Quieter Airplanes Through Science

It's luxury travel, yes, but the sound levels in the cabins of many private jets can make it hard to think, much less carry on a normal conversation. But that's all changing. In the last few years acoustics engineers and cabin design specialists have succeeded in achieving dramatic reductions in interior noise, allowing for easier conversations among cabin mates and a less tiring travel experience overall. Suddenly, a quiet cabin is de rigueur among the fashionably well-heeled and savvy executives alike.

The science behind creating a ­quieter cabin, meanwhile, is a fascinating study in human persistence and ingenuity, proving that where the will exists to do so, even the loftiest goals can be achieved.

As most every other measure of luxury private jet travel improves — speed, range, cabin size and amenities like HDTV and high-speed Internet — aircraft manufacturers have started placing a far greater emphasis on cutting cabin noise as a way to stand out from the rest of the bizjet crowd. Only recently have manufacturers succeeded in quieting interiors to the point where passengers are really starting to take notice.

The quietest business airplane on the planet right now is the Gulfstream G650. That should come as no surprise given the big twinjet's generally well-accepted standing as the ultimate private jet. (The rare buyer who has a VIP version of the Boeing 787 Dreamliner on order might disagree, but there's no denying the Gulfstream is one sweet ride.)

You might be surprised to learn that the G650's low cabin sound level (well below 50 decibels in flight, by unofficial estimates) was achieved, in part, by bringing in acoustics experts from parent company General Dynamics' nuclear submarine division in Groton, Connecticut. The sub specialists applied what they know about making ultra-quiet submarines — literally a life or death proposition to the crews who must go undetected underwater — to the business jet interior. The results were nothing short of amazing.

John Maxon, who heads Gulfstream's state-of-the-art acoustics test facility in Savannah, Georgia, was one of the sub specialists brought over to achieve the dramatic improvements in cabin noise in the G650. He helped enlist a couple of other submarine noise experts to join a team that has grown to a dozen dedicated engineers at Gulfstream who work on nothing but cabin acoustics research.

"There's so much thought that has to go into making the cabin quieter," Maxon said. "Acoustic noise sources include turbulent airflow, the engines, environmental control system noise, even the overhead gasper that controls the air above the seats. We worked hard with the cabin distribution system, the air ducts, pumps, everything you can think of, to make significant improvements to reduce all that noise to almost nothing."

Business jet makers these days are putting just as much emphasis on range and payload capability as cabin comfort. Quieting the interior starts with smart use of materials like fabrics and carpet.|

Into the Acoustics Lab

Maxon wouldn't divulge how much money Gulfstream spent to create its acoustic test facility (pictured above), but you can bet it was a bundle. The lab was erected about a decade ago and then torn down and built again from scratch to make it better. Some of the software used by Gulfstream acoustics engineers was brought over from the submarine division and modified for use during the G650 and G280 ­development programs.

The main acoustics test facility at Gulfstream consists of two huge rooms — an ultra-quiet hemi-­anechoic chamber and a reverberation chamber. The first is entombed in acoustic "wedges" that are ­designed to absorb all sound, while the second is a large room with lots of hard, exposed surfaces that allow sound to echo around the chamber. Researchers can produce lots of noise in these rooms — up to 140 dB, in fact, the equivalent of standing about 50 feet from a jet engine at full power.

Engineers need to be able to produce such incredible noise to test the acoustic treatments that eventually make it into completed airplanes. In the case of the Gulfstream G650, which can travel at Flight Level 500 at Mach 0.9, a tremendous amount of noise is generated on the exterior of the airplane by the rush of the air. Specialized acoustics software developed for the task allows Gulfstream's design team to predict within tenths of a decibel how loud a given interior will be before an airplane is ever constructed.

Importantly, Gulfstream can also perform cold-soak acoustic testing of materials down to minus 70 ­degrees C. Sustained testing at very low ­temperatures is critical, Maxon said, because certain materials are less effective at reducing vibration in the frigid air at high altitude. Since Gulfstream began doing cold acoustics testing, other large business jet OEMs and even NASA have followed suit.

