Quest for the quieter cabin

 - January 8, 2008, 10:51 AM
Noise is everywhere–annoying, tiring and sometimes painful. Since the early days of aviation, when a roaring, clattering engine sat on a wooden frame close to the pilot, and the wind whistled through the wire bracing like a banshee chorus, engineers have sought to make the process of manned flight less noisy. And they have succeeded, to a degree.

In the modern business jet cabin, sound expressed in the overall A-weighted filter scale of measurement, or dBA, has been reduced to somewhere in the 60- to 70-dBA range, higher in some parts of the cabin and lower in others, and differing from one aircraft to another. It is far better than in decades past, but cabin noise remains an annoying and often intrusive barrier to human conversation, which is normally conducted in the 55- to 70-dBA range. The study of acoustics is not magic. It’s a science–perhaps not an exact science, but a science nonetheless.

In past decades, the study of acoustics–a subbranch of physics–has given environmental engineers and scientists a wealth of new ways to define noise or, more accurately, sound.

Sound, simply defined, is a waveform phenomenon produced by vibration. Noise is defined as sound where the waveform has a predominantly random nature. Noise that is not random is described as “tonal.” While we tend to think of noise as an unpleasant sound to the human ear, not all noise is unpleasant–for example, the sound of waves lapping at a seashore.

The decibel expressed as dB (unfiltered) remains the standard measure of intensity of a sound with which the public is most aware. For the average human, the sense of hearing is incredibly sensitive, from almost 0 dB (the equivalent of a dry leaf rustling on a sidewalk 20 ft away) to about 140 dB (a gunshot).

On the decibel scale, a sound 10 times more powerful than 0 dB is 10 dB. A sound 100 times more powerful than the near total silence of 0 dB is 20 dB. A sound 1,000 times more powerful is 30 dB. And so the increase in sound is exponential.

Any prolonged exposure to sound above 85 dB can result in hearing loss, and hearing loss is related both to the power of the sound as well as to the length of exposure. Eight hours of exposure to sound at 90 dB may cause ear damage. It is generally accepted that 120 dB is the loudest sound which may be heard by the human ear without producing pain. Any exposure to 140-dB sound will cause immediate damage to the inner ear, as well as acute pain.

But acoustical scientists realized some time ago that given differences in individual ability to detect sound, something more accurate was called for than the simple decibel. So they came up with dBA, an overall average of frequency levels, or weighting, which roughly approximates the frequency response of the average human ear. The average human ear hears noise best in the 70-dBA range.

They also developed dB(SIL) to measure the “speech interference level” of sound. Much more difficult to quantify, it is an average taken from the one-, two- and four-kilohertz octave bands of a single measurement. According to acoustics consultant Chris Brunt of Malibu, Calif., it is not considered a good indicator of loudness or annoyance, but is a fairly good measure of speech interference, or how easy it is for individuals to communicate face to face.

Finally, as sound is measured in terms of frequency, the range of human hearing spans roughly from 20 to 20,000 Hz.

Brunt explained the Hertz scale in practical terms. Hearing in a young, healthy human, he said, ranges from 20 Hz up to 20,000 Hz. The lowest note on a base guitar is about 80 Hz.

A sound measured at 5,000 Hz is similar in frequency range to the hiss of the spoken letter “s.” At 10,000 Hz, it is a tone more felt than heard. But as regards an aircraft cabin, the predominant and most annoying sounds to the human ear in a typical jet aircraft fall in the 100- to 3,000-Hz frequency range.

The term “noise” is most accurately characterized by the randomness of the sound. However, some sounds that are not random, such as the rhythmic beat of turboprop propellers, can be no less annoying.

For decades, the industry solution to cabin noise was relatively simple–more mass. And it worked. The more mass you could pack into the cabin periphery, the more effective. With this in mind, said Brunt, “Some of the early corporate aircraft were very quiet. But mass carries a weight penalty.”

In the industry today, two distinct systems for reducing cabin noise have been developed–passive and active. Each system is effective in its own right.

