Why Skinny Skyscrapers Are So Loud (And How To Quiet Them)
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(Inside Science) -- In recent years, major cities like New York have sprouted a new breed of skyscraper so tall and thin they look like they should topple in a breeze. Apparently, many of them sound like it, too.
"On a windy day, there were literally these sounds almost like guns going off. It was loud creaking and then a 'Pop! Pop! Pop!' So I called it snap, crackle, pop," said Bonnie Schnitta, founder and CEO of the New York-based acoustical consulting firm SoundSense, recalling what she heard in the first "pencil tower" she worked on in 2016.
The buildings are perfectly safe, said Schnitta, but such noises can disturb and frighten residents. In November, at a meeting of the Acoustical Society of America in Seattle, Schnitta presented what she and her team have learned from solving sound problems in slender skyscrapers. The key, she said, is that everything must be flexible enough to move when the wind makes the whole structure sway and twist.
All tall buildings sway. "They can't not sway," said John Ochsendorf, a structural engineer at MIT who was not involved in the project. "The whole trick is to design the buildings so that the building occupants never feel the movement."
When people do feel movements, it's a comfort problem, not a safety problem, said Adrian Brügger, director of the Robert A. W. Carleton Strength of Materials Laboratory at Columbia University, who was not involved in the project. People tend to feel changes in acceleration more than movement itself, so what a person feels isn't always what's happening. For example, when planes take off, there is usually a moment when the acceleration slows, and passengers may feel like the plane is falling. Similar illusions can occur in buildings, especially when people are trying to sleep.
"You may be in an office building all day that actually moves 10 times as much as your home," said Brügger. "If you would just now lie down in your office building and close your eyes, you'd suddenly feel the floor move."
The other way building movements can bother occupants is by creating noise. Residents of skyscrapers may be reassured to know that most of the noise isn't caused by the support structures, which are designed to flex and move as a single system, said Brügger. Instead, they're caused by things like pipes, ducts and internal walls -- components that aren't responsible for holding the building up. Such parts may rub and strain where they connect to the rest of the architecture.
"These movements are usually very, very small, the ones that create these noises. Sometimes it can be as small as just the metal stud slipping from one thread of the screw to the next," said Adam Wells, an acoustical engineer at CDM Stravitec in New York, who was not involved in the project.
In general, the taller and skinnier the building, the more it sways. Classic skyscrapers don't move much because they are relatively squat. The Empire State Building, for example, is about seven-and-a-half times taller than its thinnest side, counting the spire; seen edge-on, it has about the same shape as the cardboard tube in a paper towel roll.
Now imagine taking three of those cardboard tubes and stacking them on top of each other. That's still fatter than New York's 1,428-foot-tall Steinway Tower. A less extreme pencil tower, 432 Park Avenue, is 15 times taller than it is wide. These towers owe their existence to recent advances in materials and engineering, as well as space constraints that compel developers to build up rather than out.
Pencil towers are built from materials stiffer than those in traditional buildings to compensate for their increased tendency to sway. Still, Ochsendorf expects most pencil towers probably move more than traditional buildings of a similar height.
"The flexibility goes up exponentially with an aspect ratio being so thin," he said.
From noisy to quiet
In the first pencil tower Schnitta consulted on, a couple had purchased an entire floor of the building, empty except for the exterior walls and elevator shaft. They planned to build their dream home there, living in a rented apartment two floors down while they oversaw the construction. But the noises, which included the gunshot-like bangs, were so bad they were ready to abandon their plans and move out. The building owner's representative, the architect and the contractor reached out to Schnitta in hopes of persuading the residents to continue the project.
To find out what was causing the noise, Schnitta and her colleagues took sound measurements and used accelerometers to measure the building's movements. They detected side-to-side sway, as well as twisting movements that concentrated the noises on certain levels, rendering some floors louder than others.
The bangs, they found, were caused by the difference between movements high up versus lower down. The highest parts of a building swing through the largest arcs, while the base stays still because it is fixed in the ground. The same is true on a smaller scale for individual floors.
"If the ceiling is moving more than the floor is, then there's a point where this rigid structure is straining, and then all of a sudden it has to make that motion, and it goes 'snap' or 'pop,'" said Schnitta.
To solve the problem, the SoundSense team used devices called bushings that create flexible joints between the internal walls and the ceilings and floors they connect to. When they installed the devices in a mock-up room on the floor the clients had purchased, most of the sound disappeared.
To quiet the remaining creaks and groans, Schnitta and her colleagues used tricks such as painting rubbery material on joints between metal parts of the architecture to dampen vibrations. Components such as ducts and pipes had to be isolated from the things they were connected to, with every contact point capable of movement. The couple who had purchased the floor decided to keep the property.
Schnitta emphasized that solutions must be custom-designed for each building and situation. A building's unique architecture, how the wind is channeled by surrounding buildings, and the sensitivities of the occupants all play a role.
Creating flexible joints between rigid parts of buildings is a tried-and-true approach for solving sound problems, noted acoustic scientist Mike Raley. It's similar to the springs in your car, which absorb the shock of the tires' movements so the body of the car doesn't get thrown around. (Raley's company, PAC International, produced some of the products Schnitta's team used.)
But the extreme proportions of the new skyscrapers make it all the more important to use every tool available.
"It's such a unique thing, these big, tall pencil towers," said Raley. "It sounds like you're in a sinking ship, or like a submarine that's gone too deep. It's really unsettling."