The new concrete belfry juts 93 feet above a vast palette of summer greens, high enough for breezes aloft to catch the 40 wind chimes inside, each sounding a different tone.

Those numbers strike a solemn chord in the collective American memory.

The tower looms over the site of one of the triad of 9/11 tragedies — the open field in Somerset County where United Flight 93 crashed as its 40 passengers and crew foiled a band of al-Qaeda hijackers who were attempting to redirect the aircraft, presumably to Washington and the U.S. Capitol. It is intended both as a marker and a monument to a planeload of heroes, with every chime representing the distinct voice of one lost that day.

Set to be finished in September for the 17th anniversary of the terror attacks, the column stands sentry at the entrance to the 2,200-acre Flight 93 memorial near Shanksville, about 70 miles southeast of Pittsburgh. The park has been years in the making, moving ahead in phases that began in 2011 with the installation of a plaza, with a wall of victims' names. In 2015, a visitor center, an education center, a multipurpose gathering space, pedestrian trails, and a walkway tracing the flight path toward the preserved crash site were unveiled. The National Park Foundation, which helps fund National Park Service projects, raised $46 million from more than 100,000 individual donors.

The final phase, the $6 million Tower of Voices, is the keystone of the project, which was awarded to former Philadelphia architect Paul Murdoch and his firm after they won an international design competition in 2005.

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Murdoch, 61, said his local upbringing — he's an alum of Marple Newtown High School in Newtown Square — influenced his design strategy. "Before I was interested in architecture, I had an appreciation for the landscapes in Pennsylvania," he said, and that translated "directly to the memorial of Flight 93."

The environs of the crash site have changed dramatically since September 2001, said Stephanie Loeb, a public affairs specialist for the National Park Service. Once host to a strip-mine, the area is now biologically diverse and filled with wildflowers. Thanks to ambitious volunteer tree-planting efforts, there are 40 memorial groves and even some protected wetlands.

So although the tract — three times the size of New York's Central Park — has been made hallowed ground by a tragedy, the memorializing of the site has served as an engine for rejuvenation.

"Once a common field," Loeb said, "and now look at it. It brings forward serenity, both the natural area, and now the the national significance it holds for all Americans."

After spending most of his adult and professional life in California, Murdoch said he's grateful for the opportunity to return to his roots, particularly for so consequential a project. He calls it "public place-making," which he said he learned from the Philadelphia hardscape and translated for the rural highlands of Western Pennsylvania.

Murdoch worked for several architecture firms in Philadelphia before getting his master's degree at UCLA. He recalls strolling the cobblestone streets of Society Hill and reflecting on William Penn's original plan for Philadelphia as a city of gardens, as well as Louis Kahn's reinterpretation of Center City and its best use, which influenced his still-maturing values.

"There is a beautiful scale to the city; there's a clarity to it," he said. "Even in high school, but certainly as I was living there afterwards, just walking the streets and appreciating the public spaces, some of the most beautiful public spaces I think, in America."

Murdoch found himself drawing on those images to create an unlikely public space in that remote field outside Shanksville.

He wanted to capture the voices of the passengers who frantically called their family members after the four hijackers took control and forced them to the back of the plane. The voicemails they left, the conversations they had, were their legacy. A particular inspiration was Todd Beamer, who issued the rallying cry "Let's roll!" before he and others rushed down the aisle, into the cockpit, and into history. Murdoch decided his tower would be a memorial of sound.

Later this year, the remaining pieces of the Flight 93 wreckage, currently held in storage lockers, are to be returned to the site and buried in a restricted area to which only the victims' loved ones will have access. In the distance, there will be chimes.