Shhhhhhh….It is so Quiet With Insulation From Air Interior
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.”
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.