Acoustic design for mass timber buildings

Loft Conversions

Marc Fuzellier-Hart is an acoustic consultant who created Atelier Crescendo in 2020.

Marc specialises in building acoustic design but more precisely in performance venues and timber buildings.

As well as working on building projects, Marc regularly creates content on :

 

This article summarises three blog posts written for the Acoustic Blog about some important topics of acoustic design for (mass) timber buildings. The topics are:

 

  • When you should involve the acoustic designer on your timber project.
  • The UK building regulations don’t always ensure acoustic comfort in timber buildings.
  • You need to plan for a larger budget if you want to expose the timber

 

Hopefully, this summary helps to shed light on some important aspects to know about acoustic design for timber constructions.

 

It should also help you understand the level of coordination required with the acoustic designer on timber building projects.

When should you involve an acoustic designer on your project?

 

The quick answer is: as soon as you can! But this might not always be possible.

 

You can really benefit from the advice of an acoustic designer when the design and the building layouts are decided but still flexible enough to be adjusted. This level of design usually corresponds to the concept design phase.

 

Why this timing?

To consider the low frequency weaknesses of timber

This is because the building regulations in most countries (excluding Sweden and Germany) don’t cover a range of frequencies large enough to take account of all the weakness of timber constructions.

Therefore, the acoustic consultant will set a brief specific for timber and generally time is required to educate the design team and make them aware that more and, possibly, different materials need to be used.

To consider the need for more mass

This is not only because achieving performances at low frequencies requires mass, but also because timber is light.

So you will need to add more mass to increase the sound insulation performance of the building elements (walls and floors).

Therefore, you should plan early this need for mass to avoid getting any costing surprises later in the design.

To potentially help decide whether dry mass or wet mass solutions are going to be chosen for the project

This is because acoustic solutions with dry mass and wet mass can be very different. Changing from one to another can lead to redesign and therefore increased costs.

To adjust the layout of the building if needed

Some layouts can require expensive acoustic measures.

For example, two noisy and sensitive spaces next to each other are likely to require thicker and denser wall or floor constructions to control the noise transfer between both rooms.

Therefore, if you relocate them near less noisy and sensitive spaces, you will save on material (and maybe also on workmanship).

To take account of the space needed for the floor and the wall constructions

Timber is pretty light compared to other masonry materials.

So instead of relying too much on adding mass to the wall and floor constructions, to increase their sound insulation performance, you need to rely on cavities. Examples of systems with cavities are raised floors, plasterboard ceilings or also plasterboard linings.

It is also good practice to decouple the building elements. Although you can use (thick) resilient materials, the best way to decouple elements is to disconnect them (i.e. leave gaps)!

The consequence of doing this is that you will reduce the space available in your building. Unless you plan for it early!

For the structural and the acoustic engineers to start coordinating their work

It is important for the structural engineer(s) and the acoustic engineer(s) to start working together early. So that they can coordinate different aspects such as:

  • the connection and fixing methods.
  • the presence/absence of cavity breaks (especially for the CLT slabs).
  • Stiffening some wall or floor constructions to increase their sound insulation performance at low frequencies.
  • The vibration isolation of some equipment or some building elements (like mass timber stairs).

To make the building more sustainable

Generally, acoustic products and solutions are not very sustainable.

Although you can’t avoid them, you can still optimise (i.e. minimise) their use with good early planning.

To leave time for further acoustic testing to inform the design

Although there is intense R&D currently undertaken by specialist suppliers and manufacturers, the availability of solutions tested on (mass) timber elements is pretty low.

Therefore, it is possible that a specific design solution put forward for a project requires further acoustic tests to ensure it achieves the desired requirements.

Tests can be done:

  • on the site, with a mock-up for example.
  • on other site with similar conditions.
  • in a lab.

Planning the tests and providing the results generally takes time, so it is best to plan the tests early enough. Hence, the need to engage an acoustic consultant during the early stages to tell you what needs more tests.

Timber is not the best material to control the sound transmission at low frequencies

 

If you speak about timber construction to an acoustic consultant (a timber specialist!), one of the first comments you are likely to get is something like:

 

“the sound insulation of timber constructions is not great at low frequencies (below 100 Hz).“

 

Even if timber constructions achieve the minimum building regulations in most countries (not in Sweden), you are not guaranteed comfortable acoustic conditions within your building.

 

This is because most building regulations are based on standards that were originally designed to test masonry constructions and don’t consider frequencies lower than 100Hz.

 

As an example, the post shows a graph two constructions that achieve very similar performances when measured and calculated in line with building regs methodologies, but have very different performances at low frequencies (i.e. below 100 Hz).

 

What about making the timber slabs visible?

It’s well known now, biophilic interior design and environments make us calmer and more relaxed. And making the (mass) timber structure visible is biophilic design.

 

However, the acoustic designer might tell you that exposing the mass timber structure, especially the underside of the slabs, is likely to cost you more than if you suspend a ceiling.

 

It seems very counter intuitive. How can omitting a material cost more?

 

In summary suspended ceilings have a few acoustic design functions, that without a ceiling, would need to be considered with separate and different design solutions.

 

These functions are primarily meant to control the following aspects:

 

  • The airborne and impact sound insulation performance of the floor constructions.
  • The transmission of sound above the partitions.
  • The transmission of sound through the penetrations created for the building services.
  • The transmission of sound through the building structure.
  • The sound reverberation inside the spaces.
  • The noise produced by the

Some materials to think about to control the airborne and impact sound insulation performance of (mass) timber constructions are:

 

  • Concrete screeds. Although very unsustainable, adding concrete to a mass timber slab is a good way to make the floor construction denser and stiffer. This contributes to increasing its the sound insulation performance.
  • Dry screeds like plasterboard, cementitious boards, dense fibre boards.
  • Fine sand or gravel screeds.
  • Dense wool or fibre boards. They provide resilience but also air cavities as they contain a certain amount of trapped air.
  • (Thick) Recycled rubber
  • Polyurethane for the resilient interlayers and the underlays.

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