What’s Old is New: Acoustical Considerations as Wood Buildings Make a Comeback

Wood construction is enjoying a revival from coast to coast. In Toronto, Sidewalk Labs is proposing a collection of mass timber buildings with the tallest one reaching 30-storeys. On the other side of the country, the University of British Columbia’s Brock Commons 18-storey Tallwood House is one of the tallest contemporary mass-timber hybrid structures of its kind in the world.

As the enthusiasm for wood construction grows, these projects are likely just the tip of the iceberg. With new changes coming to the National Building Code (NBC) in 2020, the construction of mass timber buildings is expected to rise.

Starting next year, the NBC will allow the construction of tall wood buildings up to 12-storeys from the current limit of six except in British Columbia. The province secured permission from the National Research Council (NRC) to adopt the rules right away.

For years, many people shied away from wood construction because critics claimed it posed a fire risk. Safety rules required stairwells to be built with non-combustible materials and combustion resistant roofs which made the design more difficult.

Those fears have since been extinguised as the wood industry evolved. Mass timber has taken over as the material of choice and it’s actually fire-resistant. These buildings can be defined as one in which the primary loadbearing structure is made of either solid or engineered wood such as cross-laminated timber (CLT), nail-laminated timber (NLT) or glue-laminated (glulam) timber.

Benefits of Mass Timber Buildings:

With the environmental benefits of wood buildings, it’s not hard to see why mass timber is gaining popularity. As a natural resource, it is readily available and renewable and has less impact on the environment compared to manufacturing steel or concrete. According to a study in the Journal of Sustainable Forestry, replacing steel with mass timber in construction would reduce CO2 emissions by 15 to 20 per cent.

Mass timber construction can also be more economical. While cross-laminated timber is not necessarily cheap, there can be significant savings due to greater construction efficiency. Mass timber structures are often built as components offsite and transported ready to be installed at the project site. Fewer people are required to assemble the building which amounts to reduced labour costs. In addition, the prefabricated panels and site assembly mean buildings made from mass timber are quicker to construct and as such have much shorter project timelines. For example, UBC’s Brock Commons tower was completed within 70 days after the prefabricated components were ready for assembly.

Challenges of Mass Timber Buildings:

Mass timber construction offers builders multiple advantages but there are some unique challenges when it comes to acoustics. In wood-framed buildings, lower frequency sounds transfer more easily through walls compared to a concrete one. Because it is lighter than concrete, it is harder to stop the transmission of sound which makes it easier for noise to travel.

As a general rule, more mass means better noise control. This does not mean that mass timber is a bad choice for a building that needs to be quiet. With proper acoustical design, mass timber buildings can offer the same privacy as we have come to expect from steel and concrete.

Many of the major tall wood building projects in North America such as the Wood Innovation and Design Centre (WIDC) incorporated noise control measures into the design to ensure a peaceful environment. Among the many steps taken, any openings were sealed, the amount of air space, stud spacing, and number of layers on the wall on each side were considered.

There are no requirements or building code recommendations about sound transmission in commercial spaces, but most residential codes mandate a specific sound performance or sound transmission class (STC) rating for walls and floor systems. STC is the ability of a material to reduce transmission of sound between rooms. Generally, the higher the STC number, the less sound is transmitted. However, even with a high STC rating, there can be noise issues. This is because sound doesn’t just travel from one room to another. It travels through the walls, floor and ceiling, gaps and cracks and this is known as flanking.

The following are some considerations to help minimize sound transmission in mass timber construction:

• Determine the right mass timber option: Not all mass timber is created equally so pick the one that is best suited for your needs and noise control. The options include glue-laminated (glulam) timber, cross-laminated timber (CLT), nail-laminated timber (NLT), mass plywood panel (MPP), and dowel-laminated timber (DLT). Controlled laboratory STC testing found CLT performs slightly better than other options as the laminates are cross-oriented in a panel and have less susceptibility for small holes and cracks.
• Increase mass: A panel of concrete and a panel the same size and thickness of CLT would provide significant differences when it comes to sound transmission.  As a basis, early in the design, use a minimum 5-PLY layer of CLT or equivalent.  Another common solution is to incorporate a hybrid design which combines mass timber with other materials such as concrete that are much better at stopping sound. To do this, a 1-3” thick layer of concrete or gypsum is poured on top of the wood base assembly. For further improvement, use a soft rubber or semi-rigid insulation matt between the mass timber and the concrete layer.
• Build either on-top of or below the structural mass timber panels: Simply put, wood is lighter than concrete and therefore we cannot simply rely on the base structural element as the sole vertical or horizontal separating acoustic element between rooms.  For floors, we need to either build on top or build below the mass timber to achieve satisfactory sound separation.  This can be done by incorporating a dropped ceiling below the wood structure, using a raised floor system or adding additional layers of either pre-cast/wood layers separated with rubber matts or rigid insulation. The appropriate selection is dependent on the ceiling height and desired ceiling finish.
• Ensure an isolated stud separated from mass timber walls: One of the most effective ways to prevent noise transfer is using an isolated stud wall constructed 25mm from the mass timber element. Depending on performance requirements, a 65 or 92mm standard stud size filled within insulation and 1 layer of drywall can be used for effective sound control.
• Address the flanking paths during design: Mass timber structural elements pose a significant challenge for secondary paths of noise transfer. Even a 5-PLY CLT element does not provide sufficient flanking control on its own. To avoid sound getting into the floor and then transferring structurally via vibration to the adjacent horizontal room, a build up of layers on top is necessary.  This should be incorporated into the design to ensure adequate space to allow for the additional layers. Similarly, for sound travelling through the ceiling element via a structure-borne path to an adjacent horizontal room, if not addressed during design process, the solution to the problem would be to either incorporate a drop ceiling in the source room or the receiver room which can often contradict the design goal of a project.
• Incorporate decouplers: Decouplers break the direct connection which reduces the amount of noise that travels through the structure. This includes a rubber floor underlay or mat or incorporating a noise barrier ceiling or drywall ceiling supported on a resilient channel or clips. Once the building is complete, the tenant or owner can further minimize impact noise between floors by as much as 15-20dB just by incorporating soft surfaces like carpet.

A combination of these recommendations will help to achieve even better sound control. What’s most critical is to ensure acoustical decisions are made early in the project design process as many of the controls required for acoustics tend to contradict or affect either another performance requirement or visual design element. If these items are not addressed, the walls, ceilings and floors may need to be taken apart later to incorporate various sound control methods which can result in significant costs, stress and mess.

Tim Preager is a principal with Aercoustics Engineering Limited. He has led the acoustical design for a number of innovative projects including Maple Leaf Gardens, Wood Innovation Design Centre (WIDC), the Catalyst Spokane building, Trades Technologies Facility at Lethbridge College, and Algonquin College. Tim also leads the transit related services that Aercoustics provides as it relates to vibration modelling, analysis and measurement. He can be reached at [email protected]