Passive noise-reduction systems depend primarily on thermal/acoustic sound absorbing or damping materials and systems. They are most effective in reducing sound in the high-frequency range, from about 500 Hz and up. The active noise-reduction systems are more effective in dealing with low-frequency sound levels below about 400 Hz, typical of the noise produced by turboprop engines and propellers.

Acoustic Analysis Maps Cabin Noise
The first step in creating a quieter cabin for any aircraft begins with an acoustic analysis of that particular aircraft cabin with an interior installed. Companies providing passive noise-reduction systems will frequently have on file acoustic analyses of a variety of similar aircraft types to use as a baseline.

The acoustic map is best described as a visual record of noise emanating from three main sources–engines, boundary layer (generated by wind passing over the aircraft skin and creating vibration) and interior systems (pumps, fans, and so on). Particularly high levels of boundary-layer noise are created at any point on the outer fuselage where the shape of the skin changes abruptly, such as antenna fairings.

The acoustic analysis is done under a variety of conditions, including taxi, takeoff, climb, cruise, descent, approach and landing. It will highlight any especially noisy “hot spots” to which particular attention must be paid. These usually include door seals, outflow valves and servo motors. In the process, an acoustic map of the interior is developed (see illustration below), from which a passive or active noise-reduction system (or a system combing the two) is built.

Gary Burns, manager of acoustics and vibration for Bombardier Aerospace said a client’s choice of a “harder” or “softer” interior is a major factor, pointing out that noise in the cabin is absorbed by such materials as fabric seat coverings and fabric panel coverings, but reflected and even amplified by more dense materials such as wood veneer cabinetry, mirrors and hard-composite countertops.

Danny Farnham, director of completions at Midcoast Aviation, agreed. “If noise is a primary focus for a client and they start talking about veneer bulkheads and mirrors, we try to talk them out of it. We’ve done a number of ‘soft and warm’ interiors, with a big reduction in cabin noise as a result.”

New developments in material composition and better designs for the strategic placement of passive materials have done much in recent years to quiet business aircraft cabins.

Brunt said a BBJ interior for which he was a consultant has a dB(SIL) level of 40 in the forward crew-rest area. He said it is now typical for BBJs to be delivered with an average noise level in the 55- to 56-dB(SIL) range.

Rick Penshorn is president of Associated Air Center, a major facility specializing in the interior finish of large business jets and bizliners. Penshorn said the Dallas Love Field-based completion center recently delivered a BBJ with a 52-dB(SIL) cabin noise average. But, he added, it came at a price–a 1,000-lb weight penalty.

The passive system, despite appearances, is no less complex than an active noise-reduction system.

According to Mike Lucas, senior aircraft noise control engineer at E-A-R Specialty Composites, there are four basic elements involved in passive noise reduction systems: barriers to block acoustic energy; absorbers, such as carpeting, seats and other soft products that absorb sound; structural damping materials (directly on skin panels, bulkheads and trim panels) to reduce vibration, such as that caused by wind buffeting; and vibration isolators, such as isolation mounts for headliner panels and other components.

Flight Environments president and CEO Eamon Halpin emphasized that the art of building acoustic insulation is not as simple as merely adding mass until the noise goes away. The trick, he said, is the right mass in the right place; hence the need for an acoustic mapping of the cabin to determine which mass and how much of it goes where. A thermal-acoustic package for a Hawker 800XP, said Halpin, may have as many as 900 pieces. A similar package for a Boeing Business Jet might have up to 3,000 pieces.

But a passive noise-reduction system is not limited to the intercostal sealed bags, each of which is custom-created in shape and composition for a particular section of the fuselage interior. It may also include selection of a particular sub-carpet foam, and sound damping material backing for interior trim panels. There may also be a muffling material applied to the inside of the cabin ductwork.

A passive noise-control system may come as a complete package, precut and ready to install by the OEM or by an independent completion and refurb center. Or it may be a modification of an existing noise-reduction system previously installed. With a major interior refurbishment, it may take the form of isolation mounts for a new galley or side panels. In some cases, it may be part of a more extensive package that includes an active noise-control system as well.

