A leaky division between the trunk and the interior heavily affects the speaker's output efficiency. If the back wave leaks into the interior space, cancellation can occur. Building a baffle or structure to fully isolate these waves can be a daunting task that must take into consideration items such as existing speaker holes (in the rear deck) and those areas designed to flow the factory wire system. In many instances sound-deadening material can address these areas and reduce the resonance of any vibrating parts.
Because most IB woofers are designed with stout suspension systems, low-frequency extension is somewhat compromised. This also carries over to power handling. With no solid reinforcement for damping, a subwoofer must be designed specifically for IB applications or its mechanical power handling may be dramatically reduced - by as much as 50 percent of its nominal power rating. Even with IB specific subwoofers, one should still exert a certain amount of caution. Because IBs are essentially "free-air" systems, subwoofers tend to lose some control of their cone movement when played below the driver's Fs (resonance frequency), much like vented systems do when played below the tuning frequency. For this reason, it's desirable to use a subsonic filter below the Fs of the speaker.
There are two main parameters to look for when purchasing a speaker for use with an IB - the Qts and VAS. Woofers with a Qts (total resistance of the driver at its resonance) close to or greater than 0.6 tend to work well in an IB environment. The second parameter, the VAS (volume of air having the same compliance as the driver's suspension), should be smaller than the total volume of the environment the speaker(s) are being placed in.
SealedSealed enclosures have certainly stood the test of time. This design was first patented by Harry Olson in 1949 and then exploited by Acoustic Research in the early 1950s. Today, sealed enclosures are very popular and are used by general car audio enthusiasts and sound competitors alike.
While they go by other names - acoustic suspension and closed-box systems - sealed boxes are self explanatory in their basic design. Being completely sealed, an enclosure of this type isolates the front of the speaker from the rear. This is important since a woofer produces two waves simultaneously - one from either side of the cone. Consequently, the two waves are produced 180 degrees out of phase from each other, therefore rendering them opposites. Being as such, if the waves were combined, the equal energy they produce would cancel each other out. It would be like two men rowing in the same boat with equal and opposite strokes of the paddles - while there is driving force, the boat would cease to move. In the case of a woofer, the cone would move but sound would theoretically cease to exist.