Bubble wrap

[J.stafford16v]

I found roof flashing tape works really well to reduce road noise, its a silver backed roll of bitumen, costs about 10 or 15 quid a roll from b&q/homebase etc and that was enough to do all my doors and have loads left over. The change was very noticeable (I lined each of the plastic sheets inside the doors with it) and was gonna go on to do more of the car, but i sold it before I got the chance!

Cheers for that m8, might just give it a go.

[8vRicey]

I also read somewhere on the net, that the underlay you put under lamianate flooring works well.

http://www.diy.com/diy/jsp/bq/nav/nav.jsp?fh_eds=%c3%9f&fh_reffacet=specificationsProductType&fh_location=%2f%2fcatalog01%2fen_GB%2fcategories%3c%7b8530244%7d%2fcategories%3c%7b8750001%7d%2fspecificationsProductType%3e%7bflooring_underlay%7d&fh_refview=summary&fh_refpath=facet_30202263&ts=1164735527819

The link above is to B&Q, who do it for £9.98 for a 10m roll. Just got to spray glue it to where you want it. 

[Heyho]

If you want to do it properly, get some quality deadening, these guys don't sell rubbish:

http://www.caraudiodirect.co.uk/accessories-sound-deadening-c-73_72.html

If you want to do it a bit cheaper:

http://cgi.ebay.co.uk/10-Sq-Ft-eDead-Original-Sound-Deadening-Dynamat-EDO01_W0QQitemZ330053905602QQihZ014QQcategoryZ1498QQssPageNameZWDVWQQrdZ1QQcmdZViewItem

Or go to your nearest car bodyshop/paint suppliers and ask for some bodyline sound deadening

Properly deadening your front doors can/will have a marked effect upon bass production from your speakers along with improved sound quality, as well as reducing roadnoise.

[tommk3cab]

thoretically bubble wrap should work, me and a lot of people i know use a similar thing, eggboxes as a cheap room soundproofer, with great effect. my knowledge as to why is a bit rusty but its along the lines that the sound is dispersed and lessened as it travels through certain shapes. That pointed sponge stuff works quite well too, not much use in car though.