Is there any way to reduce the noise and vibration in a C-130 Hercules? 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?
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:



AVCS Product Brochure

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