One of the bugbears of mounting speakers—and especially subwoofers—in a wall is that the drive-unit behavior depends on the less-than-rigid behavior of the sheetrock. Most manufacturers of architectural speakers get around this by using a back box to provide the correct acoustic loading. THX's Laurie Fincham (who is going to hate me for referring him as one of the grand old men of English audio) had a different idea.

What if, he thought, you used a large number of small drive-unit pairs, each in its own sub-enclosure and connected facing each other with their chassis joined so that the vibrations could mutually cancel? (Memories of Laurie's force-canceling woofer arrangement in the KEF R104/2 speaker of the early 1980s surface in my mind.) That way, the mechanical properties of the wall would become irrelevant. Reproduction of a high-level 20Hz tone to meet the THX requirement requires pumping a liter of air 20 times each second, but that could be achieved by using sufficient small drivers.

Laurie discussed the idea with speaker manufacturer BG, and the result is the BG BX-4850 THX Certified Ultra2 subwoofer, conprising four modules, each containing 12 metal-cone woofers and arranged in balanced pairs. (Double bass player Laurie calls this "Balanced Bass Line.")

Laurie is shown here holding (with help) one of the prototype modules. The 12 drivers fire into a central cavity, sealed to the top, bottom, and rear, with a narrow 26" x 4" slot opening to the room. The module is mounted between the studs and faced with sheet rock, but transmits no vibrations to the wall itself. The 48 woofers in the four modules, each with a peak excursion of 8mm at <2% THD, have a total radiating area equal to a two 18" drivers, yet fit completely inside a standard 2x4" stud-wall.

Each pair of modules is driven by a dedicated 1200W amplifier/digital crossover module, and a slave 1200W amplifier drives another two modules. Four modules are required for the THX Ultra2-certified subwoofer, which is expected to retail for around $4995 including crossover and amplifiers when it is available for sale in the second quarter of 2007. But two modules ($2995) will still qualify for THX Select certification, and while one module can be used, for $1995, this will not be THX-certified.

A dem of the 4-module subwoofer, with BG's line-source planar in-walls, was very impressive, with obviously low distortion and superb low-frequency extension. Perhaps it's time for this reporter to stop turning up his nose at in-wall speakers.