Hi Tigernuts,
I work in acoustics-- and any enclosed air space has what is called a Helmholz Resonance. The frequency of this resonance is a function of the enclosed volume. Blowing across a coke bottle neck is an example of a Helmholz resonance. It will always hoot at a particular frequency.
So does your helmet, based on its volume. So if you excite the helmet with energy from the exhaust , and particularly if the exhaust note is close to the resonant frequency of your helmet, the helmet retains this energy and vibrates with it. The T150V makes a hum where my 850 Norton makes a rumble. The Norton is further away from the resonant frequency of my helmet and does not excite the helmet as much.
I don't know about Indonesian music--sounds more like wind noise. To me, all Indonesian music sounds like wind noise anyway. The resonant contribution of your helmet would appear as a constant drone.
Cheers,
Kara


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