From Britain's Pilot Magazine
by Stephan Wilkinson
Copyright Stephan Wilkinson,
IMAGINE that an evil djinn wants to create a torture chamber to assault the senses; to turn his victims into addled idiots. A good start would be a box made of thin, flexible, drumhead aluminum sheets. Then bolt a powerful, compact machine to it that creates thousands of unmuffled explosions every minute, at the same time swinging a huge metal bat that pummels the aluminum at much the same rate. Finally, suspend the whole thing from a skyhook a mile up in the air, jam your chosen unfortunates into the cell and leave them hanging there for three hours.
Some would call it hell. We'd call it a general-aviation airplane.
Do you want to see how loud your airplane's cabin really is, and get an indication of what the noise is doing to your hearing? It requires no fancier test equipment than the radio in your car. Next time you drive to the airport, tune it to a station that provides a well-modulated level of sound (chamber music, easy-listening pops or speech rather than heavy metal). Adjust the volume to a pleasing level. When you park and shut off the radio, don't touch the volume control. If it's part of the on/off knob, mark the setting somehow.
Go fly for a couple of hours, and when you drive home, turn the radio back on and see how much added volume you need to achieve a comfortable listening level. You might be surprised! Now note the new setting. Particularly when you go out the next morning, turn the radio on to that setting for your daily commute to work and hear how loud it now sounds.
What you're seeing is temporary hearing damage, and then the recovery. What you've experienced, however, is a cumulative assault on the cilia of your inner ear - the tiny hairlike cells that vibrate in response to sound energy and communicate that sound to the brain. Excessive noise eventually wears them out permanently.
So why aren't concert rock musicians all deaf from standing amid loudspeakers powerful enough to levitate a locomotive? For one thing, they go through earplugs like popcorn. For another, according to New York City musician, pilot, and record producer Tony Bongiovi, a Twin Comanche owner who has done extensive work in soundproofing his own airplane as well as others, the noise of a concert runs up and down the scale of frequencies constantly, "but the danger of aircraft noise is that it's relentlessly focused, flight after flight, on exactly the same frequencies, since we all fly at the same power setting day after day."
Think of a loud concert as incredibly bright sunlight. Think of the drone of an IO-360 turning a two-blade prop as a lens continually focusing that sunlight on a particular spot in your inner ear.
Bongiovi feels that much of general aviation's noise problem - both inside the cockpit and on the ground below the air-plane - can be traced back to generations of pilots for whom noise was an integral part of the performance equation. They're the people who designed, marketed and originally bought the airplanes we're flying today, and for them, a silent Cessna is about as appealing as a Harley-Davidson with a BMW R60's mufflers. And left-ear deafness is a proudly worn badge of... oh, say 10 years of flying the night mail in short-stacked Twin Beeches. "I'd happily accept an additional 150 pounds of weight and reduction in performance to make my airplane quieter," Bongiovi says. "For me, it's transportation, not a thrill ride."
There are four basic sources for the noise that assaults pilots, passengers, and people on the ground below them.
The most important is undoubtedly the propeller - the baseball bat that pounds the airframe with fat pulses of air roughly 4,800 times a minute in the case of a two-blade prop, 7,200 time with three.
Next in priority probably is purely aerodynamic noise - the slipstream, turbulence, leaks, and vibrations set up by airframe protrusions. (I say "probably" because the situation varies from airplane to airplane, even among identical models, and because acoustic engineers often disagree as to the import of various noise sources. One noise can be "louder" yet less intrusive than another frequency that may be lower on the dB scale but consider-ably more annoying.)
How bad can aerodynamic noise get? If you've ever flown in a metal Schweizer glider, you'll have had a hint, and you'll know that the sailplane's beloved "silent flight" is in fact an oxymoron. Imagine what the 60-mph Schweizer's hammering and whistling and vibrating would sound like at 180, and you'll have a sense of what happens when pure airflow sets aluminum in motion even in the absence of an engine and prop.
