The following article was written by Rachel Dovey, a Northern California-based journalist, with input from Denis Blount and Peter Dodds, members of Arup’s acoustics, audiovisual, and theatre consulting group.
There are many well-chronicled upsides to building with cross-laminated timber (CLT), an innovative and fire-resistant take on the classic framing material. It’s far greener than cement or steel — trees, after all, are “made” by sunlight and water rather than through a carbon-heavy manufacturing process, and they sequester carbon as they grow. It holds great potential for rural economic development and stateside job creation. And if done right, it’s cheaper than conventional building.
One downside: the acoustics. A CLT frame is significantly lighter than one made of steel or concrete, providing less airborne and impact sound isolation. That’s particularly challenging for developers trying to construct any kind of multifamily housing, because building codes specify how much acoustic separation is needed between units.
When Arup began working on the Framework tall timber tower, one of the winners of the US Department of Agriculture’s 2014 Tall Wood Building Prize Competition planned for downtown Portland, Arup acousticians Denis Blount and Peter Dodds set out to make sure the building’s residents wouldn’t be kept up all night by their upstairs neighbors. Together with the architects at LEVER, they began drafting mitigation strategies and testing them using the Arup SoundLab.
At the time, the stakes were high and the data scarce. USDA officials selected Framework to serve as both pilot and proof that CLT can work for tall buildings — when completed, the 12-story building is expected to be the first timber high-rise in the US. Wood, of course, is often used to frame single-family homes and smaller office buildings, but since the end of the 19th century, when cities began to densify, steel and concrete have been developers’ exoskeleton of choice for buildings over five stories. Their preference was partially due to simple engineering — the so-called balloon frame design commonly used for wooden buildings at the time could only sustain a few stories — but it was also a response to the massive fires that devastated cities like Chicago, Baltimore, and San Francisco between 1871 and 1906.
CLT, however, is sometimes referred to as “plywood on steroids” and is not your great-grandfather’s timber. It’s made by gluing multiple layers of dimensioned lumber together to form slabs, then layering those slabs in opposite directions like three-dimensional latticework, and it can compete with steel in both strength and fire resistance. Still, because it’s not commonly used for tall buildings in the US, Dodds, Blount, and the LEVER team didn’t have much information to work with — and they knew their results would set a precedent in the domestic market.
When we started there was not a large set of acoustic data on how these mass timber assemblies perform ” Denis Blount Associate Principal
"We began by collecting information on what’s been done by some of our overseas colleagues.” Blount explains. (Mass timber is an umbrella term that encompasses CLT as well as nail-laminated timber and dowel-laminated timber).
Mass timber is more commonly used in central and western Europe, particularly Austria and Germany. A large mass timber structure is also currently going up in Amsterdam. But according to Blount, builders in those markets often cover the wood up to improve acoustics and/or fire resistance. Framework’s designers wanted the building’s aesthetically pleasing bones exposed, which proved challenging since thickness and density tend to equal better sound isolation. Complicating matters, the international data didn’t always translate.
“The hard thing with that data is that it didn’t necessarily account for the use of a domestic product,” says Jonathan Heppner, an associate with LEVER Architecture. “So while there might be data for acoustical testing that performed well in another international location, we have a domestically produced piece of CLT panel. The standards work differently because of the requirements for code.”
And the codes they were following relied on target numbers. For example, the International Building Code requires an STC (or sound transmission class) and IIC (or impact insulation class) rating of at least 50 each — but the team suspected that due to the new material’s unique characteristics, those numbers might not accurately reflect residents’ perception of noise.
“Because mass timber is quite a bit lighter than traditional building materials, what’s getting captured in that single number is not necessarily always indicative of how we’ll experience sound interacting with that building element,” Dodds says.
To that end, Arup’s SoundLab was indispensable. The calibrated 3-D listening space that Blount calls “the aural equivalent of a visual rendering” was developed for concert hall design, and it allows users to make decisions based on the evidence of their own ears, rather than abstract renderings and charts.
From the outset, Dodds and Blount knew that they would be relying on some kind of mass-and-resilience combination to form a sound-mitigating barrier — something massive for blocking airborne sounds and something pliable to absorb footfall. A CLT handbook has several example assemblies, many of which meet the code minimums. But many of those pretested assemblies were actually too thick, Dodds explains. They would be costlier to produce, and the building’s inhabitants would lose valuable floor-to-ceiling space.
So the two consultants began testing combinations of rubber and concrete in various sizes and densities, adding and subtracting things like a resilient underlayment to a finished floor or a pad that would act as a kind of noise-isolating cushion. They tweaked the design in the SoundLab, using recorded noises that could potentially be bothersome to Framework residents — for example, a television playing in a room or the sound of footsteps from the floor above. Because they’d gathered test data for the various assemblies, they could run the television noise through the SoundLab’s custom software and watch — or, more accurately, listen — as it took on the acoustic properties it would have in each design scenario. Eventually they landed on what Blount describes as a “CLT-plus-acoustic-topping sandwich.” To showcase the beauty of the wood, they opted for a design that layered the wooden framing with a thin layer of rubber and then a thick (but not too thick) lightweight concrete slab called Gyp-Crete, followed by the finished flooring.
Their testing data filled a definite void in the US market. And because it was USDA-funded, it will be made public so that designers and acoustic consultants working on the country’s next mass timber high-rise won’t face quite as many challenges as the Framework team.
CLT probably won’t go mainstream overnight. Despite advances in fire safety, strength, and now acoustics testing, a number of challenges remain. For one thing, it’s still something of a novelty, and from an architectural perspective, Heppner says, developers in hot urban markets tend to migrate toward lower-hanging fruit. “If an American company isn’t really super familiar with CLT and they’re not familiar with what a project needs and looks like, they’ll probably move on to other, more easily gained markets,” he says.
Whenever new building methods or materials are introduced, there’s a learning curve associated with how making these new advancements in technology can best serve the market and create efficiency, ”
Dodd explains, "Right now, we’re in a phase where the industry is figuring out how best to deliver timber products.”
Still, Framework, which is set to break ground next year, will accomplish the USDA’s goal of showing that a mass timber high-rise can be built. And Arup has helped create a blueprint proving that those tricky sound challenges — at least one potential downside of CLT — can be solved.
“In my mind, the big thing is that we’re showing the rest of the US, ‘Hey, we can build with mass timber, and we can build tall with mass timber,’” Blount says. “It’s feasible from a fire and life-safety standpoint. It’s feasible from a code standpoint. It’s feasible from an economic standpoint. And it’s feasible from an acoustics standpoint. That’s a big win.”