Hushing The NoiseIn all the talk about CSeries being more fuel efficient, you don't hear much about this consideration, or the fact that CSeries will be cleaner and easier and cheaper to maintain, but these can be important considerations for some airlines:
"He says the company launched into the Bombardier market "because we're all former Bombardier guys," but he hopes to expand applications in the future. " Hushing The Noise June 7, 2010 As mechanical vehicles, all airplanes make noise. That's a given. It's the degree of sound generated that draws people's attention. The noisiest airplane ever built? That's easy: the Republic XF-84H "Thunderscreech," an experimental variant of the F-84 that flew in 1955 with a three-blade propeller that ran at a constant 3,000 rpm, with the outer half of each blade well and truly supersonic. Only two of these banshees were ever built, and accounts at the time said they could be heard 25 miles away, that within a hundred feet of it, the airplane generated enough sound energy to pound a man to his knees, helpless.
The quietest? Truth is, it hasn't been built yet. Airplanes keep getting quieter all the time, but they were so deafening at the outset that they've had an overabundance of decibels to eliminate along the way. Now each new generation of aircraft is quieter than the last, mostly because of improvements to diminish engine noise. In business aircraft, there's almost a competition to deliver the quietest cabin, and as part of the completion process, the right noise control measures can make a huge difference.
Probably the loudest cabins extant are those of military cargo aircraft that double as personnel carriers. A C-130 Hercules cargo deck can easily exceed 100 dBA because it's designed to maximize payload, and anti-noise measures just add to empty weight. For the military, it's easier to block the noise by issuing earplugs or earmuffs.
Turboprops are noisier than jets because of the propellers' drumbeat on the sides of the fuselage. Active noise canceling is most often employed on turboprops and helicopters to generate a tone 180 degrees out of phase with the sound detected by microphones placed in the cabin. At the focus of output speakers, the combined sounds effectively zero out the noise. Noise-canceling headsets work the same way. However, weight, complexity and maintenance issues have kept active noise canceling systems out of business jets in favor of passive systems.
Years ago, Hughes Helicopter tried to promote its Model 500 to the pin-striped set by issuing passengers with David Clarks. But executive passengers generally eschew accessories like earmuffs, plugs or headsets when traveling aboard private aircraft. They expect a cabin quiet enough to hold a normal conversation without involving uncomfortable headgear and intercom switches.
On flights of more than 10 hours in a loud cabin, passengers suffer stress and fatigue because of the noise. Airlines have made some progress in lowering cabin noise, but the traditional materials used to dampen sound also absorbed moisture and added weight. Writes Chris Brunt, an applied physicist and engineer, in a paper on the subject: "Moisture absorption is a problem of enormous proportion for all airline operators [a 747 can gain thousands of pounds in added weight from moisture absorbed by the thermal/acoustic insulation materials after just a few months of passenger carrying duty]. Once acquired, this moisture is difficult to remove and substantially increases the cost of operating the aircraft." Brunt is a consultant to Flight Environments, a firm that specializes in aircraft noise reduction.
One of the newest to enter the aircraft acoustics field, Nick Houseman is president of Silentium Air, a Montreal, Quebec-based subsidiary of Zenith Jet that's introducing a new noise-reduction product for retrofit into the Bombardier 300 market. "The bigger the aircraft, the more the owner tries to reduce noise. At a higher cost, they expect quiet," says Houseman. He focuses his sales effort on the exec in back rather than chief pilots or flight department managers, "because the owner is the one with a noise fatigue factor. Nobody thinks about noise until the airplane's bought. You can't see it. So we get to the actual owner and tell him we can help make it quieter so he can enjoy it more."
The first installation under a new STC is under way now at Bombardier's facility in Tucson, Ariz., on an aircraft undergoing an overhaul. It entails removal of the factory soundproofing materials and replacement with the Silentium Air product, pre-approved under the STC, that provides a three-decibel reduction in cabin noise and entails no significant weight gain. In fact, on older models with heavy soundproofing materials, Houseman says his product, utilizing newer, lightweight materials of equal or better performance, can actually save weight. He estimates 200 hours to do a full replacement kit at $85,000 or about 60 to 70 hours (during a C check or A check, he says) for a supplemental kit that adds to the existing soundproofing at a price of $55,000. He says the company launched into the Bombardier market "because we're all former Bombardier guys," but he hopes to expand applications in the future.
The surprise alpha male of the noise-reduction übermeisters is an octogenarian with a career history that's interwoven with the modern history of business flying. He is extremely selective in the projects he takes on, and all his work is a result of referrals via the high-net-worth grapevine. If you have to ask what his services cost, you can't afford him, but when he talks about cabin sound levels in the high-40-low-50-decibel range — that's library quiet — he's not exaggerating.
BCA met up with Otto Pobanz at Midcoast Aviation's St. Louis Downtown Airport facility in Cahokia, Ill. The former 30-year chief pilot for Federated Department Stores and noted leader in the NBAA community says even back in his DC-3 days he was obsessed with noise reduction. And "obsession," in this instance, is no hyperbole; Pobanz's assault on every decibel is relentless and all-consuming.
