M&K S-150 THX surround loudspeaker system Measurements
S-150THX and S-150ACTHX: Despite their somewhat different shapes, the side and center M&K speakers measured almost identically. My comments refer, therefore, to either speaker unless I specifically mention a difference between them. I estimated the M&K's B-weighted sensitivity at around 88.5dB/W/m (center) and 89.5dB/W/m (side). The 1dB difference is inconsequential. The impedance dipped almost to 3 ohms through the midrange and hovered around 6 ohms in the mid and high treble, meaning that a good, 4 ohm-rated amplifier or receiver should used. The sealed enclosure seemed to be tuned to 80Hz.
The M&K's on-axis frequency response was commendably flat and even, with just a trace of peakiness in the high treble that I could hear on pink noise as a slight whistle. Other than a slight sibilant emphasis, its effect on movie soundtracks should be minimal, however. Both speakers also had a small peak apparent between 4 and 5kHz; though the side speaker's was more accentuated, it still didn't rise more than 1.5dB above the 1kHz reference level. As expected, the M&Ks rolled-off with a sealed-box 12dB/octave slope below 100Hz. You definitely need to use a subwoofer with this system.
Presumably due to the close spacing of the twin woofers, the off-axis response was relatively even in both horizontal and vertical planes, with only a slight cancellation notch at 900Hz developing at extreme angles. Laterally, the treble rolled-off progressively with increasing angle, which is optimal behavior. vertically, although the off-axis behavior was not as quirky as I have seen with other speakers using multiple tweeters—the foam between the each of the M&K's three tweeters does help here—quite a pattern of interference peaks and dips does develop as you move much above or below the middle tweeter. I recommend you put these speakers on high enough stands that you listen to them on the central tweeter axis.
In the time domain, all five drive-units are connected with the same acoustic polarity. The cumulative spectral-decay or waterfall plot is generally free from resonant decay artefacts, but there is a slight ridge of delayed energy apparent at 4.5kHz, the frequency on the small peak in the on-axis frequency response.
I was pleasantly surprised by the M&K 150 THX's measurements, particularly regarding its generally well-behaved off-axis behavior and the smooth, flat frequency response.—John Atkinson