Meet the Acoustician: John Maxon
Meet John Maxon, the Engineering group head of the Acoustics and Vibrations Department at Gulfstream. Maxon formerly served for 19 years as an acoustician in the development of ultraquiet nuclear submarines and has more than 27 years experience studying acoustics and vibration. The research he and his team complete in the Acoustics Test Facility (ATF) is a distinguishing factor in the evolution of Gulfstream jets and, through what’s learned there, Gulfstream has developed the quietest cabin in the industry. We asked Maxon to tell us a little more about his work in the reverberation and hemi-anechoic test chambers.
What is a hemi-anechoic test chamber?
The hemi-anechoic test chamber is the quiet chamber in our Acoustics Test Facility. The ATF is made up of two large test chambers connected by a transmission loss tunnel. The hemi-anechoic chamber is lined on all sides and the ceiling with wedge-shaped melamine foam designed to absorb noise and isolate the interior from any exterior noise or vibration that could contaminate test data. It is hemi-anechoic in that the floor is void of melamine foam so large objects and test equipment can be easily positioned. The reverberation chamber is the other side of our ATF, which as the name implies is designed to be reverberant or to echo. Conversely, anechoic means “free from echoes.”
Just how quiet is it inside the chamber?
The ambient sound level is -10 dB, or 10 dB below the threshold of hearing. The chamber is so quiet that you can hear your own heart beat.
Why so quiet? What advantage does the test chamber give to Gulfstream aircraft?
The chamber allows us to accurately test how noise is transmitted inside and outside the aircraft. Through our research we can significantly improve customer comfort and cabin flexibility. The quiet cabin of a Gulfstream allows customers to enjoy a flight of quiet work, conversation, entertainment or peaceful rest. We strive to make our cabins as close to an office or library environment as possible. Secondly, improvements to lessen exterior aircraft noise reduce the environmental impact and allow customers to fly in and out of airports with more restrictive noise regulations.
Does noise affect a passenger during a long flight?
The adverse physical and psychological impact of prolonged exposure to high levels of noise and vibration are well documented. High levels of noise and vibration can increase stress and fatigue and reduce concentration. On a Gulfstream, the low cabin noise and vibration levels allow our customers to be more productive while in flight and arrive at their destination feeling well rested.
What kinds of materials are tested in the ATF?
We try to test every material and isolate every potential noise source that can make its way onto the aircraft and be transmitted into the cabin, from interior decorative coverings to exterior wing-to-body air vents. If a component has the potential to make noise, we test it. We’ve tested operating air gaspers and noisy door latch mechanisms. We’ve mocked-up and tested full cabin air distribution systems. We’ve tested aircraft acoustic folding curtains to determine not only the best materials to block sound but also the best way to stitch the materials together to ensure that the sound doesn’t leak through the stitching. Basically, we test every material used in the production of an aircraft for its potential to either block or absorb cabin noise.
How do the facilities help in research and development and set Gulfstream apart from the competition?
Having our own ATF has allowed our acoustics and vibration group to take innovative ideas from conception to on-wing components in a fraction of the time it would have taken had we been required to use outside facilities. We can quickly prototype a design idea, test it, make design changes and retest. This allows validated improvements to be implemented into the design cycle, or our existing product line, quickly. Additionally, our facility has been upgraded to include the unique capability of performing extreme cold temperature testing which simulates changes that occur after being cold-soaked in the low temperatures found at cruising altitudes—where noise is transmitted differently.
Do you find the real world a noisy place?
Actually, yes—I am much more aware of noise. When it’s part of your job to determine the source of those “funny sounding noises” and fix them, you become much more aware of not only the loudness of the noise, but the quality of a noise. I am much more sensitive to annoying tones, squeaks and rattles.
If a tree falls in a hemi-anechoic chamber, does it make a noise?
Not as much noise as it would in the forest, but it would sure make a mess of the chamber!
The decibel is the unit commonly used to measure the intensity of sound. Broken down, the word decibel means “one-tenth” (deci) of a “bel,” the unit of measure that pays homage to Scotland-born inventor and sound pioneer Alexander Graham Bell.
Decibels (dB) are measured on a logarithmic scale as a ratio of the sound intensity, as opposed to a linear scale. In similar fashion to the way the Richter scale is used to measure seismic activity, small increases in number represent exponential increases in sound intensity. For example, on the decibel scale the sound of silence is 0 dB. A sound 10 times more powerful is 10 dB. Yet a sound 100 times more powerful than near total silence is 20 dB—an increase 10 times as powerful. A sound 1,000 times more powerful than near total silence is 30 dB. The sound of a jet engine is about 1 trillion times more powerful than the smallest audible sound of a pin drop.
The work in Gulfstream’s hemi-anechoic chamber is focused on helping to reduce interior cabin noise so a whisper can be heard amid the sound outside the cabin of two jet engines and the rush of air from near Mach 1 speed only inches away.
In fact, most sounds we hear, from the purr of a cat to the roar of a jet engine, are simply moving air. When a sound is created it makes the air vibrate, and these vibrations carry the sound to the ear. Some everyday sounds and their average decibel ratings are:
0 dB: near silence and the lowest threshold of human hearing
10 dB: a pin drop
20 dB: rustling leaves
30 dB: whisper
60 dB: conversation
70 dB: vacuum cleaner
110 dB: rock band
140 dB: jet engine at takeoff
160 dB: shotgun
194 dB: sound waves become shock waves
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