“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.
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.”