What does Isobarik Mean?

Isobarik: Also known as compound loading. By using two low frequency drivers (generally mounted face-to-face and wired electrically out-of-phase or mounted front-to-back in a shallow tube and wired electrically in phase) you can halve the volume of the cabinet without reducing the low frequency extension of the subwoofer.

Source:
http://www.hometheatermag.com/glossary/

The term Isobaric is Greek in origin. "Iso" meaning constant and "Barik" meaning pressure defines the operational characteristics of this use of multiple woofers. The air space between the to woofers remains a constant pressure and the result is a single drive unit with dual motor assemblies.

Source:
http://www.maximacar.com/isobaric.htm