I would (and do) choose sound deadening material (dynamat, raammat,
b-quiet, etc.).  Much easier to work with and easy to fasten and create
that tight seal.

MDF, sound deadening, and it's been suggested to me...non hardein
modleing clay. :

Silicon beads around the mounting rings (if your set comes with them
helps too. :

--
CBFryman

You're reallytalking about two completely different things--baffles
and rattle killers. Just because something may be good at squelching
rattles doesn't mean you should go trying to mount a speaker to it.
Trying to fasten a 6.5" driver to a sheet of Dynamat, for example,
would be a sure-fire way to mess-up a perfectly good speaker when it
falls out of your door onto the asphalt.

For rattles, the idea is to add so much mass to the rattling surfaces
that it pushes the resonant frequency of the surface down below the
audible range and at the same time reduce its "Q" or propensity to
resonate. Many have used sheet damping like Dynamat and its ilk with
great success (and cost). Other alternatives are automotive
undercoating (essentially spray-on tar) which is *really* stinky,
polyfill (for simple interior panel rattles in areas where the fill
will not get wet), etc. Some even go so far as to try what I call
"Expanding Foam Nightmare" (EFN) (http://www.amazon.com/o/asin/
B0002YWRF0), which works *amazingly* well...when used with extreme
caution. EFN is incredibly messy, and if you over-apply it, you can do
serious damage to your car (I heard of an install shop in my hometown
that got a little carried away with it and wound up making the
exterior body panels on a brand-new Mustang bulge and ripple). I'd
stay away from it, unless you really know what you're doing or if you
just don't care about the potential for destroying your vehicle. :-)

For baffles...

In the most general terms, what you want is something very rigid, with
a very low resonant frequency and a low "Q" (propensity for
resonance), and of course you want to completely isolate the backwave
of the speaker from the front wave for all frequencies you expect that
speaker to play.

Plywood is generally not regarded as a terribly good substance from
which to create enclosures or baffles because it tends to be somewhat
resonant.

Fiberglass can be used to do some really wild things in terms of
cosmetics, but *technically* by itself it is not an ideal substance to
use for speaker enclosures/baffles, particularly in the midrange.
Whether or not the average person will notice, I'm not sure, but what
I've seen done by some *very* top-notch installers is to lay strips of
Dynamat (or similar) between the layers of Dynamat as an attempt to
"deaden" the panels. It seems to work--I've never heard a car from the
people I'm thinking of that didn't sound absolutely stellar.

For the average do-it-yourselfer, nothing really beats MDF--it's
relatively inexpesive, easy to work with, and has a very low "Q". The
only real drawback to using it (other than its sawdust, which ain't
good for your lungs) is its weight.

-dan

I guess maybe I misunderstood your question.  I took it as covering u
the holes in your door skin, in which case I'd say sound deadener.  Bu
if you're actually mounting the speaker to it, such as if you would nee
some extra space for the motor so it doesn't hit the window or changin
size of the speaker, wood (namely MDF) is the best.  But you may nee
to seal the edges of that to both/either the speaker or the door ski
itself, in which case I'd use something like rope caulk

--
KU40

I just found out that cotton is a good alternative to fiberglass for
sound absorption... it's probably a lot cheaper and it is easier to
find.