Then comes the engine's exhaust, even though you'd intuitively think the bark of the tailpipes takes a bigger bite out of cabin quietude than does aerodynamic noise. "If you use a Bonanza as an example," says Olen Nelson, president of an innovative California aircraft soundproofing company called Aero Sound Shield, "the exhaust is released right about below the rudder pedals, and the airflow is going to push each pulse right up against the bottom of the fuselage. It's regenerating a lot of noise right through the belly skin, just as though a bass drummer were pounding on it and you were sitting inside the drum."
It's hard to say exactly what exhaust noise "is," but the intuitive sense that one is literally hearing the explosions inside the engine is really not true. Much exhaust noise is created by the sudden expansion of extremely hot gases into cold air, which is one reason why short stacks are louder than long tailpipes: the air has a chance to cool considerably inside the tailpipe.
Another major component of exhaust noise, particularly when an airplane engine is bugling through tubular headers rather than an ordinary cast-iron automotive exhaust manifold, is the impact of the shock wave of every single exhaust pulse upon the thin stainless-steel pipe wall nearest the exhaust valve. Again, imagine a ball-peen hammer whacking away at the metal 1,200 times a minute.
Finally, our fourth and probably least critical source of cabin noise is engine vibration, though we should count that exhaust-pipe ball-peen hammer as part of this. If you could run a Lycoming or Continental on the ground with no prop and a totally silent exhaust system, you'd be stunned at the amount of mechanical clatter, air-induction roar and general metal-to-metal hysteria you'd hear. (Stick your head inside a well-muffled Porsche Carrera's engine bay and you'll see what I mean: air-cooled engines are real thrashers.)
OKAY, so we know what creates the noise. What do we do about it?
Noise control is no mystery. It's an established engineering discipline. Indeed, one can subscribe to the monthly journal Sound and Vibration, "the noise and vibration control magazine," - and my most recent issue coincidentally includes the feature article "The Measurement of Noise and Vibration Transmitted into Aircraft Cabins." This is not a black art.
Automobiles can be made so quiet that the Lexus LS400, for example, now has an ignition interlock that prevents the starter from being cranked when the engine is already running; early models suffered a spate of chipped flywheel teeth as owners kept trying to restart engines that were already running. But automobiles have a number of big advantages:
PROPELLERS: At worst, a propeller creates a supersonic shock wave at each of its tips, when the combination of blade length and engine rpm means the outermost part of each blade is traveling at or very near the speed of sound. No engineer would allow a prop design to do this at cruise, since that shock wave utterly destroys the lift-producing (i.e.. thrust-producing) ability of that part of the blade, but it can happen at takeoff rpm. Cessna 180/185's are infamous offenders, as are Harvards - though the North American trainer's tooth-loosening takeoff rasp is actually the product of a variety of prop and engine characteristics.
Still, one of the simplest and most effective noise-reduction techniques is dialing back prop speed, to slow the rate at which the tips are traveling and to lessen the energy of the pulses of air thrown off the blades.
"We've recently started a program of trying different propellers on our testbed Bonanza to see what different noise levels are produced," explains Olen Nelson of Aero Sound Shield. "The airplane initially had a Hartzell with long, fat blades. Changing from that to a Hartzell with shorter blades resulted in a significant noise reduction. Going to a McCauley with shorter and also thinner blades again measured quieter. We're about to test a four-blade Q-Tip prop, and we expect very significant reductions."
Prof. Howard Patrick of Embry-Riddle Aeronautical University in Florida feels that there are several means of quieting propellers. "Half the noise of a conventional propeller is simply due to loading - where you get your thrust," Patrick ex-plains. "The other half of the noise is due to the thickness of the blade - the noise of the air being pushed outward by the prop and having to come back together after the prop's passage. The thicker the blade, the greater that effect is, but with advanced materials, you could reduce the propeller noise just by reducing the thickness."
The noise reduction engendered, however, would be only on the order of three dB. "Even though that's halving the noise, it's barely perceptible to the human ear," Patrick says. "The ear is a nonlinear device. It's half the noise energy, but not half the apparent sound."
