Wilson Benesch Square One Series 2 loudspeaker Measurements

Sidebar 3: Measurements

John Marks enthused about this elegantly finished stand-mounted speaker from England, concluding that "the Square One is a standout performer that I think absolutely deserves a very high Class B (Restricted LF) rating in our 'Recommended Components.'"

High praise indeed. I asked John to send me the review samples so I could perform my standard set of measurements. As always, I used DRA Labs' MLSSA system and a calibrated DPA 4006 microphone to measure the speaker's frequency response in the farfield, and an Earthworks QTC-40, with its small, ¼"-diameter capsule, for the nearfield responses.

The Wilson Benesch Square One Series 2 is specified as having a voltage sensitivity of 87dB/2.83V/m. My estimate was a little lower, at 85.7dB(B)/2.83V/m, perhaps due to a lack of low-treble energy in the frequency response (see below). The specified nominal impedance is 6 ohms, with a minimum value of 4 ohms, but, as the solid trace in fig.1 shows, the Square One's impedance drops below 8 ohms only briefly, in the lower midrange, and the minimum impedance magnitude is 6.77 ohms at 235Hz. The phase angle of the impedance is generally low, which means that, taken with the highish impedance, the Square One will be very easy to drive.

Fig.1 Wilson Benesch Square One, electrical impedance (solid) and phase (dashed) (2 ohms/vertical div.).

Small discontinuities in the impedance traces between 500 and 600Hz and just above 1kHz hint at the presence of resonances of some kind. I did find a resonant mode at 508Hz on the top panel (fig.2), though this is a little lower in frequency than the impedance wrinkle. There was also a stronger mode, at 359Hz on the side panel (not shown). I don't believe either mode will affect the speaker's sound quality.

Fig.2 Wilson Benesch Square One, cumulative spectral-decay plot calculated from output of accelerometer fastened to center of top panel (MLS driving voltage to speaker, 7.55V; measurement bandwidth, 2kHz).

With a rear-facing acoustic bass radiator (ABR) and two downward-facing ports, the Square One's low-frequency alignment is complex, though Wilson Benesch says that the ABR absorbs and controls the energy radiated by the rear of the woofer cone, and that the combination of the ABR and ports "[enhances] bass performance and extension." The red trace in fig.3 shows the summed outputs of the ports—they behave identically—measured in the nearfield. The double peak is most unusual, though the ports roll off below 50Hz with the usual second-order slope. The green trace in this graph is the nearfield response of the ABR, plotted in the ratio of its radiating diameter to that of the woofer. It peaks sharply at 100Hz, coincident with the upper peak in the ports' output, and rolls off above 110Hz and below 70Hz with a fourth-order slope. Its output is actually in antiphase to that of the woofer in the midbass, and even compensating for the fact that it's mounted on the rear of the enclosure, adding its nearfield output to that of the woofer gives a sharply defined notch at 61Hz.

Fig.3 Wilson Benesch Square One, acoustic crossover on tweeter axis at 50", with nearfield responses of: woofer (black), ABR (green), ports (red), plotted below 350Hz, 350Hz, 600Hz.

The black trace in fig.3 shows the woofer's nearfield output below 355Hz and its farfield response on the tweeter axis above 355Hz. The peak just above 1kHz coincides with the high-frequency wrinkle in the impedance traces. Above that peak, the woofer is crossed over to the tweeter (blue trace) at the specified 5kHz with second-order filter slopes—the original Square One used a first-order crossover—but there is significant overlap between these drivers. Both units are impressively flat on axis above 1.5kHz, but the overlap in the crossover region gives rise to a peak between 3 and 7kHz in the overall response averaged across a 30° horizontal window on the tweeter axis (fig.4). Concerned that this behavior, measured with sample 01240A, may not be typical, I looked at sample 01240B. It measured identically—in fact, the matching of the pair was superb, any difference between the two samples being less than 0.5dB over almost the entire audio band. Figs. 3 and 4 were taken without the grille; repeating the measurement with the grille slightly increased the height of the mid-treble peak and introduced a small suckout centered on 8kHz.

Fig.4 Wilson Benesch Square One, anechoic response on tweeter axis at 50", averaged across 30° horizontal window and corrected for microphone response, with complex sum of nearfield responses plotted below 300Hz.

