Practical noise solutions for aircraft interiors
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LIGHT AIRCRAFT INSULATION: THERMAL AND NOISE CONTROL IN YOUR AIRCRAFT (Stearman owners need not attend!)
Pegasus Aeromarine Inc. www.pegasusaeromarine.comPractical noise solutions for aircraft interiorsPresenter: Dan NewlandPresident, Pegasus Aeromarine Inc.
revised December 2009http://www.orcon.comOrcon Corporation800-227-05051Pegasus Aeromarine Inc. www.pegasusaeromarine.comWhy Insulate?For comfort. ANY insulation WILL make the aircraft thermally and acoustically more pleasant perhaps with very little weight increase. Proper selection of an acoustic system will make the most from your weight and volume restrictions.
Pegasus Aeromarine Inc. www.pegasusaeromarine.comWhat makes aircraft noise?Engine, rattles, prop and aerodynamics.BUT:a.This may be from direct noise entry or from vibration. THEY ARE NOT THE SAME AND EACH IS TREATED DIFFERENTLY!
NOISEPegasus Aeromarine Inc. www.pegasusaeromarine.comNoise characteristicsWhat are the principal characteristics we look at with noise?
Frequency and volumePegasus Aeromarine Inc. www.pegasusaeromarine.comMeasuring NoiseWhat is the primary unit of measurement of noise volume?The decibel (named after Alexander Graham Bell). It is a logarithmic scale. THIS IS IMPORTANT! A drop from 100dB to 90 dB is NOT a drop of 10% but sounds like the noise! But a sound can also be measured in terms of pressure.
Pegasus Aeromarine Inc. www.pegasusaeromarine.comHertz (Hz, also known as cycles per second). 50 Hz = 50 CPS is about the lowest level humans can hear to 20,000 Hz, about the highest frequency people can hear. What is the frequency unit?http://www.orcon.comOrcon Corporation800-227-05056Pegasus Aeromarine Inc. www.pegasusaeromarine.comWhy is my plane so noisy?You hear noise for only 2 reasons:
1.Noise entered into the aircraft through an opening.
2.Vibration in the airframe was converted to noise
Note: noise cannot go where air cannot go. If you seal an aircraft air-tight, all that you hear is vibration changed to noise.Pegasus Aeromarine Inc. www.pegasusaeromarine.comHow do I control that?This means there are only 2 broad categories of treatments. One is for VIBRATION, the other for NOISE:Stop or reduce vibration BEFORE it becomes noise (Isolate and damp)
Stop or absorb noise before it gets to the passengers (Block and absorb)Pegasus Aeromarine Inc. www.pegasusaeromarine.comThis suggests a systematic method of control, doesnt it?Yes it does! This is the logical control progression:Isolate vibration before it enters the airframe Damp vibration that has entered the airframeBlock noise with a barrier before it enters the open cabinTreat noise once it has entered the cabin with absorbers
Pegasus Aeromarine Inc. www.pegasusaeromarine.comEXAMPLE: EngineEngine mounts should treat as much VIBRATION as possible before it gets into the airframe, (ISOLATE).Once vibration is in the airframe, it can travel and radiate ANYWHERE, so reduce the vibration just like turning down the stereo volume, (DAMP, i.e. reduce height or amplitude of movement).Vibrating structure acts like a stereo speaker so get between the structure and the passengers (BARRIER between the source and receiver)Once inside, soak up the noise in the cabin (ABSORB)Pegasus Aeromarine Inc. www.pegasusaeromarine.comIsolatorsThese try to keep the vibration away from the airframe. Engine mounts are the most obvious isolators but floors boards may be isolated from the frame with foam tape, ditto anything that could rattle like doors, glove box, baggage on hard metal, etc.
Weight: not very heavyThermal: N.A.
Pegasus Aeromarine Inc. www.pegasusaeromarine.comDampersDampers control vibration once it has entered the airframe. Vibration acts like a drum head moving back and forth, dampers add resistance reducing the magnitude of the vibration.NOTE: Dampers typically only need about 50-75% coverage
Weight: Light-Moderately heavyThermal: ModeratePegasus Aeromarine Inc. www.pegasusaeromarine.comBarriersBarriers are exactly what the name implies, they block noise. Since noise can travel where air can go, barriers are non-porous and to stop it from vibrating thus making its own noise, they should be limp making waves hard to travel through.
Weight: HeavyThermal: poorPegasus Aeromarine Inc. www.pegasusaeromarine.comAbsorbersAbsorbers are the last resort and not especially effective for all frequencies since by definition, it is picking noise out of the air that has already entered the cabin. However it weighs little and does treat high frequency noise well, (lower frequency requires greater thickness, about 5 for really low frequency).
