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Re. the soundproofing for the Lancair IV
I read your email and can't comment specifically on what's in the soundproofing package since I haven't seen it but I can offer some advice. I work with EAR and others in soundproofing corporate and commercial aircraft although we are more into the actual design and installation (STC's and PMA's). EAR is a high quality manufacturer of noise suppression materials and is a great source for a variety of sound treatment materials. My company (the Orcon Corporation in Union City, California) also makes patented sound absorbing fire barrier/fire hardening materials for aircraft so we tend to be complimentary, not competitors.
Making an aircraft quiet should address three things and in this order:
1. Keep the noise out
2. Absorb the noise that gets in
3. control and quiet structure born noise coming in.
The analogy of person in a boat is good. You want to keep the water out first and what does get in, you want to get rid of.
To this end, a barrier material is used for number one and its properties are mass and "limpness". This is because of a thing called the mass law which states that for every doubling of the weight of a barrier, the noise level drops about 3dB (significant). So unfortunately, all barriers will have to weigh a fair amount to do their job. The lightest I know about is around an 1/16" thick and weighs about 20 oz./sq. yd. and they can be 3-4 times that amount. Barrier materials are frequently made from lead or barium filled vinyl because it's limp and massive, (it takes more energy to move the mass and the limpnesss doesn't easily transmit the noise that impinges on it). But a massive barrier should also not be in direct contact with a surface. They should be isolation mounted, generally with a soft foam of perhaps 1/4" thick between the noise source and the barrier. They should NOT be in direct contact with the aircraft structure. Aerodynamic noise can be the loudest in many aircraft so a couple of things can help there BIG time. Poorly shaped canopies, fittings and rivets sticking into the air flow, un-aerodynamic window trim can all cause turbulence so try and make easy smooth curves and well shaped fittings outside the cockpit (not a problem on the Lancairs I might say!), but you may not be easily able to change that stuff. So the next thing is to make sure all doors and windows are well sealed. Any air leaks are also noise leaks and may be the actual source of the noise (like a whistle) so gasket all fuselage openings such as doors, air inlets, windows, etc. Here's an experiment. Next time you are in your car by the freeway, crack the window the smallest amount you can and listen to the exponential increase in noise. You will actually tend to filter out lower frequency noise and let in higher frequency due to the size of the crack. But this illustrates how an extraordinarily small hole can let in a tremendous amount of noise. So SEAL EVERYTHING!
To tackle number 2, once noise is in a plane, absorbers are used for noise control. These are either fiberous (such as fiberglass batting) or OPEN cell foam. As a basic rule, if you have some space that isn't doing anything, poke some extra foam in there to absorb noise. Maybe under the seats, under the dash, etc., the more the better. And carpeting is an excellent absorber as is fiberous headliner. All absorbers are porous which allows noise to travel inside, and then dissipate acoustic energy by vibrating the fibers or cell walls converting energy to heat. So a fabric covered seat would probably get rid of a little more noise than leather covered ones. And the good news is that unlike barrier material, absorbers are typically very light. Most around .4-.6 pounds / cubic foot. And the absorber thickness also plays a big part. The thicker the absorber, the broader the frequency of absorbtion. To absorb extremely low frequency (the hardest to stop of all) which are at the limits of human hearing takes about 4' of foam. Not very useful but 3" will get most of the benefits and is at about at the practical limits.
For number 3, the engine mounts, prop wash and aerodynamic pulses can cause structures to vibrate and those in turn can carry the noise back into the aircraft cabin. So a good place to watch for noise entering the cabin would be through the engine firewall at the engine mount bolts and large open fuselage panels (which can vibrate like a drum head). So where possible, these should be bolted in with isolation mounts. Because the engine can transmit noise through the cabin easily, the back side of the firewall would be a great place to put some extra barrier, particularly behind the mounting bolts. And then some absorber for good measure. Another problem can be large unsupported flexible panels like doors (more true in metal planes than stiff, cored composite planes). The treatment is called dampers (NOT "dampeners"). These are soft sheets of stuff that are bonded to a large panel and help stop the vibration. They are normally only half of the area of the panel that requires treatment. So a 2' x 2' panel (4 sq. ft.) would need about 2 sq. ft. to be effective. The material that Jack Hickam describes with the foil backing sounds like this kind of material.
And what Jack said about fire resistance is very important. The material that he said was in his IV kit and was used for trucks may meet DOT specs but those guys can walk away from a burning truck, not so a plane. Commercial aircraft have to meet FAR 25.853. This means it must resist burning for 12 seconds in a 1400 degree flame and 60 seconds if mounted to the structure. And even more strengent requirements are being proposed! So while the experimentals aren't restricted to other aircraft standards, I would make sure they were certified to meet FAR 25.853. I would keep these certs. in your aircraft documents to show compliance later. It could help on your insurance and sell it later to a potential buyer. In addition, some extra fire hardening in the firewall could also make a big difference were there to be a raging fire. Our material (trade named Curlon) is being investigated by the Navy for ship and submarine fire hardening, by the department of forestry for emergency firefighter shelters, by NASCAR for race car drivers and by the FAA. Tests performed by the FAA in their full sized burn test unit in Atlantic City never burned through the material even thought the aluminum was consumed! The fire finally died when it ran out of fuel! As a plus, Curlon is also an excellent noise absorber.
If you are interested in getting more information on this, you can contact me at Orcon at (510) 476-2150 or email me at Dan.Newland@Orcon.com. And as Jack said EAR also is a good source.
Dan Newland (getting an ES kit shortly)
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