Floor Loudspeaker Reviews

Sidebar 2: Measurements

John Atkinson measured the Type MC Reference Tower, Type MC Reference 2800 LCR center channel, and Type MC Reference SUR 2800 Tower surround loudspeakers and gave me the results after my auditioning was complete and the text of the review written and turned in. I also investigated the low-frequency tuning of the Type MC SUB 1800.

The sensitivity of the Type MC Reference Tower measured 85dB/W/m (B-weighted). By a significant margin, this is the lowest sensitivity we have yet obtained for a home THX-certified front loudspeaker. The B&W FCM8, in comparison, measured 87.5dB/W/m, and the Fosgate MC-220, 91dB/W/m. (The MC-220 is now discontinued; its replacement will carry the Citation brand.) We double-checked the result just to be certain, and arrived at the same value the second time around (footnote 1). While in theory this should make the Snells somewhat power-hungry, in practice I used amplification ranging from just over 130W to 150Wpc with no apparent limitation, even in my very large (about 7000ft³) audio/video room. The Reference Tower's impedance magnitude (fig.1) is fairly high as loudspeakers go—above 6 ohms across much of the range and above 8 ohms throughout the loudspeaker's "power region." The latter is in the lower midrange—from just above 100Hz to about 500Hz. The most continuous power demands are in this band. With its impedance magnitude and phase characteristics, the Tower should not cause any problems for a well-designed amplifier.

Fig.2 shows the individual drive signals output by the Snell passive outboard crossover networks. (The corresponding measured acoustical responses are shown in fig.3.) The crossover points here are very close to the specified values, and the filter slopes are textbook-smooth and linear. The overall response of the Tower—a combination of the nearfield response of the woofers and the upper-range drivers averaged across a 30° window—is plotted in fig.4. The response here is extremely smooth, with only a minor peak at 100Hz and a small dip between 1.5kHz and 5kHz. The latter might be enough to account for the slight politeness I noted in the system, coming as it does in the region of the ear's greatest sensitivity. It is likely associated with the midrange-to-tweeter crossover, but may also be affected by the vertical measurement axis—in this case on-axis with the central tweeter, the point at which we normally take this measurement. (The central tweeter is 39" from the floor.)

The horizontal dispersion of the Towers—with response deviations subtracted so that the on-axis response appears flat—is shown in fig.5. The rolloff is smooth with increasing off-axis angles. The top octave does roll off a little earlier than usual, however, which might be expected to make the room reverberant field somewhat mellow-balanced or polite. The vertical dispersion family, not shown, indicates a smooth response from 10° below the tweeter axis to about 5° above. Above 5°, the typical vertical off-axis response irregularities of THX loudspeakers (which mandate limited vertical dispersion) begin to show up.

The step response in fig.6 indicates that the tweeters (both the supertweeter and the lower-range tweeters flanking it) are connected in negative polarity, the midranges and woofers in positive. [This was checked by examining the step responses of the individual drivers (not shown).—Ed.] Despite the profiling of the front baffle, the system is not time-aligned, with a delay of just under 2ms between the full output of the supertweeter and that of the woofers. The associated cumulative spectral-decay, or waterfall, plot is shown in fig.7. This is a very well-behaved result, with a clean tweeter response having only small, rapidly damped resonance modes.

The sensitivity of the Type MC Reference 2800 LCR center-channel speaker was calculated to be 85dB/W/m (B-weighted): identical to that of the left/right Towers. Its impedance (fig.8) is a little less well-behaved than that of the Towers in both magnitude and phase, the former dipping down to just below 4 ohms at a minimum, but it should still not be a difficult load to drive.

Fig.9 shows the individual responses of the 2800 LCR's woofers (nearfield) and upper-range drivers (at 50"), plus their combined response: the upper range averaged across a 30° window. As with the Towers, the acoustic rolloffs are smooth and steep. The overall response is a little less linear than that of the Towers, with a more pronounced rise in the bass and a broader dip across the low- to mid-treble region. Still, the deviations are relatively small, with the exception of the upper-bass rise, and that may be at least partially due to the inevitably less-than-precise splicing of the nearfield woofer responses with the spatially averaged upper-range measurements.

Fig.10 shows the horizontal dispersion of the 2800 LCR, again with the on-axis response deviations subtracted so that the on-axis response appears flat. The high-frequency rolloff is relatively smooth off-axis, though a narrow notch does develop in the low treble as the listener marches farther to the left or right. The vertical responses in fig.11 (also normalized relative to the on-axis response) are reasonably well-controlled at small angles—as with the Towers—but at progressively higher or lower positions the uneven response of typical THX loudspeakers (they're designed to limit vertical dispersion) becomes evident.

The impulse response of the 2800 LCR is close enough to that of the Tower that it is not shown here. The same is true of the step response, which indicates a very similar driver phasing and time delay.

The cumulative spectral-decay, or waterfall, plot is shown in fig.12. It is, surprisingly, even cleaner than that of the Tower. Since the same tweeters are used in both systems, the difference may lie with the different midranges in the two systems (or the resulting changes thus required in the crossover network).

The Type MC Reference SUR 2800 Tower surround loudspeaker's sensitivity measured 83.5dB/W/m (B-weighted)—slightly less than that of the front loudspeakers. Measuring the frequency response of dipolar surround loudspeakers in a meaningful fashion is a nontrivial matter for which neither we, nor anyone else we know of, has as yet devised a satisfactory solution. Fig.13 is the response obtained at the front of the surround (the baffle face with the 8" woofer). The measurement was made at the port height—a typical listening height 37.5" off the floor. The response is relatively smooth, though nothing to fax home about. The graph is useful primarily as an indication that the surround is well-designed (arguably the flattest, and certainly the most full-range, we have yet measured in a dipole surround). It is not, except in a very general way, typical of the sort of response that the listener will hear from the surround, seated in the null (at 90° to the axis used for the measured response) with a considerable amount of energy bouncing off nearby walls before reaching the listener.

Finally, I measured the response of the Type MC SUB 1800 subwoofer in the nearfield. The port's maximum output occurred at about 15Hz; the driver's minimum motion occurred at about 17Hz, implying flat response pretty much to 20Hz. Since the measurement accuracy here was likely to be no better than 1Hz in either direction, the tuning was textbook-accurate for all practical purposes. At this frequency, who's counting one or two Hz? We haven't measured extension from a subwoofer any lower than this.

Altogether, the measurements of the Snell THX loudspeaker array are as outstanding as you would expect of a price-no-object system.—Thomas J. Norton

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Footnote 1: Because the altitude at Santa Fe reduces the sensitivity of all loudspeakers, I calculate sensitivity by comparing the measured level at 50" for a given voltage input, using a noise signal, with that obtained for a sample Rogers LS3/5A that I have measured both in Santa Fe and at sea level. In the case of the Snell speakers, I double-checked my measurements, only to get the same result.—John Atkinson