- NASA's Aero-Acoustic Propulsion Lab is basically a graveyard for sound.
- Every surface inside is covered with 17,000 spiky wedges that absorb sound and prevent echoes.
Cleveland, Ohio, is home to one of NASA's stranger-looking facilities: the Aero-Acoustic Propulsion Lab.
Nicknamed "The Dome", the AAPL is about five stories high and 130 feet in diameter, wide enough to fit a baseball diamond but not much more.
For over 20 years, AAPL has functioned as an anechoic chamber — a room without echoes. It's essentially a graveyard for sound.
If you stood at the entrance and shouted "Hello" to a friend in the center, they would barely hear your voice, even though you'd only be 65 feet away, said Luke Thompson, a mechanical test engineer at AAPL.
Researchers test jet and rocket engines inside AAPL
Inside The Dome, Thompson and his team test models of new rocket and jet engine parts to see how much noise they make. NASA, commercial airplane manufacturers, and even the military use that information to develop quieter aircraft.
The source of AAPL's silence is two-foot-long fiberglass wedges — 17,000 of them.
These wedges cover every surface, lining the walls, the floor, and even the test equipment itself. They look like chalky gray Toblerones, or a giant's Lego set.
Why fiberglass wedges? Well, it's an excellent sound absorber. Builders use it to soundproof all sorts of places, from science labs to suburban houses.
The fiberglass comes in wedges because that's the shape that absorbs the most sound with the least material. While fiberglass cubes or spheres would certainly be an interesting decor choice, they'd be too bulky for AAPL's purposes.
Obviously, spiky fiberglass wedges would be hard to walk across for anyone who isn't a gymnast.
Thankfully the ones on the floor can be moved out of the way whenever Thompson and his team need to roll in the latest engine part they're testing.
People aren't allowed inside, or even near, The Dome during testing
Thompson said AAPL can test larger engine models than most other aero-acoustic labs.
While The Dome isn't large enough to contain a whole rocket, it doesn't really need to, since the engine (or a specific engine part) is the only equipment being tested.
The Nozzle Acoustic Test Rig, a spiky-looking wind tunnel 53 inches in diameter, is the machine Thompson's team uses the most. "That's our bread and butter," he said.
The NATR tests prototypes of engine nozzles, which are the tubes that exhaust shoots out of to give an aircraft thrust.
The nozzles can reach up to 1,425 degrees Fahrenheit during testing, so you can't exactly stand next to NATR and hold up a microphone to capture the noise.
Thompson said NASA doesn't even let scientists stand outside The Dome during testing, for safety's sake.
Instead, they monitor the test in a separate facility, analyzing data from the 24 microphones set up near the dome's roof (and additional microphones as needed).
So while the engine nozzle is roaring with flames, sound waves fly out in every direction. The sound waves hitting the microphones directly get recorded.
Meanwhile, the ones hitting the wedges are suppressed and absorbed, so they can't echo back toward the microphones and add a second layer of noise data. The wedges, while funny-looking, are essential to get the most accurate acoustic readings possible.
But why go through all this trouble to measure acoustics so precisely? There could be a few reasons, depending on the aircraft.
Making the world a quieter place
For example, the military has used AAPL to help design quieter jets for surveillance missions. After all, spying can be rather difficult if combatants can hear your plane screeching across the sky.
Another concern is workplace safety. NASA mostly limits the noise levels of a rocket launch to an average of 85 decibels. Continuous exposure to sounds above that range could damage crewmembers' hearing.
Meanwhile, commercial airplanes need to be quiet both for employees' and passengers' comfort.
Thompson said noise reduction is also important for people who live near airports since they have to listen to flights all day.
The Federal Aviation Administration limits the average ambient noise of airports near residential areas to 65 decibels (although surveys from residents say that's still annoyingly loud).
The Dome fills an important role in making sure new aircraft parts meet these guidelines. Despite its loud and pointy design, The Dome helps make the world a quieter place.