Wilson Audio Specialties MAXX loudspeaker Measurements

Sidebar 3: Measurements

Taking account of the system height and the mike position, the MAXX's sensitivity was close to specification: nominally 91dB in the aurally most sensitive range, perhaps 92dB in the mid and lower mid. This sensitivity is well above average, and is comparable with the WATT 5's.

For powerful, current-capable amplifiers, this speaker's high power handling of around 500W peak program means that seriously loud in-room sound levels of 112dB are possible—even a few dB more, if the MAXX is partnered with a still more powerful amp (and discretion is used). Some reassurance is given by the resistor "fuses" in the signal paths of the mid and treble units.

A health warning accompanies the higher-than-usual sensitivity: namely, the measurably difficult, amplifier-taxing load impedance. With some precision I got a minimum value of 2.2 ohms in the near-peak spectral power band of the lower midrange (fig.1). In contrast to the rather amplifier-friendly SLAMM and WITT, this more or less rules out tubed amplifiers.

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

In theory, even a 4 ohm load should not fall below 3.2 ohms, and although the MAXX's rear-panel label does admit to a "4 ohm" load impedance, one could legitimately call this speaker a 3 ohm load. In this light the sensitivity doesn't look so good; the true output for a real, as opposed to a "8 ohm" watt, is in fact closer to 87-88dB.

The load variation will certainly stretch the weaker amplifiers, while good, low-resistance speaker cable will also form a necessary part of the investment. Given the price of the speaker, most sensible, appropriate solid-state amplifier choices, from Mark Levinson to Krell, will have sufficient current capability—though I sometimes feel that even these hearty beasts can sound still lighter on their feet when stressed less heavily than this.

Given that load impedance is a significant criterion for a speaker, the MAXX's minima indicate a degree of struggle on the part of the designer. The MAXX is not alone in this, of course; many of the Thiels also dip low, the Apogees were notorious for a combination of low sensitivity and low impedance, and I found a 2.8 ohm minimum, also in the main power band, when I measured B&W's Nautilus.

The quest for high sensitivity is all very well, but in my view it should not be accomplished at the expense of undue peak current draw. If you ever felt that the peak current criteria for big amplifiers were pure indulgence, consider a 1000W amplifier momentarily clipped into a MAXX. Worst case, the speaker will draw a massive ±57A.

It's also been shown that more complex program-related signals can draw 20-30% more current than this. Think of the demands made on cables and connectors under such conditions.

Arguing in defense of the MAXX, the impedance dip was only a third of an octave wide, after which the nominal value settled at, typically, 4 ohms over the dominant part of the frequency range. The electrical phase angle was also quite low. In-box resonance for the driver pair was approximately 48Hz, guaranteeing "prime" bass down to 40Hz, even before the bass port begins to extend the low range. The port resonance was a low 22Hz, promising reinforcement down to 18Hz, depending on room-loading conditions.

Measuring the system response on-axis gave quite a lot of information about the speaker, even though this system's size and weight meant that some measurement difficulties were inevitable. The resultant graphs are my best attempt at conveying the acoustic output; full, free-field data would be still more accurate. (Measurements were made for the speaker with the chosen "review" resistors in place.)

Fig.2 shows the main response—a composite of low-frequency nearfield and reasonably well-gated axial measurement—the output was seen to have several dB of bass lift, set low enough so as not to injure the tonal balance or neutrality of the midrange, but sufficient in power to have forced me to find a new listening position. In absolute terms, I did find the MAXX to have extended bass—almost flat to 25Hz, -3dB at 20Hz—and, given room gain and a suitable environment, worthwhile bass to an absolute 18Hz or 19Hz limit: infrasonic subwoofer territory.

Fig.2 Wilson MAXX, anechoic response on reference axis at 1.4m, with individual woofer, midrange, and tweeter responses and estimate of overall LF response plotted below 200Hz.

Overall, the MAXX met reasonable ±3dB limits from 70Hz to 15kHz and was -6dB at 18Hz and 25kHz, though these figures do not really convey the smooth effect for the overall output delivered to a room. The primary midrange is very flat, meeting ±1.5dB limits from 90Hz to 900Hz. The upper mid has a mild plateau-lift on-axis, perhaps averaging +1.5dB, while beyond 2kHz the treble was mildly depressed to a similar degree, which may lend a touch of "distance" and perspective. As far as could be checked out, pair matching was better than ±0.7dB in the critical range of 200Hz-12kHz, which is a fine result.

The high treble has the usual Focal on-axis "kick," +7dB at 16-17kHz. Fortunately for most of us, this peak is placed near the edge of audibility; however, it might just be a little troublesome for younger, keener-eared audiophiles, because the tweeter in the MAXX is intended to directly face the listener. In the WATT and WITT, the treble unit is set at something of an angle, ameliorating the effect of this narrow, axially directed peak.

Fig.2 also shows acoustic crossover responses. Wilson speakers often show surprising overlaps between the drivers, yet the overall output is generally well-integrated. Take the bass section, which extends to perhaps 300 or 400Hz: there is a notch between 800Hz and 1kHz, with then a narrow directivity peak, almost a resonance, at 1.8kHz. The peak at 1.8kHz was only 12dB down on the main response, and lies underneath the treble passband.

Equally fascinating was the behavior of the tweeter, which almost meets the output of the bass units. It was only -6dB at 1.4kHz, technically considered rather low for a 1" unit with a ¾" voice-coil. I checked out the possible distortion later in the report to confirm the viability of this aspect of the design. Finally, the midrange units nominally run from 200Hz to 2kHz and kind of fill in the gap.

Stereophile's practice is to average the forward response graphs over a moderate angle. Fig.3 shows the MAXX's response, 1/3-octave-averaged from 200Hz to 200kHz, for the 0 degrees, 15 degrees lateral, and 15 degrees vertical frequency responses. This graph reveals good off-axis control and confirms the value of the main axial reference response in fig.2.

Fig.3 Wilson MAXX, anechoic response on reference axis at 1.4m, averaged across ±15 degrees solid angle.

Wilson's older, open-cell grilles more or less defied measurement. Not so the new type. I was shocked to find how audible its effects were, and the response-error graph (fig.4) bears this out. A cross-reflection path is presented to the tweeter, which adds variations to the response of up to +4.5dB and -2dB, right up to 20kHz. The grille effect is audible as an almost "ringing" quality to the treble timbre.

Fig.4 Wilson MAXX, effect of grille (5dB/vertical div.).

Nearfield measurements (fig.5) revealed the port's broadly damped output to be centered on 22Hz, with -3dB points at 14Hz and 55Hz. Clearly, the alignment was not maximally flat, and the port had significant output up to 70Hz, as the insertion of a foam plug would testify (see later). Some stray output was present up to 500H, but in reality, at probably -26dB relative to the primary response at the listening position, it is of little consequence. The right-hand trace in fig.5 is the nearfield response of the woofers. The port null can be seen at 22Hz, and the drivers cover the broad bandpass from 40Hz to 200Hz.

Wilson Audio Specialties
2233 Mountain Vista Lane
Provo, UT 84606
(801) 377-2233