His "primer" on the subject, "Noise Control in Aircraft Cabins" is a white paper authored by him in May 2000 and vetted by Dr. Andy von Flotow, a noted Massachusetts Institute of Technology-trained scientist and founder of Hood Technology Corp., which specializes in vibration management systems. Pobanz wrote the paper using lay terms that almost anyone can understand. He points out that the human ear has a limited range of hearing, from about 20 to 20,000 Hertz (Hz, or cycles per second). Human hearing is most sensitive in the middle of that range, about 500 to 6,000 Hz, and sensitivity falls off at both ends of spectrum, for which variation the A-weighted decibel scale for measuring sound, or dBA, is designed to compensate. Typical conversation generates a sound level of about 65 dBA, and noise at that level will interfere with comprehension.
Aircraft noise sources are external (e.g., engines, turbulent air flow, antennas) and internal (air distribution, hydraulics, electric motors, etc.). Pobanz manages sound levels by eliminating the noise at the source, by blocking the noise pathway — the most common method — or by absorbing the noise at the receiving end. Treating the noise at the source is always the simplest and most effective means. "Slightly imbalanced engine fans can shake the airplane at a low frequency — 90 to 115 Hz, say — so by altering an imbalance by removing six grams, you eliminate the source," he says.
Equipment noise from a pump or any rotating element will generate a tone at x Hz and multiples of that tone (2x, 3x, etc.). Random broadband noise, sometimes called "white noise," can mask distinct tones, and the ear won't detect them until they're quite intense. (Noisy neighbors keeping you awake in a hotel room? Find a space between radio stations or an empty TV channel that puts out a steady whooshing sound — the white noise — and turn up the volume until you can't hear them.)
The metal skin of an airplane is like a drum. Any large sheet of material can be set to vibrating, and passing air energizes the skin, which transmits the resulting noise through the metal airframe. Just as a drummer can dampen the vibration of the drumhead by placing his hand on it, Pobanz can add mass and elastomer damping materials to the skin to dramatically reduce the noise, being careful to select material that won't be affected by the extreme cold of the skin at altitude. Basic treatment must take the 95 dBA at the skin to a maximum of 60 to 65 at the interior surface. "It must be continuous, without interruptions, and sealed around the windows," he says, adding that a 1-percent breach can cost half of what he's trying to gain.
He can apply foam and fiberglass of various densities to create a "torturous path," as he calls it, that saps the energy out of the noise and converts it to minute amounts of heat. Barrier material reflects sound and can be used in conjunction with layers of foams and fiberglass to contain noise in a damped space. Wall panels are open-cell foam on the inboard side covered with a breathable fabric. All furniture, especially the divans in the rear cabin, has breathable fabrics that soak up sound. These materials come with adhesive backing in different weights and densities, and are easy to apply.
Pobanz has declared a one-man war on ventilation systems, which are large manifolds tucked atop headliners and elsewhere, usually riddled with dozens of holes to distribute the air to eyeball outlets and vents. "The smaller the holes, the higher the velocity and the more noise you get," Pobanz rants. In accordance with his mantra that noise reduction is attention to detail raised to a fine art, he methodically smoothes and rounds every air passageway. Each one may produce a tiny fraction of a decibel in noise reduction, but when he treats all of them, the whistle of moving air becomes a mere whisper.
Floors get an absorption layer, a barrier system, then an underpad and carpeting, with the result a kind of cushioned surface with plenty of bounce to it. But floors are a major source of cabin noise and demand an aggressive approach. Pobanz knows that the heavier the material, the better it dampens vibration, but he must balance that against the weight gain when the completed airplane goes on the scales.
One radical approach, call it Extreme Noise Reduction, creates a separate capsule within the fuselage that's held in place on soft isolation mounts that work the way Lord mounts do on a light airplane engine. Auto manufacturers often resort to rubber isolators between the cabin and the car's frame to eliminate the road noise before it reaches their interiors. Pobanz describes such a device, known as the Exo-Grid, but adds somewhat resignedly that completion centers did not accept the approach, and his current projects don't require it.
Mecaer Aviation Group of Italy has the most recent evolution of the concept in its SILENS product. Described as a "lightweight cabin noise and vibration reduction system," SILENS, designed in collaboration with AgustaWestland, received EASA approval via an STC on the AW139 helicopter. The completely passive solution is made up of an all-composite shell that mounts inside the fuselage and provides isolation from the airframe. All mounting points employ soft isolators, and there's plenty of soundproofing in the surrounding space. It forms the structure to which the rest of the interior attaches.
Sound scourge Pobanz works genuine magic on the heavy jets he has delivered, and just a casual walk through the cabin of one of his recent projects with Midcoast — we had no sound meters or measuring devices with us — yielded hints as to what the future owner of the beautiful aircraft will experience. You can clap your hands (or pop a balloon; any sudden loud noise tells the story) and the sound simply dies. Pobanz's eyes and fingertips explored the surfaces on panels and furnishings, and he seemed pleased. He'd ordered a foldout door to cover the main cabin door to reflect and muffle noises that might originate around the pressure seal. Once the cabin door is closed, the foldout door, matched to the cabinetry, can be moved into place to create a seamless gleaming swatch of exotic wood, its grain carefully matched to adjacent panels, and that one detail may be worth a small piece of a decibel.
What drives people to spend north of a million dollars for the sound of near silence? Pobanz says one captain of industry called him to ask what his new cabin had measured when they flew the final cabin-noise test with a full suite of meters aboard. "You've got bragging rights," Pobanz told him. BCA