The acoustic design implications of exposing CLT floor slabs

 

 

 

 

Are you designing a building with slabs made of Cross Laminated Timber? And would you like the underside of the slabs to remain exposed? Great idea! It will look good and feel more relaxing. 

But if you expose the slabs, you have no ceilings (of course!). This is likely to make the acoustic standard of the building harder and dearer to achieve. 

Ceilings might not always be visually appealing, especially when you can expose a timber structure instead, but they have several acoustic design functions that can lead to important cost savings. 

This post firstly presents the acoustic functions of suspended ceilings. It then summarises some design alternatives you could think about if your budget allows you to expose the CLT slabs. 

Acoustic design functions of suspended ceilings

Acoustic designers can advise for the installation of suspended ceilings to control:

• the airborne sound insulation performance of floor constructions.
• the impact sound insulation performance of some floor constructions.
• the sound flanking:
  • above partitions (through the slab and via the gap at the head).
  • via building services and structural penetrations through partitions.
  • via structural elements (like the beams) running across partitions.
• the sound reverberation within the spaces.
• the noise of building services hung from the soffits.

The sketch above pictures the above functions. 

Alternative design solutions, to using suspended ceilings, exist but very few of them combine all the above functions. Each alternative has its own cost, which overall is likely to require a higher budget than when suspended ceilings are installed.

 

Acoustic design alternatives

You could justify a higher budget to benefit from the aesthetics of the exposed timber and other qualities like smell, healthy feeling or also absorption of humidity.  

In that case, this section presents examples of solutions and materials that could help you achieve the acoustic standards you target for the building.

Note: Generally, the detailed solutions depend on the acoustic requirements, the structural design and the layout of the building. Anything shown below is just indicative.

 

CONTROLLING THE AIRBORNE AND IMPACT SOUND INSULATION 

CONTROLLING THE SOUND FLANKING ABOVE CRITICAL PARTITIONS

For any building, the acoustic designer pays important attention to the junctions between the construction elements. There is a need to control the vibrations transmitted from one element to another, up to the partitions (walls or ceilings) that re-radiate sound in the neighboring spaces. 

Timber constructions are very sensitive to such transmissions. Therefore an important part of the acoustic design is to find effective methods to decouple the elements. 

The most relevant methods depend on the structural design, the architectural design and the acoustic performances you target. Although, if you want to expose the underside of the CLT slabs, slab breaks and resilient strips are good solutions to think about from an early stage.

 

Slab breaks

Breaking the slabs above the partitions reduces the transmission of vibrations from one side of the partition to the other. Doing it might increase costs, but it can be very acoustically efficient in some cases.     

See the figure below showing where a slab break could be. 

 

Resilient strips (or resilient interlayers)

Resilient strips (also called resilient interlayers) are installed at the junctions of the CLT panels to control the transmission of vibrations in the rest of the structure. Most strips are made of elastomers but some use cardboard filled with sand. 

When exposing the CLT slabs, you install resilient strips above the critical partitions and possibly at the bottom of the partitions resting on the slab above.

See below an example to illustrate this.

To minimise acoustic bridging, resilient strips can also be used to decouple the fixings that hold the CLT elements together.

If you are looking for examples of resilient strips available on the market, visit Atelier Crescendo’s Acoustic Design Catalogue here.

Note: The anti-vibration characteristics of resilient strips depend on the load applied to them and how they are compressed. Therefore, the ideal specification for them will be the result of a thorough coordination between the structural designer and the acoustic designer.  

 

CONTROLLING THE SOUND FLANKING THROUGH THE BUILDING SERVICES PENETRATIONS

In the absence of a ceiling, the acoustic designer will generally advise you to not run the services (ductwork, cables, etc) through critical partitions (i.e. those acoustically rated and with an on-site requirement).

Instead, they should preferably run through the partitions that include a door. You will still need to carefully seal the penetrations to avoid any acoustic ‘leakage’. Dense mineral wool, plasterboard and non-setting mastic are examples of materials you can use for this. 

 

CONTROLLING THE SOUND FLANKING VIA THE STRUCTURAL ELEMENTS

In the absence of a ceiling, you are likely to require to box in the structural elements. Although it depends on the acoustic rating of the partition the structural element is going through.

Boxing in structural elements generally involves plasterboard and mineral wool inserted in the air cavities.

 

CONTROLLING THE SOUND ABSORPTION WITHIN CRITICAL ROOMS

If you expose the timber slabs, you can’t rely on the ceilings to control the sound reverberation in the spaces. Instead, you will need to use sound absorptive elements and finishes that are either suspended from the soffit or fixed to the walls and the floors. These could be:

• Horizontal acoustic rafts suspended from the soffit.
• Vertical acoustic rafts also suspended from the soffit.
• Acoustic wall panels fixed on the walls.
• Carpet.

Depending on the space, you need to find a combination of all the above to achieve suitable reverberation conditions. 

 

CONTROLLING THE NOISE GENERATED BY BUILDING SERVICES

If the building services (like FCUs, VAV boxes, etc) hung from the soffit are too loud and you don’t have a ceiling to attenuate the noise, the solutions would either be to insulate the casing of the units, encasing them (within bulkheads for example) or even sometimes fit new attenuators.

You could also think about reselecting the units for quieter ones. 

 

Final thoughts

If you expose the CLT slabs, you should expect more site inspections as any loss of quality in the workmanship will have a big influence on the acoustic performance of the walls and the floors.