The Quiet Cabin: No Simple Solution

 - December 2, 2013, 5:40 AM

Business aircraft cabins are generally not quiet. Not with the turbulent boundary-layer rush of air around the fuselage at Mach 0.85 and the whine of a couple of jet engines no great distance from the comfy chairs. Then there are the pumps, hydraulics, fans, gears, actuators, electric motors, worn bearings and air distribution through the metal ductwork, not to mention the occasional hum of the microwave and induction oven, the rattling of glasses and flatware in the galley and that giant sucking sound coming from the lavatory.

But it can be made a lot less noisy, and there are quite a few engineers, designers and acoustics experts doing their best to make it so.

First, however, a short explanation of vibration, sound, noise and hearing is in order.

When it is within the frequency range of the average adult’s hearing–approximately 20 to 16,000 Hertz–vibration can be described as sound. When it is in a higher or lower frequency range it may be unwanted, distracting, annoying, downright painful and even all of the above. Put simply, noise is sound that makes it difficult to hear what a listener wants to hear. (See new technology, below.)

To further complicate the challenge of a quiet cabin, human hearing differs from person to person, and as we age, that difference becomes more profound. As we grow older, we lose hearing at the high-frequency end of the normal hearing range. Younger people, however, are particularly sensitive to higher frequencies, which some suggest is one of the reasons infants on airplanes begin crying when the engines spool up for takeoff, the vibration producing noise in the very high-frequency range.

In the business aviation industry, the generally accepted standard measurement of cabin noise is in units of dB(SIL), or decibels (dB) expressed in terms of speech interference level (SIL). In aircraft, that speech interference level is the mathematical average of 1,000; 2,000; and 4,000 cycles per second (Hertz).

A business jet customized with the latest noise-reduction package may register at 46 to 50 dB(SIL), though not necessarily at every point in the cabin. A typical home environment might be in the 50 to 55 dB(SIL) range. A fresh-from-the-factory business jet is likely to have cabin noise levels ranging from about 62 dB(SIL) to as low as 45 dB(SIL) in some rare cases. A turboprop aircraft or helicopter cabin noise level would likely surpass 90 dB(SIL) and even hit triple digits in some cases.

To further complicate matters, noise levels can change dramatically with the phase of flight, with such contributors as flaps, spoilers, altitude, aircraft attitude, fuselage construction (aluminum or composite) and engine speed. So when acoustics specialist and consultant Otto Pobanz says there is no single solution to reducing cabin noise, he isn’t just talking to hear the wind blow, so to speak. “You have to treat each element differently,” he explains.

Fans are one example. They operate at about 7,000 rpm and produce a frequency of about 100 Hertz, a dominant, low-frequency range for which the answer might be as easy as properly balancing the fans.

The fresh air circulation system is a major source of noise. In a BBJ delivered last year virtually the entire factory system was pulled and redesigned. Sharp turns and protruding edges were eliminated, and the fresh air exit nozzles were redesigned to reduce velocity and the “whish” of air entering the cabin, but without compromising FAA requirements. In that particular BBJ, the noise reduction came in 10 dB(SIL) below the target level.

For the most part, the primary focus is on noise produced during the cruise phase of flight, on the assumption that much of the noise produced during takeoff, climb, descent and landing is temporary in nature and expected.

Where the Noise Comes From

A major “noisemaker” is the skin of an aluminum aircraft, which transmits the sound of air passing around the fuselage at low frequencies often as a rumble or roar. It typically requires a combination of vibration absorption and sound-damping materials.

Other solutions include isolator mounts that must be tuned to fit the material to which they are attached and which will perform in extremes of temperature. “You want the elastomeric isolators and thermal/acoustic blankets performing equally,” added Jeff Weisbeck, director of product management at ITT Enidine of Orchard Park, N.Y.

Elastomeric isolators are a specialty of ITT, primarily in the attachment of interior trim panels to the frame. Originally, the connectors were simple rubber, and for the most part they still are. But now they include other material formulations to create different properties so they not only absorb and dampen vibration but can also adjust to changes in the shape of the cabin at different altitudes and temperatures.

