The truth behind timber: part 4.
A sound solution.
So what does a timber building mean for acoustic design? Now that the performance of CLT and other mass timber designs is better known, we have been able to explore its acoustic credentials and how it can enhance the human, natural and economic value of a project.
Bad first impressions.
The high strength-to-weight ratio of CLT compared to concrete slab-type construction materials means that it weighs significantly less. At first sight, this is a red flag to acousticians relying on the mass law of acoustics to achieve good sound insulation. Secondly, the basic principle of decoupling building elements to gain vibro-acoustic isolation is impinged by the heavily jointed and rigid panels needed for structural stability.
CLT structures are relatively new to the UK so there is limited on-site sound insulation test data; this means many acoustic consultants often lean towards cautious designs when specifying party walls and floors construction.
The hidden credentials of CLT.
– Exposing the elements
Exposing structural elements is a common architectural feature in modern buildings. This represents significant challenges during the detailed design stage and construction. Deflection head details and movement joints, when exposed, can significantly compromise the sound insulation of a separation.
With CLT, because all panels are load-bearing, and the way they are jointed, there is less need for deflection detailing and movement joints that could cause sound leakages.
This reasoning, supported by on-site test data, would likely allow a saving on the partition’s selection ratings for the same on-site performance.
– Flanking performance
CLT is also beneficial to control direct sound transmission, not by mass but by its heavily jointed and airtight nature. However, exposing structure can increase sound flanking transmission if panels are continuous between spaces.
CLT can provide party walls that are thinner, lighter and cheaper. Statistical energy analysis predictions can show whether a single layer of plasterboard on a thin cavity either side of a 100mm CLT panel is sufficient to meet performance requirements and building regulations.
This innovative engineering approach was something we developed at Hoare Lea, and landed a nomination at the 2019 Acoustic Awards.
– Speech sound insulation
CLT is particularly efficient in development where the control of low-frequency sounds is not a key requirement. For commercial developments, for example, the combination of CLT, raised floors, and ceilings can achieve a good level of sound insulation at speech frequencies, which are the primary concern in office buildings. Where tenant flexibility is required for future fit-outs, a combination of ceiling and/or additional mass can control noise transfer between the spaces.
– Vibration control
The structural properties of CLT mean that they perform better at controlling re-radiated noise from ground-borne vibration sources, such as underground or railway lines. In contrast, it presents issues for the control of footfall vibration, impact sound insulation and control of vibration sources located upon them. However, methods to improve this are relatively simple to implement and can comprise additional layers and/or stiffening elements to make floors less responsive.
– Construction noise
With reduced foundations and the off-site fabrication of the CLT panels, the duration and magnitude of construction noise are decreased. This can lead the way to significant cost, environmental and quality benefits.
CLT presents a major advantage when it comes to protecting local amenities and reducing noise pollution to an area.
Innovation always carries risks, but with robust engineering thinking and collaboration between designers and contractors, acoustic risks on CLT schemes can be controlled. The positive outcomes go beyond current good practice and bring unprecedented value and quality to a scheme.