The face-to-face or "clamshell" configuration as it is commonly called is the most compact and, therefore, the most practical isobaric loading method to use, considering the tight confines of the average automobile. This configuration also provides the beneficial side effect of cancelling driver non-linerities.
If there is one recurring theme in engineering, it's that Mother Nature is lazy. She has made it a law that anything at rest wants to stay at rest, and, similarly, anything in motion would much rather stay in motion in a straight line. Such is life in general, and a speaker's dynamics are no exception. It's called the law of inertia, and there is no escape.
When a subwoofer does its job, it is called upon to compress and rarefy the air in the listening environment many times per second, and more often than not it is required to do so over great distances. This places a great strain on the cone itself as it fights to retain its shape in the face of intense acceleration and decceleration. Ideally, a speaker's cone would be inifinitely rigid and wouldn't deform under any circumstances, but this is not a perfect world a perfect world, and we have to deal with the consequences of fighting Mother Nature.
As the cone pushes outward, it is somewhat flattened out as it attempts to kick-start the air in front of it into motion. Likewise, the cone is deformed the other way when the cone returns and attempts to compress the air in the subwoofer enclosure. The extent of this deformation is a function of the cone's geometry, construction and the amount of power with which the subwoofer is driven. A good engineer will design his cones so that this effect is minimized, but there is only so much engineers can do if he wants to keep moving mass and/or costs low.
It is important to note that when mounting the drivers to each other and then to the enclosure, a separator of some sort must be used to space the drivers apart. If the drivers are not physically separated, their surrounds may rub against each other, which will lead to premature failure of the driver. We recommend the use of a 5/8" thick ring of Medium Density Fiberboard (MDF) with appropriately spaced holes to pass the mounting bolts or screws through.
Lay the bottom driver in the box after wiring it up (this driver should have its positive leads wired to the positive terminal(s) of the amplifier and its negative terminals wired to the amp's negative terminal(s). Lay the MDF Iso-Ring atop this driver, invert the second driver over the first, line up the mounting holes, and screw the whole assembly to the enclosure.
Assuming your driver's gaskets are clean and unscathed, and the MDF ring is equally smooth on its contact surfaces, no other sealing agents need be used to assure a good air-tight seal at the driver/ring interfaces. If you decide to use silicone or some other sealant, be prepared to go through one hell of a fight if for some reason you need to disassemble the isogroup!
Some prefer to mount one driver inside the box and its partner atop the box, using the enclosure wall itself as the spacing mechanism. However, we have found that this makes driver servicing unnecessarily difficult--rather than just undo the eight bolts/screws using our suggested mounting method, one would have to have someone come in from a removable panel on the other side of the enclosure and hold up the other driver. In short, mounting everything from the outside makes much more sense and is infinitely easier to service.
When all is securely mounted, wire the outer subwoofers (the ones with their magnets exposed) so that the (+) on the speaker is wired to the (-) on the amplifier and visa versa. This will assure that both drivers are moving in the same direction when a voltage is applied. If you hook everything up and get no bass from your new isoload, chances are that either a lead fell off inside the enclosure, or you've got a driver's polarity reversed....double check everything before powering up.
Learn about other Isobaric Enclosure Types:
|"Piggy-back" tunnel load|
|Back-to-back tunnel load|