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Is that all there is?
isolation tape
vinyl barrier
1/4" thick constrained
layer damper
Well, yes and no. The previous sections list all of the treatments but HOW they are used has a tremendous effect. There are specific strategies that can enhance the benefits of these materials.
For isolation mounting, things such as engine mounts need to be very specifically designed in order to maximize benefits. For example, let’s say two identical Lycoming engines are mounted on two different aircraft. One is in say a Mooney and the other in a Pitts. The Pitts needs a much stiffer mount to control flexing in high G maneuvers and would transmit too much vibration to be good in the Mooney. Likewise, a mount designed for a three blade prop might not perform as well if a 4 blade were added since the 4 blade would add a frequency 33% above the three blade, or 125 Hz at 2500 RPM vs. 166.7 Hz for the 4 blade.
The other vibration treatment, damping, also can be optimized. Damping treatment bonded to the outside skin of an aircraft may be exposed to extreme cold and as you might suspect, these gummy plastics tend to harden and loose effectiveness as temperatures drop below their designed range, (this applies for high temperatures also where they may become almost a viscous liquid). Dampers also need to be optimized for pressurized vs. un-pressurized fuselages since a pressurized skin tends to enhance high frequency vibration due to its being pulled tightly.
Barriers, the first line of defense for blocking noise, are fairly basic and the rules are simple: More weight stops more noise, period. The "Mass Law" states that for every doubling of the weight, noise is reduced by approximately 6 dB. But the other characteristics are important also. It must be non-porous since noise can go where air can go and it should be limp to reduce wave transmission. So Linoleum can be a pretty good barrier as can a rubber floor mat, but of course they probably won’t pass a burn test.
Absorbers, the only treatment for noise once it has entered the aircraft, can likewise be fine tuned for the application. First off, more is ALWAYS better. The thicker it is, the greater the absorption and the greater the range of frequencies absorbed. This is due to maximum absorption being achieved for a thickness 25% of the wavelength of the frequency being absorbed. Low frequencies may need 5’ of absorber but higher frequencies are easily absorbed with 2"-4" of felt, fiberglass batting or foam. Also, closed cell foams are generally better at lower frequency absorption than are open cell foams.
And finally, while a layer of upholstery foam may be only 1/8" thick and thus apparently not very effective at absorbing many frequencies, much of the noise comes from acute angles thus increasing the distance they have to penetrate and much of the noise must go through and reflect back thus doubling the effective thickness. For even more effective noise control, a layer of closed cell foam bonded to an interior panel and covered with the same 1/8" open cell foam will increase the effectiveness a lot by both absorbing noise and damping the vibration of the trim panel itself.
We hope this has been informative and will help you understand the reasons we stress a systematic approach to noise control in your aircraft.
