Soundproofing is sometimes called sounddeadening.

It is done to decrease rattling and also outside noise to a certain extend. Because less vibration = less sound/noise produced.
Especially important if you are into ICE.

2 main types of SP/SD.

1) Mass loading - Adding mass to the panel. Which is commonly known as soundproofing. That is to add weight by means of adding soundproofing mats(Dynamat, Bittumen, Cable Hunter, RAAMat, etc.)
Or we have spray on deadeners. Also known as liquid deadeners.

2) Barrier Loading - Usually in the forms of foams. Like Armaflex or Egg crates foams. It is used/should be used as an addon to Massloading for best results.
Use to absorb/deflect sound waves and thus noise.

If you want to block external noise, the best place to do is not the doors. Best parts of the car to do it is the floor pan, kicks and firewall.

Hopes this clears up some cloud.

Cheers

how about undercarriage?

quote:

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Originally posted by Ivan Liew:
how about undercarriage?

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These are the areas that sound proofing are carried out

1) 4 doors
2) Bonnet
3) 4 wheel arches
4) Car floor (below the seats)
5) Car boot

For car floor, sound proofing can be done by adding sound insulation material on the floor pan and wheel arches.

Most people usually don't go for undercarriage. If you need sound proofing the wheel arches and floor pan but don't like the idea of dismantling the car, can try SoundClad. I asked before and if i remember correctly, its $300. Its a spray on type to the undercarriage, including the wheel arches so no dismantling is required. I was also told it adds about 5kgs to the car. I did not try it so not sure how effective it is. Do also note that there will be some oil stench for at least a week due to the coating.