sound deadening in car is different than sound deadening in home or cinemas. in latter case, the walls are quiet rigid and dont vibrate. in case of cars, the metal sheet vibrates/resonates with the beats and causes annoyance. so for cars, sound deadening material achieves multiple purposes.
- blocks the road noise from entering the car
- add weight to thin sheet metal and there by prevent resonance/vibration
- create a sort of an enclosure by closing the holes in the door and hence the midbasses sound better
- in some cases where the deadening material has aluminum backing, insulate the car from outer heat.
so taking above into account, the optimum sound deadening material should have high density but should be soft, should not be too thick and should not expand on applying slight force. it should also be thin enough so as not to cause a problem when putting the door back together again.
with foam and underlay, what is achieved is the cancellation of some of the road/outer noise. underlay/jumbolon does not prevent vibrations as it does not have density/weight. also it does not close the holes in the doors properly as it expands slightly with the beats and therefore the midbass does not give the best performance.
on a budget however, having jumbolon/underlay in door is good as it does give u improvement in sound quality. foam is not recommended as no matter how much u try to prevent, water will seep into the door and the foam will become wet. short of sealing your windows so that they dont open, there is nothing you can do to prevent water from going into the doors. also underlay/foam/jumbolon should not be applied under the hood as they melt in high temps. the dynamat hood liner is a special material and it is not effected by heat that much.
those wanting to use rubber as a sound deadening material should remember that rubber used should be soft. hard rubber will bounce the sound back and will decrease the sound quality. therefore rubber mats used in gyms are better than the tire tube rubber.
