Found a snippet of info on clamshells...however this might pertain more to mounting two speakers together to act as a single speaker aka isobaric loading
About isobaric enclosures...
Again this is not a new concept, having been originally introduced by Harry Olson in the early 1950's. Technically, "isobaric" is not really an enclosure type; it is a loading method. This loading method involves the coupling of two woofers to work together as one unit. This is typically accomplished either by placing two woofers face to face or by coupling two woofers with a small chamber. The result of coupling the two speakers is that the coupled pair (iso-group) can now produce the same frequency response in half the box volume as a single speaker of the same type would require. For example, if a speaker is optimized for performance in a 1 cu.ft. sealed enclosure, one iso-group of the same speakers can achieve the same low frequency extension and overall response characteristics in a 0.5 cu.ft. sealed enclosure.
There is, of course, a penalty involved. Whenever you use isobaric loading, you are sacrificing 3dB of efficiency compared to a single driver in twice the air space. In practical terms, this is not usually a big deal since the powerhandling is doubled (two speakers instead of one) so the end result is about the same output as the single driver in the bigger box assuming you double the amplifier power.
Isobaric loading can be used within any enclosure type, including bandpass designs. The ported and bandpass isobaric designs can be difficult to design and build due to very small enclosures with large port requirements. Isobaric bandpass designs, in particular, can be literally impossible to build with certain speakers. There are some things to look out for with each type of isobaric design, such as mechanical noise and uneven heat dissipation which can present potential sound quality and reliability problems. All the methods which involve opposite cone motion require that the speakers be wired in reverse polarity relative to each other. These designs also provide a performance advantage in that their opposed cone motion averages out suspension non-linearities (differences in inward and outward suspension control,) which reduces distortion.
If you are strapped for space and can afford the extra speakers and more complex enclosure, the ability to have a compact subwoofer system with no real sacrifice in performance is well worth the extra effort and expense. On the other hand, if you have a lot of space and are looking to get the maximum amount of output without sacrificing sound quality, using multiple iso-groups can give you the best cone area/box volume ratio while still retaining good fidelity. Isobaric loading is not as popular today as it was in the early days of car audio subwoofer design. Why? Today's car audio woofers are designed for much smaller enclosures than their forebears, making the need for isobaric loading rare.
