I vented the enclosures cause I read ALOT of reviews from people that blew out their woofers from to much back pressure. When the baffle is closed off the pressure has no place to go and can tear apart a speaker.
Sent from the road while ignoring traffic
The drivers don't move a significant enough amount to create that kind of back pressure. You could seal a midrange driver along the frame and not have that as a result.
Remember, you aren't pulling any air in, so the volume of air behind the driver stays constant and that is what creates the suspension for the driver. This limits overexcursion of the driver and helps to actually save the driver from tearing itself apart. To increase back pressure would be on the "incursion" of the cone, in this scenario, the back pressure also helps to reduce the movement.
One thing that most often damages a driver is the magnet collecting metal flakes and pulling them into the vc. More often than not, that is what destroys the driver or overheating from overpowering a driver for too long of a period. This scenario happens when people turn the bass way up and expect a 6.5" driver to play like a pair of 15s.
I have never seen a driver destroy itself from too much backpressure.
Yes and no. The smaller chunks give more coverage while doing the same thing. Adding one big piece only makes the mat "one" with the door negating any benefit from the dynamat. Heavier (aka more mass) mat will change the resonance frequency of the sheet metal to a lower frequency.
Yes, it lowers the resonant freq. of the metal. Which, is a good thing. This means the thin sheet metal is much less likely to vibrate and at the very least vibrate audibly.
I'm not sure what other benefit you think there is to dynamat? In every scenario, you will find a bigger improvement with the more mat you add. Your speakers vibrate. The more mass your panels have, the less vibration you will have. The quieter your car will be. The less distortion you will get from your drivers, the clearer the sound. Using pieces reduces the amount of material available to dissipate the energy from the vibration.
The rear waves from the speaker are more on the low end. So it would actually fill out the sound if you could vent the speaker to the cab. Subwoofer enclosures do this for that very reason. In this case though it would be counter productive to add a hole to the door.
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Speakers are like the sun. They put out the same sound in a 360 degree environment. Think of the cone of a speaker as an infinite number of little dots, with each dot creating sound and sending it in each direction like light. The problem is, the rear of the speaker is out of phase with the front of the speaker. This creates cancellation when two waves out of phase with each other, run into each other. In this scenario, their is no sound. That is why a speaker not into an enclosure is so much quieter and "muddier" sounding than one in an enclosure.
Also, sound likes to run along a surface, almost like water. Even with a speaker in an enclosure, you can hear sound from behind it. The sound waves actually wrap around a baffle and head backwards. If you do not separate the front of the driver from the rear, than cancellation happens everywhere and your speaker is a lot quieter than it could be. You can test this by standing behind a home speaker. This also explains why a lot of home speakers have one tweeter, one midrange and two woofers. The second woofer is for baffle compensation, to compensate for this cancellation.
If you want to know why a sub enclosure is ported, then google it. Should be easy info to find, and it has nothing at all to do with low end coming out of the back. It has to do with wavelengths, and after working another 12 hour day, I'm too tired to type it all out knowing it is easily accessible.
If you have any questions, ask away.