While the work goes on to reduce noise produced through an aluminum cabin, a new challenge is coming from the growing number of composite-fuselage business jets. These airplanes, say industry insiders, produce higher levels of noise than those with aluminum skin. Composite materials are stiffer and lighter than aluminum and will transmit acoustic energy much more efficiently, explained Brian Joyal, director of aerospace and defense thermal/acoustic systems for 3M Thermal Acoustic Systems.

One example is the new composite fuselage of the Boeing 787 Dreamliner. ITT is providing all the interior trim isolators for the 787, for which the requirements were stringent, said Weisbeck.

Noise-damping materials are typically applied to the inside of the fuselage skin in areas not braced by stringers and ribs and therefore prone to vibration. Damping materials are also applied to the cabin flooring, and they have been used as a coating on the inside of sidewalls and overhead composite materials that are prone to reflect and even amplify noise.

In a Boeing Business Jet completed two years ago in Spokane, Wash., three-eighths of an inch of melamine foam, a quarter inch of DaxFoam and a final decorative fabric with a soft backing from Kalogridis International of Dallas were applied to the sidewalls, bulkheads and overhead surfaces. The result of that process alone, according to Pobanz, was a 3 dB(SIL) reduction in noise.

According to Pobanz, the deployment of noise-absorbing strategies in the cabin surfaces below the floor may reduce cabin noise by 3 dB(SIL). Two recently delivered Globals in for major cabin refurbishment, he said, measured 43 dB(SIL) in the aft cabin and 45 dB(SIL) in the forward cabin after particular attention was paid to noise-absorbing materials under the floor.

Aircraft manufacturers make every effort to produce aircraft with a quiet interior, as do independent completion centers. But the truth is, said one aircraft acoustics specialist, “Noise attenuation is not normally within the capabilities of the average OEM or completion center, where most of the attention is visual.”

It may be true, but there are exceptions, among them Comlux America, a Comlux Group completion and refurbishment center in Indianapolis that focuses on outfitting executive airplanes the size of the Airbus A319 and larger.

The center’s work on cabin acoustics is managed by director of engineering Daron Dryer and a staff that includes an acoustics specialist. The group takes an engineered approach and considers everything from outsourcing for customized vibration damping and acoustic absorption materials to redesigning the cabin air distribution system in an ACJ. Comlux also makes extensive use of computer modeling programs. When requested, he added, the company will bring in an independent acoustics firm to evaluate the results.

“So far, we’ve come in [quieter than] customer expectations on every airplane we’ve delivered,” said Dryer. One of the most recent requirements was for a cabin dB(SIL) of 52, and “we came in at 49. The best so far is an airplane on which we were shooting for 52 dB(SIL) and delivered it at 47.5.”

But as the cabin gets quieter, there are some unintended consequences. For example, said Dryer, “the quieter the cabin, the more privacy you give up; conversations easily overheard might otherwise be muffled by a certain level of cabin noise.”

Brian Joyal at 3M tells the story of an owner who insisted that his aircraft stateroom be the quietest zone in the airplane. It came in approximately 10 dB(SIL) below the rest of the cabin and the owner found the transition from his stateroom to the rest of the cabin disturbing. “We advise against having one cabin zone quieter than another because the transitions can indeed be disturbing.”

“The cabin has to be quiet,” said Emon Halpin, founder and CEO of cabin acoustics supplier Flight Environments. “But there are different harmonics and resonance frequencies in different parts of the cabin and it has to be made consistently quiet throughout.

“We recently had the cabin noise levels of a BBJ we worked on tested side-by-side by two independent analysts. They took readings in the stateroom, forward lounge, dining area and aft seating area, and every reading from both companies came in at 46.2 dB(SIL). That’s the lowest we’ve achieved throughout an entire cabin.”

In Search of Things Lighter and Better

Weisbeck said ITT is currently in the test phase of a new type of damping material it calls Enidamp, which he said provides “very good low-frequency damping,” especially for aircraft flooring. “You spend a good part of any flight with your feet resting on the floor, and we feel the vibration there is a fatigue issue.”

At Flight Environments, the company is deep into work on its first private 787. “We’re using some new and less conventional materials and doing a lot of acoustic chamber testing,” said Halpin.

