M&K S-150 THX surround loudspeaker system Page 3

S-150 Tripole surround speakers
The SS-150 surround speakers are a novel design that Kreisel calls "Tripoles." "Think of it as a dipole surround speaker that also includes a very high-quality direct radiating speaker," he explains. The sidewalls of the speaker angle-in slightly to the forward-facing front panel and contain a dipole array of 1" soft-dome tweeter and 3*" woofer/midrange. The front panel has an additional 5* driver that has reduced output compared with the dipole drivers.

Actually, the SS-150s are flexible to a fault: they can operate as true dipoles, with no information added by the direct-radiating driver, or as direct-radiating surrounds, with no information added by the dipole speakers, or you can adjust the output of either the dipoles or the direct radiator for a host of blended operations.

I thought I understood the THX concept of the "null" pretty well: If you locate dipolar speakers so that the listener's ears sit between the two sets of radiating drivers, the sound doesn't appear to emanate from the speaker boxes, but rather from throughout the room. I asked Kreisel how the addition of a direct radiating driver was supposed to improve on that.

"The theory behind THX is that every movie has been mixed in a dubbing studio using an array of multiple surround speakers for playback in theaters, which also have multiple surround arrays," he explained. "The reason theaters have multiple surrounds—in a line-source around the room, if you will—is to provide uniform illumination of the surround information to every seat in the hall. It's like using fluorescent lights instead of spotlights. What THX knew was that it's hard enough to get interior decorators (or even serious movie buffs) to accept two loudspeakers in the back of a room, much less more than that, so they attempted to duplicate the effect of multiple speakers with dipoles—or in the case of Pro Logic, which is monaural, dipoles plus decorrelation.

"In practice, sometimes it works, sometimes it doesn't—it's dependent on room acoustics. Of course, we're not using true dipoles in THX—two drivers out of phase with one another is not the same thing as a single diaphragm radiating in both directions—especially those speakers with the drivers at a 90-degree angle as opposed to a 180-degree angle. In fact, the theory was to have some direct energy perceivable at the listening position and most dipoles do not accomplish that. The Tripole is, therefore, in line with the theory behind the THX use of dipoles."

MX-150 THX subwoofer
As M&K is the granddaddy of subwoofer manufacturers, you'd expect their MX-150 to deliver—and deliver it do. It has two 12" drivers mounted in what M&K calls a "push-pull" configuration—that is, one driver mounted in the nonported cabinet conventionally, firing forward, and the other loading into the enclosure, with its rear hanging down toward the floor and its polarity reversed in order to keep the room-radiation in phase. You wouldn't know any of this by looking at the enclosure, however; all the naughty bits are discretely concealed behind the black cloth grille.

The rear panel has a three-position bass-level control (with calibrations at reference, -3dB, and -6dB), a switch choosing between variable-level and fixed-level THX inputs, left/mono and right-channel RCA inputs, a phase-reversal switch, and a low-pass filter switch that chooses between a built-in 80Hz, 24dB/octave roll-off or no low-pass filtering at all (for use with processors having their own built-in filters).

Implementation in a THX system is dirt-simple—just plug the subwoofer output from the THX controller into the left/mono RCA input and set all adjustments to THX. Although only one MX-150 is required to meet THX spec, I used two in parallel for my auditioning. (I split the processor's single subwoofer output using an RCA Y adapter.)

For a non-THX system, M&K makes a variety of passive high-pass filters and volume controls that optimize integration of the MX-150 subwoofer to satellites, no matter what provisions your processor or preamp might have—or lack.