Why we need to think beyond building regs for the sound insulation of CLT constructions
There are many advantages to using Cross Laminated Timber (CLT) as an alternative material to concrete, masonry or steel to erect building structures. However designing with mass timber is challenging for acousticians because, compared to most masonry materials, it is light and has a relatively high stiffness (in-situ concrete is approximately four times denser than CLT).
As sound insulation is generally increased with mass, CLT build-ups need more mass (with for example a concrete screed, plasterboard, dense boards, fine gravel, etc) to reach sound insulation performances required by British building regulations.
But is it enough?
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In fact, the sound insulation rating method used by the building regs was initially designed for heavy structures and don’t take account of the low frequency weaknesses of CLT structures. Technically speaking, building regs criteria generally look at frequencies from 100 Hz until 3150 Hz, when the weaknesses of CLT appear below approximately 200 Hz.
In other words, when assessing the sound insulation of CLT constructions, the frequency range studied should be larger than for heavy constructions by including performance requirements at frequencies lower than 100 Hz.
What is the consequence of ‘just’ achieving building regs?
Discomfort is likely to be experienced by the occupants who will hear the low frequency content of airborne sound sources (such as hi-fi systems, musical instruments, noisy appliances, building services, etc) or of the structure-borne noise generated by impacts and vibrating sources on the floors (such as people walking and building services).
This has been highlighted by several research studies, including ACOUBOIS , who raised the need to consider frequencies lower than 100 Hz for the impact sound insulation of timber floor constructions. The study suggests looking at the index L’nT,w + C50-2500 (i.e. impact sound pressure level covering frequencies from 50 Hz up to 3150 Hz) to have a better indication of the occupants’ satisfaction and their comfort.
Note: ACOUBOIS was undertaken for residential buildings only and suggests more similar studies to confirm the findings.
Figure 1 below helps to picture the need to extend the frequency range with an example. It compares two types of floor construction, one is a CLT base and the other one is a concrete base. Both have similar materials on top and the CLT base floor construction includes a suspended ceiling. Whilst they both achieve the same impact sound pressure level performance (Ln,w 59 dB), the CLT floor construction presents sound insulation weaknesses at low frequencies that are not within the range of building regs.
Note: the lower the impact sound pressure level, the better. Ln,w is the laboratory impact sound insulation performance of a floor construction. It is generally used by the acoustic designer to design a suitable floor construction and achieve an on-site performance in terms of L’nT,w in line with the relevant British acoustic standard).
It is therefore crucial to be aware of these sound insulation weaknesses when setting the acoustic brief during the early stages of a building project. The design team should locate the areas subject to discomfort due to low frequency sound and, if necessary, select sound insulation criteria to consider frequencies below 100 Hz.
Auralisation (e.g. audio demonstration) is generally a good method to render the differences between several criteria and help you choose the most relevant set for your project.
Where required, the acoustic consultant will advise on a suitable design to improve the sound insulation of the partitions at low frequencies.
This post has been largely influenced by the review of the German  and Swedish  building regulations that already include airborne and impact sound insulation requirements below 100 Hz.
 Acoubois – https://www.codifab.fr/actions-collectives/bois/acoubois-performance-acoustique-des-constructions-ossature-bois-1310
 DIN 4109-1 (2018) – Sound insulation in building construction – Part 1: Minimum requirements
 Boverket’s Building Regulations (BBR)
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