The decibel scale is not arithmetic but geometric. "The energy of the noise doubles with every three-dB increase," ex-plains Terry Carraway, a Maryland occupational health consultant who happens to fly A-10 Warthogs for the U. S. Air National Guard. "Government standards consider 80 dB to be acceptable - a sound that can be experienced for four hours without danger of any permanent hearing damage. So 83 dB is good for two hours, 86 dB for one hour, 89 dB for half an hour, 92 dB for 15 minutes and 95 dB, which is typically the sound level in a light-aircraft cockpit, for seven and a half minutes."
Patrick is pursuing research on ducted props with active noise-canceling technology as part of the prop shroud--literally, loudspeakers broadcasting a signal 180 degrees out of phase with the major prop frequency and thus canceling it. "You get some noise-blocking effect just by having the duct there, but it's mainly a place to put the speakers," Patrick explains.
EXHAUST: Once you've done what you can about prop noise, one's thoughts inevitably turn to muffling the exhaust. After all, didn't light de Havilland - and a variety of other between-the-Wars types have mufflers? Well, not really. Their long tailpipes might well have had some muffling effect, but generally, the reason for such installations was to carry exhaust gases away from open cockpits, which often were low-pressure areas and attracted the fumes.
Opinion varies widely on how effective automotive-type mufflers would be. Some think they do a little good at the cost of a lot of performance. Others, such as Olen Nelson of Aero Sound Shield, feel that even modifying the tip of the tailpipe could make a big difference. "You can do a lot by closing up the end of the exhaust pipe and redirecting the exhaust through a lot of slots or holes, which spreads the exhaust pulse out and raises the frequency of the sound," Nelson says. "We're just starting to experiment with it, but there's no reason it shouldn't work."
Borla Performance Systems of Oxnard, California makes exhaust systems for everything from formula racecars, Italian exotics, and high-performance motorcycles to package delivery trucks. (In one of their biggest recent contracts, Borla has replaced all the exhaust systems, from headers to tailpipes, of the entire U. S. fleet of UPS vans.) Borla also does design and consulting work for Chrysler and Ford. Alex Borla is a pilot - a Beech Baron owner - who feels that aircraft mufflers can make a big difference, and Borla is currently experimenting with such devices.
The company has instrumented the Baron so they can run muffling tests on one engine while leaving the other one stock while making simultaneous noise measurements at exactly matched power settings (confirmed through strain gauges on the engine mounts). Problem is, Borla's exhaust systems are too good. "We don't employ any baffles in our automotive and motorcycle mufflers," Alex Borla explains. "Everything is straight-through. As a result, we're able to tune the exhaust system all the way out to the tip of the tailpipe. With a baffle-type muffler, as soon as the exhaust pulse hits the first baffle, the tuning effect is over."
On an airplane, however, tuning the exhaust will buy you trouble. "If the product we make enhances the power of the engine, we can't get an STC on it," Borla points out. "I know from just looking at the manifold on the IO-520 in the Baron that I can get at least a 12 to l5 percent increase in power. Which is 30 or 40 extra horsepower, and that's a big number. I can also bring the engine internal temperatures down and convert that horsepower gain into performance and better mileage."
But to sell an aviation system, Borla would have to dumb down his product, "And that's tough to do, with the patented design that we have. But the way the FAA regs are written, I'd have to almost recertify the airplane if I used it."
Still, Piper has contacted Borla for help in designing a muffling system for the European market. "Piper is very concerned about the European market," says Professor Patrick of Embry-Riddle, "and there's every reason to think the environmental laws in the U. S. will eventually become just as strict. All that U. S. manufacturers can do right now to meet those noise requirements is to use reduced power. Or fit mufflers, but that doesn't do a thing about propeller noise."
Whatever Borla does with his mufflers, they won't be heavy external fitments. "You see all sorts of appendages hanging out of Bonanzas and stuff in Europe," Borla says. "They're after-thoughts - basically a knee-jerk reaction to a noise problem - and I think they definitely impede the performance of the airplane substantially. You probably lose 15 percent of your performance by fitting them, but the only other choice is that you can't fly without them."
