The Tymphany Linear Array Transducer (pictured on this page) is one type of design that can be incorporated into flat panel televisions and flat panel sound bars.
Instead of the traditional subwoofer driver cone, the Tymphany creates sound by employing a linear array of smaller diaphrams encased in an open tube-like structure. There are two "motors" - one on each end of the tube that drives half of the diaphrams. Each motor drives their half of the diaphrams in opposition to the other. The actual air flow is passed out through the side openings on the tube.
The result of this structure actually produces a credible amount of bass for such applications as flat panel televisions, sound bars, digital sound projectors, and other applications where a thin subwoofer profile is needed.
For more specifics, check out the Tymphany Website.