Designing a Quieter Cabin

Gulfstream carefully chose the materials it used in the G650 based on its acoustics testing. The manufacturer won't divulge exactly how quiet the airplane's cabin is (because decibel figures can vary from one G650 to the next depending on the cabin furnishings a buyer has selected), but unofficial numbers have put it around 47 dB. There even appears to be some competition among private jet owners who argue about whose airplane is quietest. Typically a G650 will be about 5 to 6 dB quieter than a G550. Apparently that's something to really crow about at your more exclusive country clubs.

Kidding aside, that's a significant reduction, and it's the main reason why G650 owners can rightfully say they own the quietest production business jets on the market. With its lower cabin altitude (just 2,675 feet at FL 410) combined with huge oval windows that let in lots of natural light, the G650 carries passengers who arrive feeling much more relaxed and refreshed.

Unlike some manufacturers that have tried active noise-cancellation techniques, Gulfstream has relied on materials science and super efforts to cut noise wherever possible, whether it be a pump, actuator motor, fan or even something as simple as a microwave oven in the galley. The secret behind Gulfstream's success is just that — and much of what the company has done to reduce cabin noise is patented and its engineers are sworn not to divulge too much.

A problem with active noise-­control systems, said Maxon, is they add complexity and a little weight. And if an active noise-cancellation system were to break, the results for the poor passengers would be unpleasant indeed. The bill the jet owner received to repair the system would likely be unpleasant as well.

The extra weight of such a system isn't a trivial matter. In fact, Maxon said, making a business jet cabin quieter isn't all that difficult if weight is no object. Of course, in any aircraft design, weight is paramount to a manufacturer's success, as it impacts range and cargo-carrying capability. Considering the G650 is über-quiet and has range and payload capability in spades, it's safe to say Gulfstream got the balance just right.

G650 buyers, of course, owe a debt of gratitude for their quiet cabin to other jet manufacturers, namely Bombardier and Dassault Falcon Jet. Both companies had achieved impressive noise reductions in their newest models when they were brand-new, namely the Global family and the Falcon 7X. Recognizing the increasing importance of turning down the volume on cabin sound levels, Gulfstream made acoustics testing a major focus for the G650 program. Embraer upped its game as well, in the next-generation Legacy 450 and 500, and Cessna also began placing a greater emphasis on cabin comfort and noise reduction.

One of the big advantages of the HondaJet's over-the-wing engines is reduced vibration and noise translated to the cabin.|

Trial and Error

The acoustics testing carried out at the major business jet OEMs allows designers to evaluate various materials to determine how well they dampen sound before ever building the actual airplane. For instance, fabric sidewall coverings and seat upholstery absorb noise better than leather does. Cabinet veneers, glass and other hard materials reflect rather than absorb sound. Hard surfaces can also reflect sound to a particular part of the cabin, making it noisier, say, at the rear of the cabin.

Once a production prototype flies, the engineers can validate their results and fine-tune the changes as certification testing progresses. But merely testing the materials that go into the cabin isn't good enough. All business jet manufacturers also work closely with engine makers to reduce engine noise vibration to as low as possible. They superbalance the turbo­fans to make them as quiet as can be, hone the shape of the nacelles to cut noise further, and even seek ways to quiet APUs to make them barely discernible to passengers in the cabin or people outside the airplane on the ground.

The sleek HondaJet now undergoing certification flight trials is a case in point. It benefits from having the airplane and engine manufacturers fall under the same corporate umbrella, in this case the Japanese auto giant that brought us the Accord. The GE Honda turbofan engines that power the HondaJet to an impressive 420 knots are also incredibly quiet — as anybody who witnessed the jet fly at Oshkosh over the summer can attest.

One of the real secrets to the HondaJet's quiet cabin is its Over-The-Wing Engine Mount (OTWEM), a radical break from traditional aeronautics, engineered by Honda after more than 20 years of extensive research and development in Japan and the United States. Combined with natural laminar flow aerodynamics and a carbon-fiber composite fuselage, the HondaJet boasts remarkable performance and surprising space inside the cabin.