Surprisingly, the active noise-cancellation (ANC) system is a concept that has been around since the 1930s, but was relatively impractical until the advent of high-speed, lightweight computers. Shelby Carr of Quiet Flight in Dallas described active noise cancellation as a system that chases, identifies and actually destroys the frequency and amplitude of unwanted noise in the lower range between about 80- and 360 Hz. Microphones strategically placed throughout the cabin pick up noise produced in a designated range.

Noise in the form of an analog signal is sent to a control box, which identifies the phase, amplitude and frequency. The computer then recreates the identical amplitude and frequency but shifts the phase 180 deg, converts it back to an analog signal and sends it to a set of speakers, also placed strategically throughout the cabin. Because the noise from the speakers is 180 deg out of phase with the original noise, it effectively destroys the original noise.

A more recent variation is the standard active noise-cancellation system commonly referred to as the “shaker can,” but more accurately as “active tuned vibration absorbers” (ATVA). The process is similar to the original speaker-based system, but the out-of-phase signal is sent back to the ATVA units, which produce an identical but out-of-phase vibration to destroy the propeller and engine noise. According to Mike Turner, director of marketing at Elliott Aviation in Moline, Ill., an ATVA system is particularly effective at eliminating prop-tip noise.

The disadvantage of the ATVA system, according to Carr, is that while it may destroy unwanted prop and turbine noise transmitted through the fuselage, it is less effective in coping with noise transmitted through windscreen and cabin windows.

Elliott is licensed by Cambridge, England-based Ultra Electronics to install its speaker-based ANC system as an aftermarket item in the King Air 200 and 300 series, and the same system is being installed as standard by Raytheon Aircraft in new King Air 350s. Raytheon is also offering an ATVA system as an option on the King Air B200.

The Total Cabin System
Bombardier, embarking on an ambitious noise-reduction program in 1996, decided to create a lightweight thermal/acoustic insulation package and trim-panel configuration that it hoped would deliver “the quietest corporate business jet cabins in the marketplace.”

Bombardier also took its program a step further, introducing an ATVA noise-reduction system as optional equipment on the Challenger 604. The company refers to it as an active noise and vibration control (ANVC) system. It is available–either as a factory option or a retrofit item on the 604. Bombardier is now working on a passive noise-reduction package for its new Challenger 300, and noise analysis is now being done using aircraft S/N 200004 in the certification flight-test program.

Both Lord and Ultra Electronics are major suppliers of active noise-reduction systems. Lord was recently selected to work with Nordam of Tulsa, Okla., to create a noise-reduction system for the Embraer Legacy. The package includes elastomeric isolators to attach interior sidewall and ceiling panels; elastomeric rod ends to attach interior boundaries and bulkheads; and isolators to mount credenzas, cabinets, galleys and lavatory fixtures.

No less important, said Brunt, are the “fluidlastic” tuned vibration absorber (TVA) mounts from Lord that attach the engines to the airframe. The TVA is tuned to create a disturbance frequency that causes it to resonate. “The resonating TVA generates a force back into the structure that destroys the unwanted vibration,” according to Lord engineers.

The two disadvantages of active noise-cancellation systems of both types, according to Brunt, are their expense and lack of reliability. When they work, he said, they work very well, “but they are in constant need of maintenance and tuning.”

While the additional weight of an active system is a penalty, the cost may be a greater consideration. The installed cost of an Elliott Aviation Sound Management System for a King Air 350 is in the $56,000 range. For a King Air B200 it is about $46,000.

As to additional weight, it will vary from one aircraft to another, depending on the complexity of the system and the number of microphones and speakers or active tuned vibration attenuators required. On the Challenger 604, the ATVA system adds about 62 lb.

New Technology
Companies such as E-A-R and Flight Environments are constantly searching for new materials, or new ways to use old materials, that will prove more effective in reducing cabin noise.

The difficulty is not always in finding materials that do a good job of absorbing or reflecting noise, but in finding materials that are also light in weight, that meet FAA requirements with regard to toxic outgasing and exhibit adequate burn-through properties. But most research money is now going into further development of active noise-reduction systems.

