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A first look at the powerful final phase of the Flight 93 memorial

This 10-story chimed instrument commemorating the victims of Flight 93 is an unprecedented feat of design and engineering.

The first thing you notice is the wind. It accompanies a quiet chorus of chirping birds, rustling leaves from surrounding trees, and the whoosh of muted traffic on a nearby highway. It makes for a deceptive tranquility, belying the horror of United Airlines Flight 93, which crashed near Shanksville in rural southwestern Pennsylvania field after passengers and crew thwarted hijackers’ attempts to hit the U.S. Capitol on September 11, 2001.

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Since then, the Flight 93 National Memorial has slowly taken shape over an elongated 2,200-acre expanse in Shanksville, leading from Rt. 30 to the crash site, and funded by $46 million from 112,000 private donors. It’s composed of a landscaped honeybee garden, a Memorial Plaza, a Wall of Names, a visitors and learning center, and a sandstone boulder marking the point of impact. The park’s final and most majestic phase is unveiling September 9 (you can watch it live here) that ties this chapter of the 9/11 tragedy to the area’s ever-present wind.

[Photo: National Park Service]

The Tower of Voices is a 93-foot-tall musical instrument of 40 chimes representing the 40 passengers and crew members lost on the flight, many whose last contact came through phone calls to loved ones. It stands atop a 10-foot-tall mound and will eventually be encircled by white pine trees. More than a monument, it’s an entire landscape, sculpted with sound.

“We wanted to create a landmark that could be seen from the highway, that combines what happened on 9/11 with the natural beauty of the site,” says Stephen Clark, superintendent of the National Park Service’s Western Pennsylvania Parks. “It will serve as a welcoming beacon to visitors, to experience the harmonies of the individuals, and put them in the right frame of mind about honor and courage. From there, they will move onto display objects, cell-phone recordings, stories recounting their efforts to overpower their hijackers, and get to know the 40 as people.”

The tower also represents an unprecedented engineering and design feat, one that combined physics, hacking, and a touch of madness. It required a design team of wildly varying skills, including a musician and tuning theorist; chimes artist; sailors; machinists and fabricators; acoustical, structural, aerospace, and wind engineers; and simulation experts to craft digital chime mock-ups for 3D recordings, figure out tower shapes through computational fluid dynamic models, and run wind tunnel tests of acoustic conditions in different wind directions and velocities.

[Rendering: bioLINIA and Paul Murdoch Architects]
“The tower was one of the original features of the design, but it wasn’t a foregone conclusion that it could happen,” says Paul Murdoch, of Los Angeles-based Paul Murdoch Architects. His firm has helmed the entire park design since winning a 2005 competition that garnered 1,100 international entries. “It was too unprecedented, and required a certain evolution of confidence and institutional capability to make it possible. It’s the largest and most complex in terms of design and emotion. We’re still trying to figure the last things with the chime assembly. Had we started with the tower, I don’t know if we could have done it.”

The challenge of the chimes

Structure. Early on, Murdoch and his team nixed a number of simpler designs. They didn’t have the right sound. Funneling wind through tubes would sound too eerie and, “we didn’t want the clanking sound of striking metal. Besides the damage, it wasn’t what we wanted symbolically,” he says. “We wanted a resonance emanating from a tower.” The solution was to create vibrations from bending 5- to 10-foot-long aluminum tubes, all with 8-inch diameters and half-inch-thick walls. The ringing mechanism involves quarter-inch-thick aluminum plate sails attached to 15- to 25-pound stainless steel chime strikers inside the tubes. Wind rushes against the sails, which move the strikers with enough force to temporarily bend the tubes, creating a rounded pitch reverberating with over- and undertones.

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“Since tuning would change with wind speed and temperature, by keeping all the tubes the same diameter and thickness, they would all contract and expand in relation to each other,” says Murdoch. “So while the absolute sound changes, the relationship sound stays the same.”

[Early audio simulation. Rendering: National Park Service]

Sound. To figure out the combination of musical pitches, Murdoch hired musician and tuning theorist Sam Pellman, who was also a friend of one of the United 93 families. The amount of wind variability ruled out a composed piece. Instead, Pellman arrived at two modified scales, one harmonious and one dissonant, to represent the passengers and crew acting together against a backdrop of violence and tragedy.

Pellman’s take was, “I see this as being able to engineer possibilities,” says Murdoch. “It was truly a work of love for him; he had the right spirit and capabilities for it.” Sadly, Pellman’s contribution would become the tower’s 41st voice, after he died last fall from being hit by a motorist while cycling. “He was there at the soundbreaking of the tower—a full-scale chime that people could ring—and had the chance to see the memorial and initiation of the construction phase.”

Simulation. Configuring the chimes required numerous types of simulations. A sound lab tested decibel range against the area’s ambient sound, strike frequency, and the best acoustic placement for plaza seats. (The decibel range runs 0 to 65, and best acoustics are 30 to 50 feet, at a 45-degree angle from the tower.)

A small-scale physical mock-up last summer tweaked the suspension system (elements of which its designer had perfected at Burning Man). It was a maddening four days in 95-degree heat changing the way the chimes were suspended (cable vs. rope), striking and rebuilding the model, testing individual components—down to the range, weight, and mass of strikers—for why they did and didn’t work.

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An element of suspense hovers over how it ultimately will fare. “I’ll let you know in September,” laughs Murdoch. “One of the great challenges of this, besides never having done it before, was not being able to completely simulate this because of the complexities and variability. We simulated what we could digitally, worked it out mathematically and computationally, and got feedback from physical testing.”

[Rendering: bioLINIA and Paul Murdoch Architects]

Closure and continuance

For the families, the tower’s completion represents a continuance for their loved ones.

“This was such a unique partnership. We were really offered a seat at the table,” says Families of Flight 93 president Gordon Felt, a special needs camp owner and educator, who lost his older brother, Edward, in the crash. A computer systems engineer, Edward was en route to his company’s headquarters in San Francisco.

“We know that they formed a battle plan and fought for that flight and to get home,” says Felt. “The message is: This could have been any one of us. Could I have done what they did? Could I have stepped up? That’s the inspiration.

“Paul captured the majesty of the environment and the juxtaposition of a rural, peaceful place in stark contrast to the devastating violence,” he adds. “This is the tower that greets visitors, that speaks to them as they come, and then is woven into the background noise. The way I look at it, it’s standing guard, defiant as our loved ones once were, and ennobles the spirit of the memorial.”

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About the author

Susan Karlin is an award-winning journalist in Los Angeles, covering the nexus of science, technology, and arts, with a fondness for sci-fi and comics. She's a regular contributor to Fast Company, NPR, and IEEE Spectrum, and has written for Newsweek, Forbes, Wired, Scientific American, Discover, NY and London Times, and BBC Radio

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