Avantgarde Acoustic Uno Series Two loudspeaker Measurements
Like Bob Deutsch, I also had problems with AC noise when I measured the Uno. (For logistical reasons, I measured a different sample, delivered to Santa Fe by Casey McKee.) The DRA Labs MLSSA system I use inhabits an ancient 486 computer, and it turned out that the monitor and computer were plugged into a surge suppressor with sufficient series resistance to introduce a ground loop. And at an estimated 102.5dB(B)/2.83V/m voltage sensitivity, the Uno faithfully revealed the problem.
As well as its astonishingly high sensitivity, the Avantgarde speaker will be an easy load for the partnering amplifier to drive: its impedance (fig.1) hardly drops below 8 ohms, with a mainly resistive phase angle. The small peak at 19kHz will be due to the tweeter's primary dome resonance, but the impedance plot is otherwise free from the narrow peaks usually seen with a horn-loaded speaker.
Fig.1 Avantgarde Uno, electrical impedance (solid) and phase (dashed). (2 ohms/vertical div.)
Fig.2 shows the nearfield response of the subwoofer module with the crossover set to 220Hz (top curve) and 90Hz (bottom curve) and at the three indicated intermediate frequencies. (The extension switch was set to 20Hz for these measurements, and you can see that the module's -6dB point indeed lies at 20Hz.) The first thing to note is that, at the extreme settings, quite large rotations of the control introduce very little change in the response, while in the center, even quite small rotations make a large difference. Optimizing the subwoofer setting will need a delicate touch on this control. Second, note that at the more extended settings, the subwoofer extends almost an octave higher in frequency than the indicated "220Hz." This has implications for the speaker's setup that can be seen in the next graph.
Fig.2 Avantgarde Uno, nearfield woofer responses with crossover set at its maximum (220Hz), minimum (90Hz), and three intermediate frequencies.
Fig.3 is the Uno's quasi-anechoic response, taken at the manufacturer's recommended microphone distance of 63". (A similar measurement taken at my standard 50" distance differed only in the low treble, and then only regarding the heights of the peaks and dips.) The measurement axis was again the manufacturer's recommendation: midway between the center of the midrange and tweeter horns, a sensible 36" from the floor. The lack of energy in the lower crossover region is most likely due to interference resulting from the overlap of the midrange horn and the woofer. It suggests that the woofer would integrate better at this relatively close distance if its electrical polarity were to be inverted. Unfortunately, the need to return the measurement sample of the Uno to its Austin, Texas owner before I could process the measured data meant that I could not further explore this aspect of the speaker's performance. But if you're bothered by a thinness in the Uno's lower mids, I encourage you to experiment by changing the polarity of the speaker leads connecting the woofer terminals to the upper-range units.
Fig.3 Avantgarde Uno, anechoic response on design axis at 63", averaged across 30 degrees horizontal window and corrected for microphone response, with the nearfield woofer response plotted below 150Hz.