i am a huge proponent of door treatments.
let's talk and show what that means.
Seals
Door seals are vital, as mentioned above. You need to recognize that you are changing the factory system which has seals on the rear and front of the speakers. the plastic door panel IS THE FRONT SPEAKER BAFFLE in a stock door. The seal between the speaker and that door panel is what provides midbass. A lot of stock systems have good midbass response simply because the speakers are efficient and as well sealed as possible. When people drop in aftermarket speakers, they almost always neglect that stock seal. All it takes is a $4 roll of closed cell foam weatherstripping around the front of the woofer. That seal alone, even without deadening, will make a good difference and is the bare minimum for any speaker install. it is so important, i think all speakers should come with that in the box!
we also want to turn the door cavity into a sealed enclosure. we want to close up the large holes that provide access. i use wood or sheet metal with screws. note any wood used should be waterproof.
Deadening
This concept is basically to reduce how long the metal rings out. for example, rap your kuckles on the metal in several places. listen to how long the sound lasts after you hit it. each part of the car sounds different and it's worse in the middle of large panels. simply adding stiffening will help a lot. on a budget, just taking treated plywood and siliconing it to the outer door skin (inside the door) would make an audible difference. viscoelastic and elastomeric sound damping products do a great job with less weight. You don't see people putting wood inside their doors, but that doesn't mean it won't work. think about it this way - if you build a speaker enclosure, what do you use? would you use thin sheet metal with holes and a plastic front baffle? nope. so our effort is to turn this hollow metal cavity into a valid speaker enclosure that is rigid and non-resonant.
absorption
this is also neglected in doors but adding sound absorption is equally vital. we need to not only isolate the rear wave from the front but also absorb it. if we don't, the rear sound will bounce around inside the cavity then come back out through the speaker cone later. preventing the rear wave from coming back to the cone is one of the most vital aspects of speaker enclosure design for midrange and midbass. I achieve this with bags of fiberglass batt insulation. a cheap solution that has worked for 50 years. the bags just keep it from getting wet and are thin as possible (i use 1mil plastic).
examples:
http://www.caraudio.com/forums/car-audio-build-logs-cars-trucks-suvs/536049-2001-accord-ex-sedan-its-long.html
Silver Flute 6.5
replaced with German Maestro Status
But the Silver Flute gave more midbass when ran in a 3-way. so i'll likely put the Silver Flute back in teh doors as midbass and move the German Maestro to new kick pods.
