The Sound of Silence
The metal door swings out as heavy and quiet as a bank vault’s. Inside is a second door, equally massive. This one swings inward, into a stark, metal-walled chamber painted a dull beige. Suspended from heavy chains high in each corner are large metal panels angled toward the floor.
On the other side of one wall, separated by a few well-insulated inches, is another chamber outfitted with long, triangular white foam wedges mounted floor to ceiling on every wall. Hundreds more hang from the ceiling like uniform stalactites. The light settles in pale blue pools at the base of each wedge, giving the room a quiet, almost arctic feel.
These two chambers, the first a reverberation room and the second, a sound-absorbing hemi-anechoic one, are the heart of Gulfstream’s Acoustic Test Facility in Savannah, Georgia. This is where Gulfstream acousticians and engineers spend thousands of hours a year testing and analyzing aircraft parts to determine how those that create noise and vibration can be made quieter and how materials meant to absorb noise and vibration can be improved.
Nothing escapes analysis, whether fabrics for headliners, latches on doors or carpeting for flooring. The cumulative effect creates an aircraft cabin so quiet it serves as a productive office area or as a setting for conversation where no one has to raise a voice to be heard, even while massive jet engines generate thousands of pounds of thrust just inches away.
“The Acoustic Test Facility, combined with our analytical and testing capabilities, has helped us achieve the ultimate quiet flying experience for our customers,” says John Maxon, engineering manager of the Acoustics and Vibrations Department.
A Good Soak
The 8,899-cubic-foot/252-cubic-meter reverberation chamber also has the capability to cold soak test samples, meaning the temperature of a sample can be dropped to as low as minus 60 degrees Celsius to duplicate temperatures at high altitude. The luxury of being able to cold soak in the controlled environment of a lab is also a sustainability effort; Gulfstream does not have to fly aircraft at altitude for hours to achieve the cold soak for testing like other manufacturers.
“Based on our ability to test at cruise temperature conditions, we’ve learned how some things are not affected by the cold that we thought would be and how some things are affected in ways we didn’t expect,” says Kris Lynch, lead test engineer. “That has helped us make some very important decisions.”
The G280 has the quietest cabin in the midrange class. Referring to the large-cabin G650, Maxon says, “Even flying higher and faster we’re still quieter than any other similar-sized business jet.”
The group’s success can be measured in the number of patents the group holds. Maxon and engineer Tongan Wang, who develops statistical energy analysis models to predict cabin noise, earned two patents for thermal-acoustic materials that attenuate noise and one for a technique to mitigate structure-borne noise transmitted into the cabin. The acoustics group has five more patents pending.
Make Some Noise
Gulfstream’s in-house lab allows acousticians to test, adapt and retest, which wouldn’t be possible if materials went to an outside facility.
“We’re developing solutions in the lab real-time instead of just getting results,” Lynch says. “Sometimes we can make changes within a matter of hours.”
Acoustic testing works like this: The test sample is set up in a transmission loss tunnel, a rectangular opening about 16 square feet/1.5 square meters between the two chambers. The reverberation chamber’s hard surfaces allow noise to bounce around, which achieves a uniform sound level.
This is a chamber made for making noise. Two concert-sized JBL speakers blast sound through the room, and high-pressure jet hoses add high-intensity sound. The technique creates up to 138 decibels of noise, the equivalent of a jet engine operating 50 feet away.
All the sound and fury from the reverberation chamber travels through the test sample to the hemi-anechoic chamber. The foam, rather than reflecting noise as the metal walls do, absorbs sound before any reflected noise can contaminate the measurements. That allows acousticians to measure only the sound transmitting through the sample.
Test results are incorporated into analytical aircraft models to predict the most effective configurations to reduce noise during typical cruise conditions-—all before ever flying the aircraft.
The lab serves additional needs. Environmental controls engineers use the chamber to pump air through mock-ups of the cabin ducts. That determines which configurations deliver the highest volume of air most efficiently and quietly.
Blocking sound in an aircraft creates more challenges than designing for other needs, says George Lesieutre, professor and head of aerospace engineering at Penn State University, State College, Pennsylvania. Lesieutre is also director of Penn State’s Center for Acoustics and Vibration, one of the leading such facilities in the U.S.
Typically, a quieter environment is created by increasing insulation or installing heavier, noise-absorbing materials. That adds weight, the last thing wanted when designing aircraft to be lighter and fly faster and farther.
Acoustic analysis of aircraft must look at everything—from how air travels over the fuselage to how wings are mounted—because it affects how sound and vibration travels. Often, sound generated outside the aircraft is carried through the body of the aircraft and into the cabin.
Lesieutre credits Gulfstream with being aggressive in exploring new techniques to reduce noise.
“Gulfstream is notable because it has an R&D group that is looking at this problem,” Lesieutre says. “They are very future-looking. They are doing novel things, and I think that’s what you want.”
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