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So what about absorbers?
a "glass / foam sandwich"
(1/2" open cell foam, between
two layers of 1/2" fiberglass)
1/4" felt
closed cell foam (tan) &
open cell foam (white)
Absorbers are not as effective as barriers in stopping noise from getting inside a plane but they have one tremendous benefit in that they do not need to weigh much to do their job. They are also one of the most versatile noise treatments since they do many other jobs as well. They tend to be inexpensive (OK, in airplane dollars), they generally are great thermal insulators (thus doing double duty), and finally, when placed under upholstery they add a plush look while reducing reflected noise, (echoes).
Absorbers are typically added in two areas, between the fuselage frames, and on upholstered trim panels.
Metal aircraft are the only type of aircraft that permit adding absorbers between the fuselage frames.
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The space between the frames is clearly the best place for thermal insulation so if you only needed to keep your plane warm, this could be your one treatment and you would get some noise attenuation as a side benefit. | |
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But if you wanted to acoustically treat your aircraft, this could be one of the primary treatments even if it is generally less effective than dampers or barriers. | |
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Absorbers placed in the fuselage structure treat noise twice: once as the noise goes from the skin into the aircraft, and again as it bounces around inside the cabin. |
Composite aircraft present a special challenge thermally and acoustically, at least for sidewalls, (floors, bulkheads and firewalls are treated the same as aluminum).
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If you have a honeycomb cored aircraft, the amount of thermal insulation is minimal let alone the acoustic damping since you have two layers of very thin fiberglass sheet over air filled cells. | |
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Another challenge is that ordinarily, there is not any framing to stuff insulation in so, for composite aircraft, you need to place the insulation all on the surface and then upholster over it. |
That is why we at Pegasus Aeromarine Inc. have two types of insulation, that which can be upholstered over (foam or felt based) and that which cannot, (fiberglass batting). If you have a composite aircraft, you will have to make a decision as to which type of insulation to add, bond in place or removable by snaps, hook and loop, etc.
For the fuselage, acoustically, direct bonding of a gummy foam with good vibration damping to the fuselage is the best solution but that must be weighed with the down side of having to damage the foam if it ever needs to be removed.
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One solution for foam absorbers that are less likely to be damaged if they even need to be removed, is to use a dense felt bonded to the fuselage then direct glue foam layers on top of that. If removal became necessary, it would tear away some fibers but should leave most of the blanket intact. | ||
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Direct bonding will help damp vibration while a removable system will likely add only absorption or perhaps also a layer of barrier. | ||
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Another important need is for the blankets to conform to a typically complex curved surface. | ||
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This may be best done in two or more thin layers, each bonded to the other layer and perhaps trimmed like the cover of a baseball to allow these complex shapes to be molded. | |
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If each layer is applied and bonded to the other layer, it will allow the resulting blanket to fit perfectly and have a fitted shape. | |
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Do NOT cut the seams in the same place! Instead, overlap all seams so that each is backed by another layer to add continuity. | |
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This foam or felt and foam blanket may then be upholstered for a beautiful fit but it must be remembered that this will still have less insulation than an aluminum aircraft. | |
For trim panels, foam or felt and foam blankets can make a HUGE difference in noise since it can treat sound on so many levels and it may be the most cosmetically appealing item on your entire aircraft. If sealed from the rest of the interior where noise cannot leak out around the edges (flanking noise), it will act as a barrier. If covered with foam and upholstery, not only will it look plush, it will absorb reflected noise inside the cabin.
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Trim Panel foam and upholstery when mounted against a backing will also damp residual vibration thus further reducing noise and stopping the trim panel from acting like a stereo speaker. | |
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Note that for acoustics, a trim panel is most effective when the padding and upholstery are bonded to a backing panel of aluminum, plastic or other semi-rigid sheet. This may also add more thermal insulation and increase the interior comfort level. But as with barriers, (actually in this case it IS a barrier), it MUST be isolated from the vibrating frames of the aircraft if it is to be best utilized. | |
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Isolating a trim panel from the vibrating frames of an aircraft may be as simple as mounting the trim to the frames with hook and loop (Velcro), a material that does not transmit vibration well since it is after all a type of felt. Or some types of gummy foam gasket materials may also be placed between the panel and the frames to hold them away from the surface. |
As mentioned above, there are two main types of absorber material, foam and fiberglass batting. Both have advantages and disadvantages.
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Foam is semi-rigid and for composite aircraft without framing, this is a benefit in installation. It is easy then to direct glue foam to a plastic fuselage and glue the upholstery fabric over that. The semi-rigid nature of the foam though is prone to allowing it to vibrate thus reducing its effectiveness in isolating vibration, even while absorbing noise. | ||
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Fiberglass batting would be too weak and fall apart if direct glued so it should NEVER be directly upholstered over. | ||
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Since fiberglass has poor integrity, it needs to be sealed into a bag and if it gets wet, it tends to get thinner and it may become "clumpy" over time or may even fall apart. | |
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But even with these limitations, it has a number of benefits. Made into a blanket by bagging, it works well when fitted between the frames of an aluminum aircraft. | |
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Also, batting is very limp and can be compressed easily to 10%-20% its original thickness thus making it ideal for getting into tight places or under wiring and hoses. And being so limp, it does not transmit vibration easily thus it is a good isolator. | |
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Further, it tends to be slightly better at absorbing noise than most foams, does not release as much smoke in a fire and is less expensive. For these reasons it has long been a favorite of large aircraft manufacturers like Boeing and Airbus. | |
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For myself, I like making a sandwich out of one or two layers of fiberglass outside of a single layer of foam. This cannot be upholstered over but it holds up better than plain batting and is rigid enough to stay in place without needing to be affixed to the fuselage. This allows a lot of thickness variation while maintaining its shape and does not compress very much in its manufacture. | |
