A better application is to simply try minimizing the area or partially sealing off "leaky" areas.

The concept of sound-proofing is easy. Think of your car as a bubble that is insulating you from outside noise. The more holes there are, the more noise there is. On why the likes of German car manufacturers like Benz and BMW can make their car so quiet (compared to, let's say, a Kia Pride) is their attention to detail in trying to seal the car interior as much as possible from the outside. The likes of Lexus use double-paned glass for example. For Mercedes, this sometimes means using rubber grommets as much as possible to keep noise out as well as thickening some parts of the chassis.

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A better application is to simply try minimizing the area or partially sealing off "leaky" areas.

The concept of sound-proofing is easy. Think of your car as a bubble that is insulating you from outside noise. The more holes there are, the more noise there is. On why the likes of German car manufacturers like Benz and BMW can make their car so quiet (compared to, let's say, a Kia Pride) is their attention to detail in trying to seal the car interior as much as possible from the outside. The likes of Lexus use double-paned glass for example. For Mercedes, this sometimes means using rubber grommets as much as possible to keep noise out as well as thickening some parts of the chassis.