Here is what you are dealing with if the sound is coming from outside the cabin (which is where you ears are located) - best thing is to stop it before it can hit a panel on the inside and make it resonate...
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Here is what you are dealing with if the sound is coming from outside the cabin (which is where you ears are located) - best thing is to stop it before it can hit a panel on the inside and make it resonate...
You need to pay attention to these frequencies (which are just wavelengths in drag) and amplitudes above let's say 60 dB or so.
You want good noises to reach your ears:
Induction sound, exhaust sound, some chain noise (so you'll know it has a chain driving the cams, not some wek-kneed rubber belt) - all good
and ALL from the rear
I'm not sure if trans. noise is good, and noise from the CV joints is definitely bad.
Other noises from other directions like the front or sides are all going to be bad noises. Only exception would be noise from speakers in a stereo system.
So, consider that when you think about sound treatments.
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The low tech way is find yourself a panel that seems suspicious. Rap it with your knuckles and listen. if it sounds dead, it is likely ok, if it seems to 'ring' then it probably IS ringing - i.e. resonating.
An example is a drum - when you hit the surface, it rings. If you glue some gum onto the drummer's drum it won't ring as much. Try this sometime as an experiment.
NOTE - the figure in post #1 does not show sound from the panel vibrating
The sound will be lower in freq. and the amplitude will be reduced.
This what sound dampening materials do, aka sound deadener. So stick a piece over about 25% of the panel. That should stop the ringing. Do all the panels in the cabin.