[Munro]

After a bit more researching I have put some “Skins acoustic sound wave diffusers” in my doors. It sounds highly technical for what is basically foam cut like an egg crate with a sticky back. It Cost £20 for 2 x 250 x 250mm sheets which seems quite expensive for what you get, but worth it as there is a definite improvement in speaker sound quality.

There are other similar solutions, but I chose this product because it is a purpose designed acoustic diffuser, waterproof for car door use and has a sticky back for ease of application. Further details on this link:

http://www.caraudiodirect.co.uk/skinz-acoustic-wave-diffuser-p-5482.html

“Acoustic sound wave diffuser

Reduces back waves, ensuring a tighter punchy mid/bass speaker performance.

Skinz wave diffuser is an ultra dense foam damping material used extensively to control acoustics in the recording studio industry. Now specially treated to reject moisture its benefits can now be achieved in car door applications.

Skinz Wave Diffusers sinusoidal profile helps elimate rear sound waves from reflecting back through the speaker cone causing sound cancellation. By attenuating unwanted sound waves from behind the speaker, greater sound and energy is gained. Thank to its excellent elasticity, flexibility and damping properties, Skinz Wave Diffuser will help deliver a far more open dynamic midrange with a much tighter mid/bass performance from any speaker system.”

It’s dead easy to install – simply stick the 250 by 250 panels on the outer door skin behind the speakers. I had to trim the corners off the panel for a good fit. I also cut the panel into halves to help pass it thru the speaker hole and manoeuvre it into the correct position.
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Some other useful links:

Another similar product “deflex”:
http://www.deflex.co.uk/cgi-bin/deflexshop/deflexshop.cgi

Sound proofing – Comparison of products on the market:
http://www.sounddeadenershowdown.com/

And of course Rasmus thread on soundproofing if you get really serious about soundproofing:
http://www.civinfo.com/forum/how/5241-sound-isolation.html

Some pictures: