Inside one of NASA’s strangest test sites where sound disappears

• NASA’s Aero-Acoustic Propulsion Laboratory is basically a graveyard for sound.

• Each surface inside is covered with 17,000 barbed wedges that absorb sound and prevent echoes.

• Nicknamed “The Dome,” AAPL provides highly accurate sound level readings for aircraft and missiles.

Cleveland, Ohio, is home to one of NASA’s odd-looking facilities: the Aero-Acoustic Propulsion Lab.

Nicknamed “The Dome,” the AAPL is about five stories tall and 130 feet in diameter, wide enough to fit a baseball diamond but not much more than that.

For more than 20 years, AAPL has served as an anechoic chamber – a chamber without echoes. It’s basically a graveyard for sound.

If you stood in the doorway and shouted “hello” to a friend at the center, Luke Thompson, mechanical test engineer at AAPL, said they could barely hear you, even though you were only 65 feet away.

Researchers are testing jet and rocket engines inside AAPL

Inside The Dome, Thompson and his team are testing models of new rocket and aircraft engine parts to see how much noise they make. NASA, commercial aircraft manufacturers, and even the military are using this information to develop quieter aircraft.

The source of AAPL’s silence are two-foot fiberglass wedges—17,000 of them.

These pegs cover every surface, lining the walls, the floor, and even the test equipment itself. They look like chalky gray Toblerones, or a giant Lego set.

Why fiberglass wedges? Well, it is an excellent sound absorber. Builders use it for sound insulation in all kinds of places, from science labs to suburban homes.

Fiberglass comes in wedges because this is the shape that absorbs sound with the least material. While fiberglass cubes or balls would certainly be an interesting choice for decor, they would be too bulky for AAPL purposes.

Obviously, it would be difficult to walk on spiky fiberglass wedges for anyone who isn’t a gymnast.

Fortunately, items on the ground can be moved out of the way whenever Thompson and his team need to use the newest engine part they’re testing.

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People are not allowed in or even near The Dome during the test

Thompson said AAPL can test larger engine models than most other aero sonic labs.

Although The Dome isn’t big enough to hold an entire rocket, it really doesn’t need to be, because the engine (or a specific engine part) is the only equipment that gets tested.

The sonic nozzle tester, a 53-inch-diameter spiky wind tunnel, is the device Thompson’s team uses the most. “This is our bread and butter,” he said.

The NATR system is testing prototypes of engine nozzles, the tubes from which exhaust is released to give an aircraft boost.

The vents can reach 1425 degrees Fahrenheit during testing, so you can’t stand next to the NATR and hold the mic to pick up the noise.

NASA doesn’t even allow scientists to stand outside The Dome during testing, Thompson said, for safety’s sake.

Instead, they monitor the test in a separate facility, analyzing data from 24 microphones set up near the ceiling of the dome (and additional microphones as needed).

So while the engine nozzle is on fire, sound waves are flying in every direction. Sound waves hitting the microphones are recorded directly.

Meanwhile, those hitting the pegs are suppressed and absorbed, so they can’t echo back toward the microphones and add a second layer of noise data. Although the pegs sound funny, they are essential to getting the most accurate audio readings possible.

But why do we go through all this trouble to measure acoustics so accurately? There may be several reasons, depending on the aircraft.

Make the world a calmer place

For example, the Army has used AAPL to help design quieter aircraft for surveillance missions. After all, espionage can be rather difficult if the fighters can hear your plane screaming in the sky.

Another concern is workplace safety. NASA mostly limits noise levels from a rocket launch to an average of 85 decibels. Continuous exposure to sounds above this range may damage crew members’ hearing.

Meanwhile, commercial aircraft must be quiet for the comfort of staff and passengers.

Thompson said noise reduction is also important for people who live near airports where they have to listen to flights all day long.

The FAA limits the average ambient noise for airports near residential areas to 65 decibels (although surveys of residents say this is still disturbingly loud).

The dome plays an important role in making sure that new aircraft parts meet these guidelines. Despite its tall, tapered design, The Dome helps make the world a quieter place.

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