One of the most interesting developments is coming from Pelzer Consult. In 2011 the Swiss company announced what it called “closed sandwich” window trim panel technology that it claimed could reduce cabin noise by up to 10 decibels. Aircraft Composites Consortium, a firm created by Pelzer, Plastika Balumag and Tubus Bauer to develop the idea, has received a U.S. patent on the technology.

The Pelzer window-surround panel consists of acoustically open outer layers that allow sound to pass and 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. It is also lighter and requires less installation time, the company claims.

When Nextant Aerospace decided to rebuild the Hawker 400/400XP as the Nextant 400XT, one of the major improvements went into the customized noise-suppression package. According to the Cleveland, Ohio-based company, the solution cut cabin noise at 41,000 feet by 9 dB to approximately 66 dB, “an unprecedented low noise level for this class of aircraft, which typically exhibits ambient noise levels of around 83 to 91 dB.”

German chemicals company BASF recently had its open-celled melamine foam acoustic material applied to reduce cabin noise in an executive Russian Mi-8 helicopter. The interior work and custom fitting of the foam, coated on both sides with a water-repellant fleece, was done by Vemina-Aviaprestige of Moscow and attached to noise-sensitive points in layers 40 mm thick. “The noise level inside the helicopter dropped to 80 decibels from 85,” according to Sergey Milek, CEO of acoustic systems solution supplier StandardPlast.

BASF also noted that by using the lightest version of its foam materials, it was also able to reduce the weight of the insulation by 80 percent compared with the standard factory fit.

Some new technology is not new at all, and is not indirectly related to a quiet cabin. This past spring, yacht and aircraft interiors designer Andrew Winch introduced an AgustaWestland AW189 with “whisper dish noise cancellation technology.”

The idea dates to the years just before World War II when Britain built several large, concrete acoustic mirrors facing away from the coast. The parabolic (actually spherical) mirrors were known colloquially as “listening ears.”

The largest was approximately 15 feet in diameter and the idea was that the sound of approaching aircraft would be collected by the acoustic mirror, and focused on a microphone placed at the foci point for amplification. It would enable listeners to hear approaching aircraft from far out over the English Channel. The invention of radar quickly spelled the end of the technology, but Andrew Winch engineers revived it in a modern form last year.

The Winch design places acoustic mirrors in cabin walls opposite each other, causing sound waves created by passengers in 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.

“Within an aircraft or helicopter cabin, this involves manipulating the shapes of the panels to create zones where sound within a particular area is amplified,” explained Alex Winch. He added that maximizing a cabin’s acoustic potential through incorporating whisper dishes, as well as other materials that absorb external noise, enhances the passenger experience.

Skandia, as do competitors, places great emphasis on a customized solution for each aircraft and each source of cabin noise. “We have supplemental type certified (STC’d) kits for more than 50 aircraft models and we customize every kit, from damping material for the skin to the interior trim panels,” said v-p Jarod Triplett. Now in the test phase for STCs are acoustics kits for the Challenger 600 and Challenger 300, the Falcon 2000 series and the Global 6000.

SMAC Groupe provides both standard kits for installation by OEMs and customized packages for the aftermarket. The company has developed a water-jet technique for cutting sheets of its thermal/acoustic materials to fit. According to general manager Philippe Robert, SMAC has installed a customized kit in a Beech 1900 in as little as a single day.

Solvay Specialty Polymers of Alpharetta, Ga., is looking ahead to further development of its Radel product, a “super-tough thermoplastic foam” that meets current and emerging safety requirements.

According to business development manager Armin Klesing, while Radel foam alone is a closed cell material not particularly made to cancel noise, it can be used to build lightweight Helmholtz resonators (panels actually), and tuned for certain frequency ranges. “We are hoping for substantial progress in due course the next year,” he added.

While some independent cabin acoustics experts focus on the aftermarket trade, many also develop noise-abatement kits for installation by OEMs as standard.

“Our business is aftermarket, when customers want something more than the baseline approach,” said Triplett. “But the real goal of everyone is the same: better, lighter, cheaper.”

“Better performance and less weight is the holy grail of our industry,” added Halpin.