But wouldn't weight, cost and finding under-cowling space all be problems? Not necessarily: Borla Performance already manufactures compact carbon-fiber mufflers with stainless-steel end caps for motorcycles, and Borla estimates that a Baron-size installation would only add seven or eight pounds per engine using similar mufflers. "And if the motorcycle industry can handle the cost, the aircraft business certainly can.'
SOUNDPROOFING: A moderately expensive but increasingly effective way (due to the use of some interesting new materials) to make a lightplane cabin more comfortable is to thoroughly soundproof it. This means adding some kind of dampening and insulating material between the outer skin and the inner panels of the airframe, and Olen Nelson's Aero Sound Shield specializes in this technology.
Back when Cessnas and Pipers were being pumped out by the thousands and airplanes were sold on performance, price and sex appeal, the manufacturers simply stuffed fiberglass "insulation" into the voids between the aluminum and the Royalite interior panels. "That only works at the higher frequencies at the very high end of our hearing range, it's not helping a lot," Nelson points out.
Aero Sound Shield firmly glues foam panels backed with aluminum foil to the inside of every skin panel. "What has the most effect is the glue itself. It ties the skin down and makes it vibrate at a higher frequency," Nelson says. "If you did your whole airplane with that, you might net about a two-dB reduction. But beyond that, we make bags of insulation that contain an inch of acoustical glass, an inch of closed-cell neoprene acoustical foam and then another inch of acoustical glass. The bags are pressed to fit in between the stringers and bulkheads, filling the area completely." Since the acoustical foam is isolated between cushioning layers of glass, it takes considerable energy to reach the foam and cause it to vibrate, which is what regenerates noise.
Another effective form of soundproofing involves the addition of thicker windshield and window Plexiglas. New York record producer Tony Bongiovi claims to have sound-deadened his Twin Comanche to a level that he says is "no louder than a good sports car." In fact, his airplane is equipped with a 160-watt, 10-speaker hi-fi system that he listens to without a headset. He made complex noise studies of the airplane using extremely sophisticated recording-industry equipment and then bonded sound-proofing foam to the skin panels, but he also attributes a lot of the quietude to considerably thicker windowpanes. (Bongiovi has worked as a noise consultant for Mooney and then Roy LoPresti on the SwiftFire program and also does individual retrofit projects - most recently OJ Simpson lawyer F. Lee Bailey's Commander Shrike).
Double-paned "insulating" glass, however, does little to mitigate noise unless the panels are at least an inch apart - four inches is optimum - which means only relatively large GA aircraft have the depth to accept such an installation.
Aero Sound Shield is currently soundproofing a particularly large corporate jet - a converted McDonnell-Douglas MD-87 - for a Las Vegas hotel owner who is almost totally blind and who has compensated by developing extremely sensitive hearing. "He wants a 60-dB airplane, which is quieter than a Lexus," Nelson points out. Because there are 35 inches of stringer depth between the inner and outer skins of the cabin, Nelson feels they can achieve this with a complex, multi-Walled version of the bagged-insulation system plus almost totally isolating the cabin from the airframe via shook mounts, so it's as close as possible to floating free. "That way, it won't pick up any vibrations," Nelson avers.
Unfortunately, your airplane and mine can't accept the mass and weight of MD-87 measures. "I rode one time in a Piper Arrow that was very quiet," Nelson recalls, "but it was completely lead-lined. It could carry two people and half a load of fuel. That was fine for that guy, because that's all he ever wanted to carry, but it wasn't very practical."
VIBRATION DAMPENING: Some of the most interesting and advanced practical aircraft noise-control work is being done in the U. S. by the Lord Corporation, makers of the familiar rubber-biscuit Lord mounts that cradle many Lycomings and Continentals. But Lord's NVX Active Noise Control Systems go far beyond rubber shock mounts.
"Our traditional business has been passive-elastomeric engine mounts," Lord Market Specialist Rebecca Weih explains. "Now, however, we have three different versions of active noise and vibration control for aircraft. One is active isolation control, where we put actuators into the engine mounts, connected to a computer, that make the mounts vibrate in apposition to the engine's vibration. It doesn't stop the engine from vibrating, but it keeps that vibration from being transmitted to the cabin." This system is intended for jets, and will first appear on the Cessna citation X.
