Practical noise solutions for aircraft interiors

Pegasus Aeromarine Inc. www.pegasusaeromarine.com

Practical noise solutions for

aircraft interiorsPresenter: Dan NewlandPresident, Pegasus Aeromarine Inc.

revised December 2009


Why 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.


What 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!

NOISE


Noise characteristics What are the principal

characteristics we look at with noise?

Frequency and volume


Measuring Noise What 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 ½ the noise! But a sound can also be measured in terms of pressure.


Hertz (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?


Why 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.


How do I control that?This means there are only 2 broad

categories of treatments. One is for VIBRATION, the other for NOISE:

1. Stop or reduce vibration BEFORE it becomes noise (Isolate and damp)

2. Stop or absorb noise before it gets to the passengers (Block and absorb)


This suggests a systematic method of control, doesn’t

it?Yes it does! This is the logical control

progression: Isolate vibration before it enters

the airframe Damp vibration that has entered

the airframe Block noise with a barrier before it

enters the open cabin Treat noise once it has entered the

cabin with absorbers


EXAMPLE: Engine Engine 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)


IsolatorsThese 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.


DampersDampers 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: Moderate


BarriersBarriers 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: poor


AbsorbersAbsorbers 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: excellent


Why is frequency important?

Higher frequencies are most easily stopped (say above 800 Hz)

Low frequencies like engine and prop noise are the hardest to block


Why 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.


ABSORPTION 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.


BLOCKING NOISE WITH A ½ lb. sq. ft. BARRIER

Note large reduction in all frequencies due to barrier


EFFECT OF THICKNESS ON ABSORPTION

Performance is improved everywhere but not quite by 2X


EFFECT OF MASS ON BARRIER, doubling weight of barrier.

Twice as much barrier does not improve sound much beyond 800 Hz


What 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 regime

The more massive the barrier, the lower frequencies blocked

Above a certain frequency, extra mass does not improve acoustics in higher frequencies


What about vibration? If you were in a spaceship in orbit,

the engine firing would be a roar but you wouldn’t hear it outside the ship since sound doesn’t 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.


Controlling 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 won’t be as quiet as a cross country mount that is softer)


Vibration 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.


Can 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? It’s 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.


What 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.


Design Summary

Eliminate 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!


Design 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.


Design Summary

Clean up aerodynamics as much as possible like add gap seals to flaps, wing-fuselage gap, etc.


Design Summary

Balance 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 it’s normal tolerance.


Design Summary

Make 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).


Design 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.


Design Summary

Install barrier blankets behind firewall (with isolation “decoupler” layer, barrier and absorber) plus felt/barrier/felt blankets on floor and over nose wheel well, wing spar, etc. Total weight app. 20 lbs for a 4 place aircraft, (30 sq. ft. @ .6 lb/sq. ft).


Design Summary Install trim panel for interior

(fiberglass insulation batting is OK behind this) but be CERTAIN to ISOLATE the panel with silicone isolation mounts. Add ¼” foam and upholstery over trim panel to add absorption on the outside while being a barrier on the inside. Trim panels can be VERY EFFECTIVE but are a fair amount of work. About 8- 15 lbs. for a 4 place but very dependant on area and material.


Design SummaryInstall DAMPERS against 50% - 75% of the fuselage. If you have a standard stringer/longeron construction and say you have a panel of 20” x 10”, (200 sq. inches), attach a piece of damper of approximately 100 – 150 sq. inches in the center and ideally, of an elliptical shape. This is to cover the area of greatest vibration, the center of the panel, that is acting like a drum skin. Figure 5- 10 lbs.


Design Summary

Add a plush ¼” – ½” thick felt cover over glare shield to absorb noise radiating from canopy. Looks great and absorbs noise. About .25 lb


Design Summary Install a thicker but not

necessarily heavier blanket, in FRONT of the firewall to reduce energy hitting the firewall, (If you have not installed this blanket in your aircraft yet, I would definitely do this and make it one of the highest priorities for “cheap” performance enhancement). No change in weight.


In-between solutionsMOST EFFECTIVE AND COSMETICALLY BEST LOOKING: As above but add trim panels to interior over damper, (isolation mount trim panels!) and over absorber blankets. Trim panels must be gasketed to fuselage to seal in noise and eliminate rattles. Add ¼” foam and upholster over panels. This is a triple whammy in that it is a barrier, gasketed against rattles and an absorber. Adds about 10 lbs.~ 6-12 dB reduction. A LOT would depend on fit, isolation, weight and how much noise is coming through sidewalls.


Suggestions & Recommendations

I strongly suggest that all materials be certified to FAR 25.853 flammability standards. While not required by experimental standards, it is a very important safety issue.


Suggestions & RecommendationsEliminate hard surfaces inside the aircraft. Hard surfaces will bounce sound off of it and radiate vibrations most effectively.


Additional information: Go to my website at

www.pegasusaeromarine.com and check out the educational pages. These contain specific recommendations for each area of the aircraft like wing spars, cabin floor, firewall, etc.

We have “Generic” kits for 2 place tandem, 2 place side x side and 4 place aircraft in 3 levels of treatment and weight

http://www.pegasusaeromarine.com/

Practical noise solutions for aircraft interiors

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