i'm not picking on you here, this is a good discussion for the forum.
Exactly what issues are you going to create on a car door using vibration damper? You may create theoretical issues adding mass at points with a resonant material but treat enough doors and you'll see that it isn't a practical concern at all. How are you going to squelch resonance by applying vibration up to the fixed edges of a closed termination system? The real big picture is not doing things that waste money, make future bodywork more complicated than it needs to be and doesn't improve things.
there is nothing theoretical about physics. finite element analysis is what the industry currently uses to predict resonance frequencies using modal analysis if i had a spare 40 hours, i'd do FEA on a car door. when you sparsely place deadener you create nodes which means you have several resonance frequencies, and all of them are higher in frequency. as you knock across the door skin you hear a change in frequency - all of which will be excited by the speaker. i am in favor of fewer resonance frequencies and having them be lower in frequency.
rapping your knuckles is the cheap way of replicating the ATSM test that automobile manufacturers use when they actually try to make a car quiet. They have a calibrated "hammer" which taps on the metal and records the resulting noise, decay, and resonance frequencies. i have a few acoustics publications on the subject i can look for - some were ASA presentations/research.
the real picture is to make a car sound good and not like a tin can. it's not a cheap process and it requires some sacrifice. besides, even good deadener can be removed with a heat gun - making the theoretical body work a sinch. //content.invisioncic.com/y282845/emoticons/smile.gif.1ebc41e1811405b213edfc4622c41e27.gif
"too much" is relative. the strutural deadening in a $100,000 Wilson Audio Alexandria loudspeaker may be considered "excessive" by some, and "neccessary" by others.
the audible benefit of multiple layers of deadner verus sparce placement is very obvious - but some just can't justify spending $800 in deadener. certainly you reach a point of diminishing returns, but you will benefit from additional damping and mass. no debate there.
Most vehicles don't have 3" clearance between the outer skin and glass. If yours do, 3" is going to attenuate 1 kHz and above. Is that really where the problem is? If you keep the front and rear waves from interacting, the only problem the rear wave can cause is after it reflects off a concave surface, interacts with the side impact intrusion prevention beams and strikes the back of the cone. The odds of any of that resulting in problems that are audible or measurable in a vehicle are remote. It makes a lot more sense to focus on those things that will make a significant difference.
available depth depends on the vehicle. i have at least 2" on the door of my accord - the structural steel tubing (for side impact) are about an inch in diameter. some vehicles have even thicker doors, some thinner.
http://www.cmainc.net/ProductDetails.aspx?FileDownload=true&AttachmentNum=1&ProductID=1162
Refer to absorption coefficients of the 2"x2# 4.0 mil PVC with A mounting. That's 2" thick of 2 lb/cu.ft. fiberglass encased in 4.0mil thick PVC with type-A mounting (directly to wall). Good absorption down to 250Hz octave band. don't forget the PVC acts as a membrane absorber as well - further enhancing midrange and midbass absorption.
If you keep the front and rear waves from interacting, the only problem the rear wave can cause is after it reflects off a concave surface, interacts with the side impact intrusion prevention beams and strikes the back of the cone. The odds of any of that resulting in problems that are audible or measurable in a vehicle are remote.
this is not a remote problem. it is a problem that plagues most cars as well as many home speakers. back wave reflections cause phase interference that takes the form of coloration - it adds to the sound the speaker is trying to reproduce. the problem is clearly audible, and when properly addressed, the difference is striking. the reflecting plane of a door is concave, sure, but the focal point is so far beyond the door it will appear to be a flat surface to most of the frequencies reproduced. the side impact bars will only diffuse a small portion of the rear wave as well as a narrow band of frequencies.
I have always wondered how the window being down affects the speaker with no deadner or any access holes sealed closed on the door itself?
once you lower the window you create a gap in the weatherstripping at the top. you also create a hard reflecting plane behind the driver. the gap at top will reduce measured/heard midbass response and the reflecting plane will increase distortion (in the form of phase interference through the cone).
------
Increasing the transmission loss of the speaker enclosure is a good step towards a better sounding system - regardless of the speakers chosen. I would rather spend less on speakers and more on vehicle treatments - because that methodology will result in a better sounding system.
Vehicle doors are about the worst type of speaker enclosure known to man. no one builds speaker enclosures out of thin sheet metal for very good reasons. combating the myriad of automotive issues requires a lot of effort and some financial investment. You don't have to buy name brand deadener. There are dozens of viscoelastic damping compounds on the market, and most are not automotive but for commercial equipment enclosure treatments. while the acoustic benefits of damping is not well tested in the automotive market, the commercial construction market is full of third party transmission loss tests for acoustical products. I have hundreds TL tests that I refer to when making recommendations. looking at the performance of those systems can give you a better idea of what to expect when applying similar materials in an automotive market. having dozens of acoustics texts as reference material helps as well.
we are here to enjoy music. while we will mostly disagree as to what we consider good music, we should be able to agree on the benefits of a good installation and properly treating the vehicle.
in the end, the cheapest and best performing solution is probably sealed enclosures of adequate airspace made from MDF and fiberglass. just eliminate the doors as an enclosure. then seal the openings and introduce a layer of decoupled MLV for transmission loss to control road noise. you still want to seal the doors since the best noise isolation is achieved with dead air space and decoupled surfaces.