Lord's second level of defense against noise is called active structural control, and it's also a big-league program: it would probably cost on the order of $40,000 to $45,000 to retro-fit to a King Air-class aircraft. Small actuators are attached to the inside of an aircraft's skin, and they are made to vibrate like tuning forks, at frequencies that cancel the noise-producing vibration of the skin itself. There can be as many as 30 of them - one for each unsupported area of skin around the cabin that is delineated by the airframe bulkheads and stringers - and they're all connected to a small central computer that constantly adjusts the frequencies at which the actuators vibrate.
Except for the cost of such a system, active structural control would work splendidly in light GA aircraft, for one of the major "noise propagation paths" in an airframe is any large area of thin skin paneling set to vibrating by prop pulses. The aluminum acts almost like huge loudspeaker cones. Nor does it help that the tail cone of most lightplanes is shaped like a megaphone. The flatter the skin panel and the greater the area of unsupported aluminum, the more effective the skin is as a noise propagator. (Flat-bottomed, slab-sided, minimally bulk-headed Piper Cherokees and their offspring should therefore tend to be louder than the equivalent Cessnas and Beechcraft, though I don't know of any studies proving that surmise.)
Lord is working on a third level of noise-cancellation technology that will have light-aircraft applications, and it's called active noise control. The active noise-reduction headsets that increasing numbers of us are wearing work by broadcasting inaudible signals inside each earcup that exactly cancel out the two or three predominant low-frequency tones that otherwise would be assaulting the pilot's ears. Lord active noise control essentially removes the earcups and broadcasts the same signals from half a dozen or more speakers inside the aircraft cabin, and the effect is much the same, except that the entire cabin becomes the earcup.
This system has already been commercially installed in a King Air and goes for around $35,000, but Lord hopes to soon let the benefits trickle down. "We plan to develop a more generic system," Rebecca Weih says. "Right now, the technology is pretty much custom-designed for each aircraft, but we need a system with which we can tell an FBO, 'Find a space to put the computer, and here's how to determine the best locations for the speakers and the sensor microphones.' From what we can tell, people with sophisticated singles and light twins might be willing to spend $5,000 to $10,000 on such a system, and we need to drive the cost down to that."
Weih agrees that lowering aircraft noise levels is not something that old-timers care much about. "We've found that the very experienced King Air pilots aren't particularly impressed by our active noise-control systems," she admits, "probably because they're already half-deaf. You have to get medium-experienced pilots who are bothered by the noise because they can still hear it. "
THE ENVIRONMENT and THE FUTURE: Fortunately, many of the things that we can do to make lightplane cockpits and cabins more livable also lower the noise level for people on the ground--particularly propeller changes and exhaust muffling. Crowded Europe is a decade or more ahead of the wide-open-spaces United States in terms of anti-noise legislation - aircraft enthusiasts would say it's a decade behind us - but such legislation is as inevitable here as is the appearance of catalytic converters on aircraft engines.
Some local anti-noise regulations are already in place. "If you take off from our local airport, at Torrance, California, in a 210 and don't pull that guy back to 24 square at 500 feet, you'll get a letter," Alex Borla says. "Do it twice in a row and they won't let you back in."
"I'm based at Santa Monica Airport, and they monitor every single flight that goes out of here," says fellow Californian Olen Nelson. "The neighbors complain a lot, and I think we're eventually going to get pushed into more and more anti-noise regulation."
At Sugarbush, Vermont, an area dotted with environmentally conscious summer- and winter-resort homeowners, the busy local soaring operation has equipped its fleet of Cessna L-19 towplanes (essentially a military version of the 180) with four-blade Hoffman composite props of considerably reduced diameter. They've lost some performance but gained a reduction in climb-out noise.
