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Why the emphasis on vibration?
The largest single contributor to noise in an aircraft is vibration and the majority of that is from the engine (although exhaust and aerodynamic noise are contributing factors).
So most of the noise we hear started out as vibration that was carried along the rigid structure of the aircraft before being converted into audible sound.
Literally, your aircraft is acting like a stereo speaker but instead of an electromagnet moving the diaphragm, the shaking of the engine, prop and aerodynamics are moving the structure.
This shaking in turn compresses the air in waves that we hear as noise. By reducing or eliminating vibration, you literally are doing the equivalent of turning down the volume of a speaker.
This is a fundamental concept and cannot be emphasized enough! To wit: If all structural vibration were eliminated in a sealed aircraft, it would be COMPLETELY SILENT.