The trace below 300Hz in fig.4 shows the complex sum of the nearfield responses of the woofer, ABR, and port. Some of the apparent peak between 90 and 200Hz in this graph will be an artifact of the nearfield measurement technique, but the alignment does appear to be underdamped overall. The ultimate rolloff seems closer to third-order, 18dB/octave, rather than the usual 24dB/octave of a ported design. But with a –6dB point of 80Hz or so, the Square One doesn't offer significant low-frequency extension, I feel.

Fig.5 shows the Wilson Benesch's horizontal dispersion, referenced to the output on the tweeter axis. The contour lines are even across most of the audioband, indicating a well-controlled radiation pattern, though the Square One is a little more directional above the cursor position at 3200Hz than is usually the case with a small speaker. This might well work against the audibility of the crossover-region peak in larger rooms, though the top octave will sound rather too mellow in those rooms. In the vertical plane (fig.6), the use of first-order crossover filters makes the speaker's response very dependent on the listening axis. This graph suggests that placing the speakers on high stands so that the listener sits between 5° and 10° below the tweeter axis might give the optimal treble balance.

Fig.5 Wilson Benesch Square One, lateral response family at 50", normalized to response on tweeter axis, from back to front: differences in response 90–5° off axis, reference response, differences in response 5–90° off axis.

Fig.6 Wilson Benesch Square One, vertical response family at 50", normalized to response on tweeter axis, from back to front: differences in response 45–5° above axis, reference response, differences in response 5–45° below axis.

Turning to the time domain, the Square One's step response on the tweeter axis (fig.7) reveals that both drive-units are connected in positive acoustic polarity, with the decay of the tweeter's step smoothly blending into the start of the woofer's step. But the latter is disturbed by an undulation with a period of just less than 1 millisecond, visible in the cumulative spectral-decay plot (fig.8) as a ridge of delayed energy centered on the frequency of the on-axis peak. Other than that, however, the spectral decay is impressively clean from 2kHz up.

Fig.7 Wilson Benesch Square One, step response on tweeter axis at 50" (5ms time window, 30kHz bandwidth).

Fig.8 Wilson Benesch Square One, cumulative spectral-decay plot on tweeter axis at 50" (0.15ms risetime).

I'm puzzled both by the Wilson Benesch Square One Series 2's measured performance and by John Marks's praise for its sound quality. This is a not-inexpensive loudspeaker, and while its fit'n'finish are to a quality commensurate with its price, it's difficult for me to see how its rather peaky treble response is outweighed by its shallower-than-usual low-frequency rollout, well-controlled horizontal dispersion, and clean decay in the treble.—John Atkinson

COMPANY INFO
Wilson Benesch Ltd.
US distributor: The Sound Organisation
1009 Oakmead Dr.
Arlington, TX 76011
(972) 234-0182
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COMMENTS
spacehound's picture

That makes me VERY wary right from the start. A well-written and informative review, though of course the manufacturers say all the usual things about how wonderful they are and their design skills. Does anyone NOT do that?.

Would I buy them? Unlikely. A very small room I would give them a listen purely on the strength of your review and because you have drawn my attention to them. But all small speakers give so much away over big ones that unless you want them for multi channel video rear speakers they are not worth considering. And in that position expensive ones are hardly worth buying unless you want
particularly accurate surround explosions. And most systems equalize the rear speakers to the front ones anyway. Not that I have any great interest in such things.

Interesting that they are bolted to the stands though. That can make a BIG difference. In fact the bottom of my (short) stands are spiked to the floor with intervening large coins (low value ones!) and screwed to the floor though the carpet too, with one big screw on each side of each stand. That improves things slightly more. They cant move at all.

kensargent's picture

A passive radiator is generally regarded as a vent substitute. In other words, it acts much like a port, but in a more-controlled manner. I have never seen another design that uses a radiator AND a port. But the article indicates that they both, unusually, locate on the bottom of the speaker. Although this is not the first bottom-vented speaker I have seen, it is the first I've seen that "vents" the vent substitute. How does that work? Are there multiple venting circuits? The WB web page just mentions that the ABR remains, but does not show a mechanical schematic. I am fascinated!

John Marks's picture

johnmarks@sorc.com

jm

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