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We are using C-130 B and E Models with T-56 A-15 engines and Hamilton Sundstrand 54H60-91/117 propellers. A lot of vibration and resonance are felt on these military version aircraft even after using solid state synchrophasing and conducting prop balancing.

I wanted to know if there is some system to make the aircraft quieter. Except for shutting down the engines of course :)

Are there any resonance controlling items to make the passengers more at ease?

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    $\begingroup$ Have you tried loudspeakers and AC/DC? $\endgroup$ – user12007 May 14 '18 at 9:59
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    $\begingroup$ Or noise cancelling headsets for everyone? $\endgroup$ – CrossRoads May 14 '18 at 12:21
  • $\begingroup$ Line the cabin walls with Sound Proof material, such as aircraftspruce.com/menus/ap/soundproof.html or other similar material approved military use. Of course, the walls may already be lined with something. $\endgroup$ – CrossRoads May 14 '18 at 16:49
  • $\begingroup$ I really like the "shut down the engine" solution $\endgroup$ – Manu H May 14 '18 at 21:04
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    $\begingroup$ I worked the C-130E/H my whole stint in the Air Force. To this day, I have never slept as soundly and as comfortably on an airplane as I did on those birds. When the props are synced, the hum and vibration was so relaxing. I used to string a hammock up from one side to the other and just rock away like a baby. I honestly don’t think there is anything to make it quieter as the military never really cared much about how much noise we were exposed to. $\endgroup$ – Frank May 15 '18 at 0:15
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Active vibration control systems exist for some aircraft, though I doubt an ex-military type would have anything off the shelf. There is a system from Lord (the engine mount people) that is intended to counter the vibration from a helicopter main rotor to improve passenger comfort, among other things. This is their claimed reduction:

enter image description here

AVCS Product Brochure

They focus on helicopters but sell components as well, and may be able to customize something that could work for you.

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