Sound Ideas

Acoustics experts are finding ways to make business jets quieter than ever.

Racing through the lower stratosphere at Mach 0.85, pushed along by roaring jet engines and outfitted with pumps, actuator motors and fans, the business jet is naturally noisy.

How noisy? Experts measure cabin noise in decibels of Speech Interference Level, or dB SIL. Straight from the manufacturer, with only a standard acoustics package, the typical business jet’s cabin noise level is about 62 dB SIL, or roughly the equivalent of the din in a crowded restaurant. The quietest unmodified business jet today, claims Gulfstream, is its G650, whose noise level is reportedly around 45 dB SIL. The manufacturer would not confirm that figure, however, noting that the level depends on several factors, including the customer’s choice of materials.

For example, fabric sidewall coverings and seat upholstery absorb noise better than leather. And veneers, glass and similar hard materials reflect rather than absorb noise. While the harder materials do not, as is often thought, amplify sound, they may well concentrate the reflected noise in certain areas.

To further complicate matters, perceived noise level varies from person to person. As we age, we lose hearing at the high-frequency end of the normal range. Younger people, on the other hand, are particularly sensitive to those frequencies. Some suggest this is one reason why infants on airplanes begin crying when the engines spool up for takeoff and the vibration produces high-frequency sounds.

Otto Pobanz, an expert on aircraft cabin acoustics, believes that no single solution exists for the problem of cabin noise. “You have to treat every source of noise differently,” he says.

An aircraft’s fresh-air circulation system offers a case in point. In one recently delivered BBJ, the entire system had to be pulled and revamped to achieve the target cabin noise level. Sharp turns and protruding edges inside the ductwork had to be eliminated and the fresh air exit nozzles had to be redesigned.

The aluminum or composite skin of an aircraft also poses challenges. It transmits the sound of air passing around the fuselage at low frequencies and, to reduce this noise, technicians apply sound-damping materials to the inner skin. They then add materials to absorb the remaining vibration so it doesn’t pass through to the cabin.

About a dozen well-known companies provide such services. Each has developed its own damping and absorbing materials, technology and techniques.

These companies pay attention to the ability of the noise-absorption package to resist moisture. Moisture absorption is not a problem as long as the bags in which the material is encased remain sealed, according to George Tsopeis, vice president of operations at Montreal-based Zenith Jet. “This is another reason that the best people to install a cabin acoustics kit are the people who designed and built it,” he says.

No less important than moisture resistance are the isolator mounts that attach the inner shell to the aircraft fuselage. They may be made of various materials, depending on the sound frequency of the vibration they are intended to dampen. The isolators must not only be “tuned” to perform in concert with the particular acoustic blanket and skin-vibration dampers, says Jeff Weisbeck, director of product management at ITT Enidine of Orchard Park, New York, they must also be formulated to adapt to changes in the shape of the cabin at different altitudes and to the dramatic alteration in temperature from the ground to the typical cruise altitude of 41,000 feet.

Many companies produce cabin noise-reduction kits and ship them to aircraft manufacturers for installation as standard equipment. Each kit is typically created for a particular airplane model.

Often, though, a buyer may ask for a noise level well below what the standard equipment delivers. This is frequently the case with large jets such as Bombardier’s Challengers and Globals and the bizliners from Airbus and Boeing, all of which have cabins completed by independent centers.

Those centers outsource the noise-attenuation job to a cabin-acoustics specialist. The work may include an acoustic mapping to determine the source of the noise and the best material for reducing it. Suppliers normally build the acoustic package and ship it to be installed by the completion center. Certain suppliers, however, recommend that their own technicians be involved in the installation. And some of them insist on it.

“You can design and build a great system, but if it’s improperly installed, you can lose up to 3 dB SIL in noise reduction,” says Emon Halpin, founder and CEO of Flight Environments of Paso Robles, California.

While aircraft manufacturers and independent completion and refurbishment centers make an effort to produce aircraft with quiet cabins, this generally isn’t a top priority for manufacturers and completion centers. But there are exceptions–among them, Comlux America, a Comlux Group completion and refurbishment center in Indianapolis that focuses on outfitting executive aircraft in the bizliner category. Daron Dryer, director of engineering, said his acoustics department considers everything from outsourcing for customized vibration damping and acoustic absorption materials to redesigning the cabin air distribution system. And the department will frequently bring in an independent acoustics analyst.

“So far, we’ve come in under customer expectations on every airplane we’ve delivered,” he said. One of the most recent requirements was for a cabin dB SIL of 52, he added, “and we came in at 49. The best we’ve done so far is an airplane on which we were shooting for 52 dB SIL and delivered at 47.5.”

Such levels are impressive, but keep in mind that a quieter cabin can involve tradeoffs and unintended consequences. For example, sound-dampening materials can add weight, which means less range. Moreover, said Dryer, “the quieter the cabin, the more privacy you give up; conversations easily overheard in a very quiet cabin might be muffled by a certain level of noise.”

The Latest in Cabin Noise Reduction 

Virtually every cabin-acoustics supplier is developing materials aimed at further reducing cabin noise. Here’s a look at some recent and forthcoming innovations:

• ITT Enidine is testing a damping material it calls Inidamp, which it claims provides “very good low-frequency damping,” especially for aircraft flooring. “You spend a good part of any flight with your feet on the floor and we feel the vibration there contributes to fatigue,” says Jeff Weisbeck, director of product management at the Orchard Park, New York company.

• While ITT Enidine ponders floors, Switzerland-based Pelzer Consult is looking at windows. In 2011, the company announced what it called “closed sandwich” window-trim panel technology. Pelzer says the technology can reduce cabin noise by up to 10 decibels. The Pelzer window-surround panel consists of acoustically open outer layers that permit sound to pass and subsequently be better absorbed. In addition, a decoupled sealing film prevents airborne noise from making it through the open honeycomb from the fuselage inner wall into the interior.

• German chemicals company BASF recently had its open-celled melamine foam acoustic material (see photo on facing page) applied to reduce cabin noise in an executive version of a Russian Mi-8 helicopter, a machine not known for being even remotely quiet. “The noise level inside the helicopter dropped to 80 decibels from 85,” according to Sergey Milek, CEO of acoustic systems supplier StandardPlast. 

• Solvay Specialty Polymers of Alpharetta, Georgia, has introduced a foam product that it claims offers superior dimensional and thermal stability in flight and excellent acoustic insulation properties when combined with multiple materials. “We are also examining how to construct a sandwich panel that can be ‘tuned’ to a particular frequency,” said business development manager Armin Klesing. “We are hoping for a breakthrough within the next year.”

• Last spring, yacht and aircraft interiors designer Andrew Winch introduced an AgustaWestland AW189 helicopter with “whisper dish noise cancellation,” a passive technology dating back to before World War II. The idea from Andrew Winch involves incorporating acoustic mirrors in opposing cabin walls, causing sound waves created by passenger conversation to be reflected and amplified in a specific space. It permits passengers to converse quietly within the amplified area without being heard by others in the same compartment, a desirable feature for anyone requiring privacy.  

Kirby J. Harrison ([email protected]) is a senior editor at Aviation International News.