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Dan,
Some observations about our composite aircraft and noise:
Keep in mind that the following information is for a slow-built, Pre-preg glass, thin canopied (3/16), 1989 Lancair 320 kit.
1. After the soundproofing described later, cruise flight (defined as 25 inches, 2500 rpm, 180 KIAS, Hartzell 70 inch CS prop) would yield between 101 and 103 Db (slow response scale, Radio Shack noise meter).
2. The greater noise reading is obtained by opening the cockpit cooling vents (air rush plus prop/engine). These are NACA ducts on the fuselage sides at about 2 feet aft of the firewall that feed air at "calf" level and thru an instrument panel "eyeball" vent just in front of each stick (face air).
3. The measurements were taken near the pilot's right ear.
4. For comparison, my 96 Corvette yields a reading of 98 Db at 70 mph traveling on asphalt pavement with 20,000 miles on the tires and not counting the screams coming from the passenger.
Sound conditioning components:
1. The interior is fully upholstered with sheepskin seats and various automotive type wools over foam with "plastic" backing for shape control. The baggage area below the waterline and the forward floors are carpet covered.
2. The 18 pound E.A.R. soundproofing kit was installed. This consisted of self-stick dense foam and dead aluminum coated sheets.
3. Material was applied to the back of the firewall and on the fuselage back to the seat backs, including under the seats and the nose wheel well.
4. The forward "D" section of the stub wing was soundproofed to eliminate a "sound-box" effect from that area.
5. Remember that the firewall consists of aircraft plywood, 2 sheets (in my case) of Fiberfrax and a sheet of stainless steel. The wood is a natural sound barrier.
Some conclusions drawn from a few experiments:
1. Most of the noise comes thru the 3/16ths inch canopy. I tried soundproofing this, but it was hard to see out. Note that later kits were supplied with ¼ inch canopy material and that should go a long way to reducing the noise transmitted thru the canopy.
2. The cockpit air intake's noise contribution would benefit from either a quieter location or longer sound conditioned ducting.
3. If the baggage bulkhead is removed, one can hear a great deal of noise from the tail cone. Don't fly without the baggage bulkhead in place.
4. Fiberglass is a hard surface and can reflect and enhance "noise".
And, the ultimate conclusion is that you would have to pry my Bose headset off of my cold dead ears - I will never give them up.
I know the ES is a different animal and I applaud your use of lighter sound conditioning material. I wonder if there is a light weight "flock" spray that could reduce noise or its reflection in unexposed interior areas?
Good Luck.
Scott Krueger
N92EX
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