Weight: Very lightThermal: excellentPegasus Aeromarine Inc. www.pegasusaeromarine.comWhy is frequency important? Higher frequencies are most easily stopped (say above 800 Hz)
Low frequencies like engine and prop noise are the hardest to blockPegasus Aeromarine Inc. www.pegasusaeromarine.comWhy is does it matter?Most of the noise in an aircraft is lower frequency thus harder to control. Example: 6 cylinder at 2400 RPM firing every other revolution (120 Hz power strokes). So you can expect a lot of noise at 40 Hz and even more at 120 Hz. You will also see noise spikes at 240, 360, and 480 Hz etc. Pegasus Aeromarine Inc. www.pegasusaeromarine.comABSORPTION OF NOISE ADDING 1 OF FOAM
Note half noise at 8000 Hz but only about 5dB (at best) in lower frequency so total is reduction in high frequency, not much in low.Pegasus Aeromarine Inc. www.pegasusaeromarine.comBLOCKING NOISE WITH A lb. sq. ft. BARRIER
Note large reduction in all frequencies due to barrierPegasus Aeromarine Inc. www.pegasusaeromarine.comEFFECT OF THICKNESS ON ABSORPTION
Performance is improved everywhere but not quite by 2XPegasus Aeromarine Inc. www.pegasusaeromarine.comEFFECT OF MASS ON BARRIER, doubling weight of barrier.
Twice as much barrier does not improve sound much beyond 800 HzPegasus Aeromarine Inc. www.pegasusaeromarine.comWhat did we learn?The thicker the absorber, the lower the frequencies and loudness absorbed Barriers can make a HUGE difference in the difficult low frequency regimeThe more massive the barrier, the lower frequencies blocked Above a certain frequency, extra mass does not improve acoustics in higher frequenciesPegasus Aeromarine Inc. www.pegasusaeromarine.comWhat about vibration? If you were in a spaceship in orbit, the engine firing would be a roar but you wouldnt hear it outside the ship since sound doesnt travel in a vacuum. But once you were inside the capsule, you would hear the engine just fine. That sound is noise traveling in the structure then being converted to noise in the air of the capsule. Pegasus Aeromarine Inc. www.pegasusaeromarine.comControlling vibrationThe importance of vibration cannot be overstated so it pays to do it right!
Install the engine with the right mounts for your engine/prop/style of flying (an aerobatic mount for the same engine wont be as quiet as a cross country mount that is softer)Pegasus Aeromarine Inc. www.pegasusaeromarine.comVibration SolutionsClean up the aircraft aerodynamically as much as possible. Use gap seals when possible and do tight fits for moving parts as much as possible (consistent with clearances needed for normal movement, ice buildup, etc.). Install the doors (including landing gear) carefully so as not to hang out in the slipstream.
Pegasus Aeromarine Inc. www.pegasusaeromarine.comCan vibration (and noise) be stopped completely?Not in a practical sense for aircraft. It takes weight (mass) to stop acceleration and enough to mass to stop vibration is very prohibitive.
Ever wonder why you can get thicker and heavier windscreens to make quieter cabins? Its called the mass law that states every doubling of the mass drops the noise by about 6dB. Likewise, noise absorption requires thickness and barriers require mass. Pegasus Aeromarine Inc. www.pegasusaeromarine.comWhat is the next best solution in dealing with the vibrations? Reduce the magnitude with materials that affect the way the fuselage, bulkheads and wing spars vibrate.Pegasus Aeromarine Inc. www.pegasusaeromarine.comDesign SummaryEliminate ALL gaps and openings into the aircraft! Use soft closed cell foam or silicone caulking to close EVERY opening since we now know that once air tight, noise CANNOT enter DIRECTLY into an aircraft!Pegasus Aeromarine Inc. www.pegasusaeromarine.comDesign Summary Add or replace foam or rubber cushions under any structures that can rattle like cowlings against the fuselage, oil filler doors, trim panels against the fuselage, etc.
Pegasus Aeromarine Inc. www.pegasusaeromarine.comDesign Summary Clean up aerodynamics as much as possible like add gap seals to flaps, wing-fuselage gap, etc.Pegasus Aeromarine Inc. www.pegasusaeromarine.comDesign SummaryBalance engine and prop (very expensive so this is rarely done) but balancing the engine is the first thing done on a business jet, typically to 10% of its normal tolerance.Pegasus Aeromarine Inc. www.pegasusaeromarine.comDesign SummaryMake certain you have the correct engine mounts for your engine, prop and style of flying, (a cross country plane should NOT have the same mounts as an aerobatic plane even though the engine and prop may be identical).
Pegasus Aeromarine Inc. www.pegasusaeromarine.comDesign SummaryInstall absorber blankets either foam, (can be directly upholstered over and looks great!) or fiberglass and bagging film. Thermal will be great too at very little weight, 1 of .6 lb/cu. ft glass = .05 lbs/sq. ft. so a typical 4 place plane with 125 sq. ft. of area might only weigh 5-7 lbs.Pegasus Aeromarine Inc. www.pegasusaeromarine.comDesign SummaryInstall barrier blankets behind firewall (with isolation decoupler layer, barrier and absorber) plus felt/barrier/felt blankets on floor