Seventeen years ago, the 40 passengers and crew members of United Airlines Flight 93 diverted their hijacked plane from its intended target of the U.S. Capitol, crashing into a rural field in Shanksville, Pa. Though this act of heroism likely saved hundreds, perhaps thousands, of more lives from the Sept. 11 attacks, everyone aboard the plane was killed.
This weekend, those passengers and crew members were honored in the dedication of the Tower of Voices, a 93-foot-tall precast concrete memorial tower featuring 40 wind chimes located on the grounds of the Flight 93 National Memorial, which also includes a visitor center and plaza. Made of polished aluminum tubes, the chimes range between 8 and 16 inches in diameter and between 5 and 10 feet in length to create their intended specific and distinct musical note.
Because this memorial is first structural chime system of its kind, according to the National Park Service, global design and engineering firm Arup was brought on to help achieve the acoustical elements conceived by the memorial designer, Beverly Hills, Calif.–based Paul Murdoch Architects.
Below, Elizabeth Valmont, acoustics group leader of Arup Los Angeles, discusses how the firm achieved the desired sounds of the tower.
ARCHITECT: What was the inspiration for the tower design?
Valmont: The 93 foot tall tower utilizes 40 wind chimes to honor the passengers and crew members aboard United Airlines Flight 93. Each wind chime has a distinct tone due to its length, tuning, and placement in the tower representing the critical and lasting role of the voices heard through phone calls and voicemails that took place during the flight.
How many mock-ups were required to achieve the desired sounds of the chimes?
Prototype testing was critical to the project’s inception and fabrication process to design chime assemblies that created the desired sound in the wind. Arup and the design team conducted three preliminary mock-ups—preceding additional testing by the fabricator—in different locations to conduct acoustic measurements and recordings, and to assess striker mechanism dynamics to determine the performance specification of the chimes.
Our first prototype took place in Arizona, where chimes were mocked up with a traditional external striker. Subsequently, architect Paul Murdoch, AIA, decided to go with his initial design, which featured chimes in a cascade down the tower. This meant the striker would have to be housed inside each chime, rather than externally. This change led to feasibility studies in Arup’s Los Angeles SoundLab, where we explored and identified ideal striker locations, weight, and material attributes.
The next prototype was created to develop a reference design in advance of fabrication. This testing took place in Simi Valley, Calif.—a location with similar wind conditions to the Tower of Voices site in Pennsylvania—to observe the internal striker performance under natural wind conditions.
What were the greatest engineering challenges in constructing and assembling the tower and its chimes?
This is the largest grouping of wind chimes in the world and the first to use strikers within the chimes themselves. Each element of design and construction needed careful, technical consideration. The C-shaped tower was designed and orientated for optimal airflow. Music theory was applied to select specific musical notes to create an atmosphere of contemplation. One of the most significant challenges was designing an internal striker mechanism capable of generating the desired chime strikes. That included changing the striker mechanism throughout the design process, and using prototype mock-ups along with acoustic measurements to establish the desired sound level and quality. We wanted to realize Murdoch’s vision of creating a meaningful acoustic experience from the moment visitors arrive at the tower plaza.
What role did Arup's SoundLab play to help complete the project?
The Arup SoundLab played an integral role in designing and completing this project and worked in collaboration with Murdoch. Early in the project process, Arup acoustically modeled his tower design options to create 3D audio simulations of the 40 chimes for him to assess. We also used Arup’s SoundLab for the Murdoch to confirm acceptable sound level targets for the chimes in the plaza. These auralizations of the chimes suspended within the tower allowed the design team to hear a representation of what they would sound like at the site before they were built.
What impression do you hope visitors leave with after visiting the Tower of Voices?
Our team has been working on this project for approximately two years and the visitor experience has always been a top priority in our work. While we recognize that each visitor’s impression will be unique, we hope that everyone who has an opportunity to hear the soundscape at the Tower of Voices site will feel that the Flight 93 National Memorial properly honors the sacrifice of the 40 brave passengers who were on that flight as well as their families.
This interview has been condensed and edited for clarity.
