

The new Pennsylvania September 11 memorial is an engineering feat full of wind chimes.

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bioLINIA/Paul Murdoch Architects

Honoring the heroes of United Flight 93 takes a whole lot of stainless steel. But you also need some serious computational analysis—and a stiff breeze.

At appropriately 93 feet tall, the Tower of Voices memorial in Somerset County, Penn., will be the world's tallest wind chime tower when it opens this September (though only 8 chimes will be installed for the opening ceremony this Sunday.) This tribute to the 40 heroes of Flight 93 who died on September 11, 2001 was the work of L.A. architect Paul Murdoch. Along with his team of consultants, Murdoch used computer modeling, physical prototyping, and intricate material testing to ensure the tower design provided both a powerful visual and an ear-catching audio display.

Then they had to figure out how to build the thing.

Engineering Sound

The Tower of Voices is perched on a mound overlooking the 2,200-acre site. A lighted stone path winds through the trees to the tower and around the mound, allowing visitors to go inside. The tower not only holds a place of prominence, but pushes the 40 individual wind chimes higher into the air.

The tower itself is 548,000-pound concrete and steel structure consisting of 53 precast pieces varying in weight from 25,000 to 31,000 pounds, includes connecting beams at 40-degree angles to represent the branch angles of hemlock trees surrounding the site. The foundation required 3-inch diameter steel rods grouted inside 7-inch diameter steel casings embedded 15 feet into solid bed rock, some extending 83 feet below the surface of the plaza on a reclaimed mine.

But even with such a robust and artistic form of concrete and steel, the key element remains the 40 wind chimes, each tuned and engineered to provide a pleasant sound.

It wasn’t a simple process.

Digital recreation of the Tower of Voices memorial.
NPS Photo

Chiming In

Murdoch brought on a chimes artist, who added an acoustic engineer. Murdoch also enlisted help from a tuning theorist and a musician. More help came from Arup, a design and engineering firm, which provided SoundLab testing, computer modeling, and multiple physical mock-ups.

Arup's Los Angeles acoustics group leader Elizabeth Valmont led the acoustic design of the chimes, and started the project by creating acoustic renderings. “Once we broke into what was really at the crux of how the visitor experience will happen, we had to break down the dynamic variables involved,” she says

The strategy was twofold: how would the chimes work mechanically and audibly.

The collective chimes will always produce a unique song because the chimes move with the wind. The 8-inch diameter aluminum tubes that make up each chime have half-inch thick walls and vary in length from 5 to 10 feet, weighing up to 150 pounds.

Stainless steel chime strikers vary between 15 and 25 pounds, moved by quarter-inch thick aluminum plate sails powered by the speed and direction of the wind.

The sound design team determined the two-octave range for the chimes, and instead of tuning all 40 to different pitches, they narrowed down the tunes to key foundational notes within the range, realizing that the sound of each one would be perceived differently because of tuning and location.

“We will have 40 different sounds,” Murdoch says, “even though they are not necessarily tuned differently. A middle C on one column down low oriented a certain way will sound different than another middle C higher on a different column and oriented a different way.”

A Changing Vision

One of the Tower of Voices 40 chimes, some of which weighed up to 150 pounds.
Gregg Payne Art + Design

The tower and the chimes took different forms during design. Murdoch opened the tower itself to ensure a more 360-degree experience for the wind to approach the tower from different orientations and speeds. “Just trying to calibrate the degree of solidity versus openness was compositionally one of the big challenges of the tower,” he says.

From there, he worked on the shape in order to best funnel the wind and create turbulence inside the tower to help activate the chimes in different ways, all while using computer modeling to stand in for the wind.

The hemlock-sloped beams help create the wind flow, tie to designs throughout the memorial and eliminate wildlife habitat potential inside the tower. With the complex curving and sloped beams, using precast pieces became the obvious choice for construction.

"The goal is to strike the chimes' fundamental frequency. Our job was more about seeing if we could hit those notes."

Designing the chimes themselves, though, proved the toughest task. The 30-piece chime assemblies each needed to attach to the tower and the consultants needed to help configure the materials and size of the chimes in order to produce the desired sound.

From how the wind entered the tower, how it moved the sail, how the chime tube and mechanism reacted to the striker, the 30 elements that make up the chime all lead to the acoustic output. “All of those variables have to create the sonic output desired,” Valmont says. “The goal is to excite the chime’s fundamental frequency. Our acoustic testing of the physical chime mockups enabled us to support the fabricator’s musical instrument design process.”

The end result is a tube with an interior striker, vastly different than a traditional striker on the outside. The chime assembly ties to the tower with a precast corbel anchored with a plate and bar. A suspension bar with stainless steel rods and joints requires precise positioning to not distort the sound and the sail and striker attach at the bottom of the chime to produce the most vibration. The bottom of each chime is tethered to the next corbel down within the tight assembly to ensure it doesn’t hit another chime.

Putting It All Together

Paul Murdoch Architects

To make it all work, the team started with digital analysis with computational fluid dynamic simulation and conceptual analysis, but moved to physical testing. Too large for a wind tunnel, they built full-size chimes and mounted them in an Arizona forest. Last summer a four-day test in Simi Valley in Southern California furthered testing of what was necessary for the internal striker to bend the chimes.

“We needed to test what kind of sail would work inside the striker to make this happen,” Murdoch says. “The tests were very much about motion and the components needed to generate the sound.”


Moving chimes into the Los Angeles-based SoundLab allowed the team to use a controlled environment to study the relationship between the striker and the chime. This testing established the feasibility of the internal striker and the placement of it inside the tube. Using the SoundLab, including playing recordings of ambient noise from the actual site, allowed everyone to make decisions in real-time without having to set up installs in Pennsylvania.

With traffic noise, crickets and wind through trees from the site playing in the lab, “we had a different idea of what to expect at each listener location,” Valmont says. “Physical testing and using the SoundLab as a tool was critical.”

The final result was the 93-foot-tall Tower of Voices containing 40 individual wind chimes. It took a lot of work, but it more more than worth to remember the 40 heroes of a Flight 93.

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