Does the building structure influence the acoustic efficiency?

Julie Leduc

Thursday, 15 March 2018

Achieving acoustic performance in a building depends on a combination of several factors. The structure of the building and the quality of workmanship during the installation/application of each of the materials are all elements that will influence the acoustics of each project and will provide optimal acoustic to its occupants.

Five main acoustic principles

 First of all, in order to understand the role of the various elements that make up a structure, it is necessary to understand five main acoustic principles:

  • Mass
  • Resilience (decoupling of materials)
  • Treatment of cavities
  • Sealing
  • Flanking

Influence of each material on acoustic performance

 Here is the description of a typical assembly of a wood structure that will allow you to better visualize the materials described in this article:

  • Floor covering (not covered in this article)
  • Acoustic membrane (not covered in this article)
  • Concrete slab
  • Resilient material
  • Wood subfloor
  • Wood joists
  • Acoustic insulation
  • Sealing (polythene or sealer)
  • Resilient channels or gypsum suspensions
  • Gypsum

In addition to being a ceiling finishing material, gypsum board adds mass to the assembly. The use of 2 gypsum boards rather than one can offer a superior acoustic performance of 2 to 3 dB (decibels). In addition, a 5/8" thick board is preferable to the 1/2" panel. Once again, the objective is to have as much mass as possible.

The use of resilient channels or gypsum suspensions is one of the most influential factors for optimizing the performance of the assembly. The addition of a resilient channel improves performance by an average of 3 to 5 dB and the gypsum suspension provides an additional 3 to 5 dB to the resilient channel, which is considerably high.

The assembly must be as sealed as possible, because where air passes, so can sound. It is necessary to follow the mechanical principles of acoustics to soundproof the perimeter of the conduits, the openings of the built-in lights or even the passage of the electrical wiring. The openings in the ceiling and through the assembly can reduce the performance by 10 dB, which is not negligible.

For best acoustic efficiency, when using blown acoustic insulation, it is best to fill the cavity between the beams at least 2/3 of its height (ideally 100%). Batt acoustic insulation, meanwhile, may fill less voids compared to the blown acoustic insulation, therefore may offer less performance. However, if properly installed, it will provide equivalent efficiency.

Wood joists most frequently encountered are type "I" or "open web". It is the rigidity of the wood joists that contributes mainly to the acoustic performance. The choice of wood subfloor also seems to influence performance. According to some studies we have consulted, OSB panels tend to perform better and offer 1 to 2 dB and more.

It is important to decouple the concrete slab with a resilient material. The acoustic contribution provided by the use of a membrane under the concrete slab is major and differs according to the type of product used. According to our expertise on different structures and some studies, including the one made by FP Innovations on a mass timber structure, a membrane under the concrete slab can offer up to 10 dB, depending on the material that composes it. The standard products used under concrete slab are wood fiber ("Tentest"), which would offer the weakest performance and rubber, which would offer the best performance. Moreover, in order to reduce the lateral transmission of sound (flanking), the load bearing walls should not be in contact with the floor they support.

Also, it is important to choose a concrete whose mechanical capacity is adapted to the weight that it will have to support. Concrete plays an important role in the mitigation of airborne noise as it is a mass addition to the assembly. The type of concrete chosen for the slab and its density have little influence on the acoustic performance of the impact noises. On the other hand, the weight of the concrete gives rigidity to the structure.

Finally, it must be understood that acoustics is a precise but not linear science. It governs rather according to a logical mathematics. That being said, we cannot add the performance of each element of an assembly to conclude a final result. Rather, it is likely to understand the importance of each material on the acoustic performance of a building to make better choices. There is no miracle product; the combination of the best solutions is the key to achieving optimal acoustic efficiency. Do business with professionals who will guide you through this process.

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