Barry Controls of Burbank, Calif., has been in the business of noise reduction for some 50 years. The company is now a subsidiary of European oil conglomerate Total-FinaElf and is closely aligned with sister company Paulstra Vibrachoc of France. “It’s a good marriage,” said Max Maggi, director of engineering for Barry Controls. “Paulstra works primarily on noise-reduction programs for helicopters and we are focused primarily on fixed-wing, but there is considerable technology exchange between the two companies.”

Most recently, Maggi said, Barry has been working with Raytheon on a noise-reduction system for the Premier I “to make it the quietest cabin of any entry-level business aircraft.”

Sources report that the Paulstra Vibrachoc research facility near Paris has developed a new noise-control system for helicopters that will go into production in September. Barry Controls is expected to market the package in North America.

One of Lord’s more recent contributions is a system designed not only to balance a propeller but to keep it in balance. “Where to place the weights is a difficult engineering problem because optimal placement really depends on the aircraft’s speed,” according to Becky Weih, manager of aerospace sales. “Our system is integrated into the hub of the propeller and moves the weights around as the aircraft is changing speed, so the propeller is always in balance.”

In October, Lord announced it had reached an agreement in partnership with Bell Helicopter that would allow the company to do complete rotor-head assembly, tail assembly, remanufacturing and blue-coat repair service. Lord will also provide elastomeric tail-rotor feathering bearings, Frahm dampers and numerous elastomeric components for pylon isolation but will not be involved in the final-assembly process. “Since Lord manufactured many of the critical components, it made good business sense to provide the complete assembly,” said Lord account manager Andy Stevenson.

There is also research being done at dozens of universities in the U.S. and abroad. Bombardier, in developing its noise-reduction system for the Challenger, worked closely with Canada’s Concordia and Sherbrooke Universities to characterize the acoustic properties of skin-damping treatments, insulation and trim panels.

At Virginia Tech, research is going on into reducing vibration produced by propellers and turbine fans that experience random changes in vibration due to a turbulent and irregular airflow field. The project is designed to develop modeling for the random changes and for calculating the responses and reliability of such systems.

According to Bill Connor, a consultant in acoustics, another idea that has promise but remains “way out on the horizon” is application of the current active noise-control systems to existing cabin panels. “This would be a massive breakthrough,” said Connor, who noted that at least two major research labs are at work on this.

Why You Need A Quiet Cabin
Why is a quiet cabin desirable? The obvious answer is so that passengers are not distracted from normal conversation or annoyed by random sounds. But a more important function of a quiet cabin is the health of the passengers and crew.

Loss of hearing is not the least of the health problems associated with a noisy cabin.
While the average sound level in most fixed-wing jet cabins is well within OSHA guidelines, this is not true of many fixed-wing turboprop or helicopter cabins. In a turboprop cabin, sound levels are often in excess of 80 dBA, and often not much better in many parts of the cabin of a modern jetliner. In some helicopter cabins it may be well over 90 dBA, most of it directly attributable to the transmission. Even in a helicopter with a well designed passive noise-reduction system, it is likely to be no better than the low 80-dBA range.

OSHA considers that “a time-weighted average of 85 dBA is the ‘action level’ at which hearing conservation measures must be implemented.”

Burns of Bombardier told AIN that cabin noise contributes to physical and mental fatigue, and can have a negative effect on an individual’s decision-making process.
According to Quiet Flight’s Carr, people who complain about fatigue after only two or three hours in a noisy airplane are actually feeling the cumulative effect of hours of exposure.

What aircraft are good candidates for a noise-reduction makeover? According to acoustic consultants, most new business jets are adequately insulated for sound. Many OEMs, in fact, are making use of pre-packaged thermal-insulation systems from independent companies such as E-A-R and Flight Environments.

According to acoustic consultant Brunt, used aircraft are the major market for noise-reduction packages. Older Learjets, and in particular, older turboprops are prime candidates. Portions of the cabins in aging turboprops often have noise levels well above 105 dBA. Even in older business jets, the dBA may top out in the low 90s.

The decision as to whether to make the cabin quieter is not always driven by comfort. But in a particularly noisy airplane it may be a question of what value you place on your hearing.