For decades, general-aviation supporters have used the argument, "We were here first," when homeowners complain about airport noise. Unfortunately, that is an increasingly irrelevant defense. In Manhattan, loft-seekers move into light-industry areas of the city and then complain about smelly dry-cleaning plants that were "there first." Invariably, the apartment-dwellers win. In the Midwest, suburban sprawl butts up against century-old farms that reek of fertilizer, and it's the farmers who are forced to clean up their acts. As Los Angeles has grown over the years, it has rolled over trap-shooting ranges, sprint-car racetracks, rocket test sites and other noise producers, and complaints that "we were here first" are invariably ignored.
"People want quieter cars, quieter apartments, less noise from the freeways," Olen Nelson points out. "Because we're moving closer together, people have become much more sensitive to whatever privacy and quiet they can get. People try to upgrade the quality of life wherever they live, even when it's near an airport. That's the natural thing to do."
Even reducing aerodynamic noise will help. "An airplane is a noise radiator," says industrial hygienist Terry Carraway, "and if there's less noise for it to radiate, there's less to bother people on the ground."
Soon, we may be forced to make our aircraft quieter - if not for our own good, certainly for that of people on the ground below. It'll require a high-performance lightplane probably made of leak-free, clean composites (even airframe add-ons such as antennas make aerodynamic noise) that are stiff enough to resist vibration but not so solid that the airframe itself becomes one big noise pathway. The engine will be liquid-cooled, driving a small-diameter, many-blade, ducted pusher prop made of advanced materials yet light and cheap enough to be usable and affordable. It'll have a light yet effective internal muffling system, not just a heat exchanger can. The cabin will be extensively soundproofed from the git-go, not as an expensive retrofit, and it will make use of active noise-reduction technology.
Trouble is, this is never going to happen in an industry that manufactures a couple of thousand airplanes a year. So look at it this way: Either general aviation becomes healthy enough to support advanced technology rather than born-again Skyhawks… or there will be so few of us flying lightplanes that nobody will care about the occasional noise we make.
Some of the sources for information or products mentioned in this article:
Aero Sound Shield
Sound & Vibration Magazine
Borla Performance Industries
SIDE NOTE: A Simple Experiment Regarding Aircraft Noise
It's interesting that when the first Bonanza was being developed, according to the American magazine AOPA Pilot, "the engineers specified a minimum of 150 horsepower at a maximum of 2,050 rpm [to] increase prop efficiency and reduce noise." Beech and Continental abandoned those limitations, of course, as soon as the need for speed became a part of the program
A simple experiment showed me why. I bought a $35 decibel meter from an electronics hobbyist store. Admittedly not a precision instrument, the meter nonetheless provided stable and useful readings, and certainly was good enough to show trends, if not absolutely exact dB numbers,
I took my new decibel meter to 5,500 feet in my own airplane, a Falco - a fiercely noisy homebuilt with a powerful engine for its size, a pair of straight exhausts, and a large bubble canopy. I tried different power settings ranging from 2,650 rpm at 25 inches of MP all the way down to 1,900 rpm at 23 inches. Noise levels ranged from a high of 108.5 dB (2,650/25", providing a true airspeed of 181 knots) down to 101.5 dB (2,400/20"/152 knots).
It was an interesting exercise if only because the resulting curves of noise versus prop speed versus performance were complex enough that it would take a better calculus student than I ever was to graph them meaningfully. For example, going from a power setting of 2,250/22" to 2,000/23" cost only a single knot of true airspeed but raised the decibel level from102.5 to 107, perhaps because of increased vibration or a different resonant frequency caused by the lower prop speed. The lowest performance (149 knots/104 dB) came at the slowest prop speed but relatively high manifold pressure (1,900/23"), demonstrating that the efficiency of conventional prop blades decreases rapidly as speed drops.
I also tried several partial-power takeoffs, on the theory that we don't accelerate away from every stoplight at full throttle and max revs in our cars, so why should we do it in our airplanes? Bad idea. Normal full-throttle takeoff (2,700/30") produced 111 dB. A leisurely trot down the runway at 2,500/25" admittedly halved the noise energy, to 108 dB, but extended the takeoff run considerably.
However, decreasing prop rpm as soon as possible during the climb out - say at 500 feet agl - is an enormous boon to people on the ground along a runway's extended centerline and should be part of every pilot's standard operating procedure.