You might wonder if the odd engine placement also makes for a quieter cabin. It does. With the engines mounted above the wing and not directly attached to the fuselage, noise and vibration are displaced and absorbed by the wing, making the cabin a much quieter place to think and relax.

Turbine engines and wind noise are the biggest sound producers in business jets, but in turboprops you also have to contend with propellers, which create a whole new set of challenges for acoustics engineers.

Hartzell was issued just such a challenge with the new Piaggio Avanti Evo, the latest iteration of the super-cool (and fast) pusher twin turboprop. The new five-blade aluminum-alloy propellers are lightweight and highly swept. In conjunction with changes to the Avanti's ­nacelles and engine exhaust geometry, and by slowing the props from 2,000 to 1,800 rpm, cabin noise has been improved by 20 percent and external noise has been slashed by an incredible 70 percent.

Can You Hear Me Now?

Nowadays in the latest luxury business jets, reducing cabin noise is important not just for hearing the person seated across from you, but also for better enjoyment of ultra-­sophisticated and ever-more-expensive cabin entertainment systems.

I was once lucky enough to be a guest aboard a Boeing Business Jet flying from Chicago to Geneva and got to experience in-flight entertainment unlike anything I've seen or heard in flight. Midway through the trip somebody put Die Hard on the BBJ's 42-inch HDTV and turned up the surround sound to 11. When the Bruce Willis-induced explosions started, the captain came back to the cabin to make certain the airplane wasn't coming apart at the rivets.

Isolator mounts that attach the cabin's inner shell to the fuselage can help reduce noise. They must be tuned for the particular cabin they serve and be able to withstand subtle fluctuations in the shape of the airplane brought about by temperature and altitude changes. Some companies even supply cabin noise-­reduction kits, which they ship to aircraft manufacturers for installation on the production line.

Some manufacturers have explored active noise canceling technology, but such extras add weight, cost and complexity. As with everything else in aviation, it's all a matter of trade-offs.|

Other suppliers offer OEMs improved sound-damping materials, windows that are better at blocking out noise, and even foam insulators that offer superior thermal stability in flight. The trick, of course, is in choosing and installing them properly so that they provide the greatest benefits with the smallest weight penalty.

Anybody who has flown on the G650 knows that Gulfstream got it right. The cabin is so quiet that a passenger standing at the rear of the cabin can have a conversation with someone at the front of the cabin without even having to raise his or her voice. Depending on what you're discussing, you might not want everybody else on the airplane to hear that conversation, but that's a small price to pay for some welcomed in-flight peace and quiet.

How Loud Is a Decibel?

The word decibel, abbreviated dB, is named in honor of Alexander Graham Bell, the inventor of the telephone. The bel unit, created in the late 1920s, was first used to measure telephone transmission loss over distance. Divided into 10, a bel becomes a decibel.

Decibels measure ratios of power or intensity. In the case of acoustics, 0 dB is roughly the lowest sound level that a human can hear. Unlike centimeters or seconds, the decibel is a logarithmic scale. For instance, the difference between 10 cm and 20 cm is the same as the difference between 30 cm and 40 cm, but the difference between 10 dB and 20 dB is not the same as the difference between 30 dB and 40 dB. Essentially, when one sound is 10 times louder than another, there is a 10 dB difference between them.

So how loud is a decibel? Decibels can be measured in negative numbers, which are generally below what humans can perceive. This is quiet, indeed. Meanwhile, 10 dB would be the sound in a very quiet room; 20 dB is the sound of rustling leaves or of someone whispering; 30 dB is quiet conversation; 40 dB is normal conversation; 50 dB is sound that's loud enough to wake a sleeping person; 70 dB is the sound in a crowded room; 80 dB is a police siren; 90 dB is a symphony orchestra; 100 dB is loud factory noise; 150 dB is a jet engine at takeoff power; 200 dB is a Saturn rocket firing; and 250 dB is the sound of a cannon going off 12 feet away. In other words, really loud.

We welcome your comments on flyingmag.com. In order to maintain a respectful environment, we ask that all comments be on-topic, respectful and spam-free. All comments made here are public and may be republished by Flying.

Get online content like this delivered straight to your inbox by signing up for our free enewsletter.