Audio Physic Virgo III loudspeaker Measurements

Sidebar 3: Measurements

The Audio Physic Virgo III is of above-average voltage sensitivity, my estimate coming in at 89.2dB(B)/2.83V/m, which is in reach of the specified 90dB. However, its impedance plot of magnitude and electrical phase against frequency (fig.1) reveals it to be a moderately demanding load. Not only does the speaker feature a minimum impedance of 3.5 ohms at 572Hz, but the combination of 5.5 ohms and -42 degrees at 78Hz will stress low-powered amps if the user wants to rock out on bass-heavy music.

Fig.1 Audio Physic Virgo III, electrical impedance (solid) and phase (dashed). (2 ohms/vertical div.)

The saddle at 44Hz in the magnitude trace gives a clue to the tuning of the bass radiators, which in turn implies modest bass extension. There are no wrinkles in the impedance traces that would suggest the presence of mechanical resonances in the cabinet panels. Fig.2, calculated from the output of an accelerometer fastened to the center of a side wall above the woofer-radiator opening, reveals a mode present at 310Hz as well as a couple a little higher in frequency, but these are all low in level. The speaker's front baffle was rock-solid.

Fig.2 Audio Physic Virgo III, cumulative spectral-decay plot calculated from the output of an accelerometer fastened to the cabinet's side panel. (MLS driving voltage to speaker, 7.55V; measurement bandwidth, 2kHz.)

The traces in fig.3 were all taken in the nearfield, and reveal considerable overlap between the two side-facing woofers (red trace) and the front-firing midrange unit (green). The passive radiators (blue) also offer output into the midrange, but it is fair to note that the close proximity of the woofers may have resulted in upper-frequency leakage into their apparent response. The woofers show only a slight notch at the nominal radiator tuning frequency of 44Hz, though the output of the radiators does peak as expected in this region. There are various other small peaks and notches apparent in these traces; I wonder if they are due to interference between the twin woofers and radiators. The black trace in fig.3 is the overall sum of these individual responses, mathematically summed taking both acoustic phase and the different distances from a nominal farfield measuring point. It is pretty flat overall, with the slight hump in the upper bass presumably stemming from the nearfield measurement technique.

Fig.3 Audio Physic Virgo III, nearfield responses of the midrange unit (green), woofer (red), passive radiator (blue), and their complex sum (black), taking into account acoustic phase and distance from the nominal farfield point.

This trace is also shown to the left of fig.4, spliced to the farfield output averaged across a 30 degrees horizontal window on the tweeter axis. There is a slight excess of energy in the lower and mid-treble, but the top-octave response is slightly shelved-down, something I have seen before in speakers using versions of this ring-radiator tweeter. But overall, the Virgo III's quasi-anechoic response is both smooth and flat.

Fig.4 Audio Physic Virgo III, anechoic response on tweeter axis at 50", averaged across 30 degrees horizontal window and corrected for microphone response, with the complex sum of the nearfield responses plotted below 300Hz.

To my surprise, given its narrow baffle and small-diameter midrange unit, the Virgo III was quite directional in the treble, which can be seen in fig.5, the speaker's plot of horizontal dispersion. There is a distinct step in the radiation pattern just above 1kHz, but the dispersion is actually well-controlled, with no flare apparent at the bottom of the tweeter's passband. The tweeter itself, however, becomes very directional above 10kHz, which will make the balance sound rather airless in very large or well-damped rooms, which BD did note. In the vertical plane (fig.6), the response stays remarkably even over quite a large angle.

Fig.5 Audio Physic Virgo III, lateral response family at 50", normalized to response on tweeter axis, from back to front: differences in response 90 degrees-5 degrees off-axis, reference response, differences in response 5 degrees-90 degrees off-axis on driver side of baffle.

Fig.6 Audio Physic Virgo III, vertical response family at 50", normalized to response on tweeter axis, from back to front: differences in response 15 degrees-5 degrees above axis, reference response, differences in response 5 degrees-10 degrees below axis.

Fig.7 shows how this all added up in my own listening room, which is of moderate size but not overdamped. The Virgo III's spatially averaged response—120 individual 1/3-octave responses taken over a window 18" deep and 40" wide and centered on my ear position, each speaker driven alone—is remarkably flat through the upper midrange and treble. The tweeter's beaming above 10kHz results in a rolled-off top octave, which I did hear in my own auditioning as a slightly polite balance. However, the placement of the woofers near the floor results in much less of a "floor-bounce," interference-caused suckout in the lower mids. The lack of measured energy in the 50Hz and 63Hz bands is typical of my room, but the Virgo III does not offer much in the way of low bass, not being too different from the stand-mounted Monitor Audio Silver S2 speaker I reviewed in the August issue. Again, this correlates with what BD found in his auditioning.

Fig.7 Audio Physic Virgo III, spatially averaged, 1/3-octave response in JA's listening room.

In the time domain, the Virgo III's step response (fig.8) shows a sharp, positive-going spike from the tweeter, followed a fraction of a millisecond later by the positive-going output of the midrange unit. The output of the woofers cannot be distinguished in this graph, but its negative-going step—the woofers are connected in inverted polarity—coincides with the negative-going undershoot of the midrange unit, implying good frequency-domain integration. Finally, other than a low-level mode at 7.7kHz, the Audio Physic's cumulative spectral-decay plot (fig.9) is extremely clean, which suggests a grain-free, transparent treble presentation.

Fig.8 Audio Physic Virgo III, step response on tweeter axis at 50" (5ms time window, 30kHz bandwidth).

Fig.9 Audio Physic Virgo III, cumulative spectral-decay plot at 50" (0.15ms risetime).

Overall, this is good measured performance. The III appears to be an excellent successor to the original Virgo, which was one of my favorite speakers in the late 1990s.—John Atkinson

Audio Physic
US distributor: Immedia
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