M&K S-150 THX surround loudspeaker system Page 2
Given the thought that has gone into something as basic as mounting hardware, you'd assume that the S-150 satellite speakers would be thoroughly considered—and you'd be right. The speakers are compact, measuring 12*" tall by 10*" wide by 12*" deep. They are not, however, conventionally boxlike—the face of the speaker angles 45 degrees, putting the three vertically mounted tweeters on the outside edge and farther into the room than the two midrange drivers.
There are foam pads between the two 5*" midrange cones as well as between the three tweeters; this novel driver array is designed to make the speakers as directional as possible in the vertical dimension. Says designer Kreisel, "The goal is to make a vertically directional loudspeaker, so that it doesn't cause ceiling and floor reflections. The S-150 is designed to be listened to on-axis in the vertical plane. Period. The physics that make it directional also make it have lobing off-axis, and it's the lobing that gives the speaker directivity. These speakers are designed to be listened to on-axis, plus or minus a couple of degrees." According to Kreisel, this is part of the THX specification that is not commonly understood—you're supposed to listen to THX speakers on-axis, plus or minus nothing.
There's an additional benefit that derives from the use of multiple drivers: power handling capabilities are substantially increased. For the 150 series, Kreisel designed what he calls a Phase Focused Crossover, which takes a three-dimensional approach to designing the circuit. "Normally, when you design a crossover, you pick a point in space, put your measurement device there, and design to produce optimum results at that spot. One of THX's specifications concerns horizontal symmetry: on one hand they want the speaker to be directional vertically—which produces lobing—but on the other hand, they want it to be very symmetrical in the horizontal plane, which means no lobing. To achieve that goal required the development of the Phase Focused Crossover. Other THX speakers usually have a tweeter and woofer arrayed vertically—that's almost the only way to get smooth horizontal directional characteristics.
"We developed our Phase Focused Crossover with multiple microphones and a time-based computational measurement system—[DRA Labs'] MLSSA, [Maximum Length Sequence Susyem Analyzer]—and that development has shocked most people in the industry—people can't believe that we got rid of lobing in the horizontal plane.
"It takes hundreds and hundreds of measurements and calculations to make it work—I'm not about to say too much and give away the secret. Most crossovers optimize the phase and amplitude characteristics of the driver to one specific point in space. Move off-axis—get closer to the midrange driver and farther away from the tweeter—and the phase characteristics of those two drivers have changed and the crossover is no longer optimized for that new point in space. The Phase Focused Crossover looks at the phase and amplitude characteristics of the drivers at many, many points in space and optimizes them to eliminate the dips and peaks in response. These are time-domain measurements, not frequency response. We did this work attempting to design a successful center channel, but found that it carried over to all of the front speakers."
The S-150's cabinet is made from *" MDF, coated with M&K's proprietary lacquered glass-bead finish, a 14-step painting process that, according to Kreisel, further acoustically deadens the cabinet.
The Angled Center Channel (AC) version of the 150 changes the angle of the face-plate: it rakes back from the top toward the bottom, while the tweeter and midrange arrays remain vertically aligned. This makes the AC-150 easy to focus on the listening position from the top of a large monitor. But turn the speaker upside down, and it becomes simple to focus it upward when the AC-150 is mounted below a screen. The top of the speaker also incorporates a five-degree rake-back to further reduce parallel-wall-induced reinforcement nodes. It's magnetically shielded to simplify top-of-monitor placement.
All the 150 series speakers have one pair of knurled metal binding posts that facilitate connections with bare wire, spade lugs, or bananas. Kreisel does not feel that benefits accrue from biwiring. "If you design the crossover properly, you don't end up with strange loading characteristics in the first place."