What you are talking about when sound proofing is in fact sound wave absorbtion. I would go into the physics of it all but (really can't be assed) basically sound wave absorbtion aims to reduce the reflected sound from coming back to the source. As you know speakers make sound and you hear that sound projected outwards from the cone. Well there is just as much sound pressure being produced and directed from behind the cone and in a car audio environment that means aiming at the car's front doors.

Dynamat adds weight to the panels and in doing so raises the amount of sound energy required to physically vibrate the panel, or send it into a harmonic oscillation, hence your music will sound better as there is less background noise from vibrating car doors/boots and your panel is made stiffer as well which means less energy is lost trying to bend the panel. That's why your bass will sound better and louder. It also partially stops the reflected wave from interferring with the speaker cone milliseconds after it was first produced.

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No more GC8, sad face.
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