and mass in the party wall. A larger void and greater, isolated mass will both improve the sound insulation. Common Robust Detail EWT-1 (which is typically made up of 2x100mm studs with a 40mm gap, a layer of plasterboard and gypsum plank on each room face and a minimum of 2x60mm sound absorbing insulation within the cavity) has a large void between the structural materials. The void benefits the wall, meaning when the sound energy impacts the plasterboard wall linings, the resonant frequency of the cavity is sufficiently low that it is not measured on the standard sound insulation test. As mentioned before, the low frequencies that travel through are not within the human range of hearing.
Timber frame construction can attribute its good performance down to a combination of materials being put together in the right way. The balance of materials, voids and joints combine in a way that can give a high performance.
Sound absorption Higher frequencies are a lot more complicated exhibiting increases and decreases in sound pressure level, which causes vibrations that are passed on through the wall. Within timber frame, these are partially stopped with the insulation in the wall, which not only acts to avoid heat loss but absorbs higher frequency sound that resonates within the cavity by turning the sound energy into heat energy.
Barry Cope says to achieve good overall sound insulation performance, it is important to perform well across all of the frequencies.
Timber frame is not without its problems, albeit they are rare. If foundations are not built exactly to specification, the resulting foundations can be too small. The result of which means the timber frame party wall cavities will not be wide enough. Resonance within the cavities will therefore occur at a slightly higher frequency and become measurable during the sound insulation test, reducing the overall performance of the wall.
Supporting Services Chiltern Dynamics is UKAS-accredited to undertake both Sound Insulation Testing and Air Tightness Testing. Visit www.chilterndynamics.co.uk or telephone 01494 569800. TRADA Technology offers a design and consultancy service to prepare for sound insulation testing. Visit www.trada.co.uk or telephone 01494 569725.