Acapella High Violoncello II loudspeaker Measurements
I used DRA Labs' MLSSA system and a calibrated DPA 4006 microphone to measure the Acapella's frequency response in the farfield, and an Earthworks QTC-40 for the nearfield and spatially averaged room responses. As with the Focal Maestro Utopia that I reviewed in the July 2010 issue, the sheer bulk of the High Violoncello II precluded my being able to take it outside and place it on a high stand for the acoustic measurements. I therefore performed those measurements in my listening room, firing the speaker along one of the room's diagonals to move the sidewall reflections as far back in time as possible following the arrival of the direct sound at the measuring microphone. The restricted dispersion of the midrange and treble horns (see later) was a help in this respect, reducing the amplitude of those boundary reflections.
Acapella specifies the High Violoncello II's sensitivity as 93dB/2.83V/m. My B-weighted estimate on the tweeter axis was even higher than that, at 94.5dB(B)/2.83V/msignificantly higher than average. The speaker's impedance drops to 4.5 ohms at 150Hz, but the Acapella is otherwise an easy load for the partnering amplifier, with a generally low electrical phase angle (fig.1). The magnitude rises in an almost linear manner with increasing frequency, but as the tweeter is actively driven, the large difference between the impedance at low and high frequencies will not result in the expected treble boost when the Acapella is driven by a tube amplifier possessing the usual high source impedance.
Fig.1 Acapella High Violoncello II, electrical impedance (solid) and phase (dashed) with Midrange switches set to "0" (2 ohms/vertical div.).
The solid impedance-magnitude trace in fig.1 is free from the small glitches that would imply the existence of cabinet resonances. However, there is a discontinuity in the dotted phase trace between 300 and 400Hz, and I did find a moderate resonant mode at 400Hz on the woofer enclosure's side panel, and another at 465Hz (fig.2). The lower-frequency mode could also be detected on the rear panel, where it was higher in amplitude. While both modes could be heard with a stethoscope while I played the half-stepspaced toneburst track from Editor's Choice, I couldn't hear anything amiss in the midrange from the listening position.
Fig.2 Acapella High Violoncello II, cumulative spectral-decay plot calculated from output of accelerometer fastened to center of woofer enclosure (MLS driving voltage to speaker, 7.55V; measurement bandwidth, 2kHz).
Looking at the individual responses of the three sections of the Violoncello II (fig.3), the two front woofers behave identically, and roll off smoothly below 50Hz or so, with the second-order, 12dB/octave slope typical of a sealed enclosure (red trace). The woofers' high-frequency rolloff is smooth, with no apparent resonant modes apparent. Below 350Hz, this trace was taken in the nearfield, and so inevitably features the boost in the upper bass that results from this technique. The crossover between the woofers and the midrange horn (blue trace) appears to be set closer to 600Hz than the specified 800Hz, with asymmetrical filter slopes. The midrange unit offers a basically flat response within its fairly narrow passband, from 600Hz to 5kHz, and rolls off rapidly above that range. The ionic tweeter (green trace) comes in with a third-order acoustic slope and is also flat within its passband. It is still putting out energy at full level at the 30kHz upper limit of this graph.
Fig.3 Acapella High Violoncello II, acoustic crossover on tweeter axis at 50" with Midrange switches set to "0," corrected for microphone response, with nearfield woofer response plotted below 300Hz.
For these measurements the tweeter-level control was set to 8:00, the position recommended by Acapella. As suggested by fig.3, this results in a level about 5dB too hot in the speaker's overall response, averaged across a 30° horizontal window on the tweeter axis (fig.4). That the speaker didn't sound as tilted-up at high frequencies as this graph suggests is due to the fact that the tweeter is quite directional (not shown), meaning that the speaker's power response will be more rolled off in the top two octaves than with a direct-radiating HF driver. The two Midrange switches were both set to "0" for this measurement, and there are slight depressions at both ends of the midrange horn's passband that might correlate with the lack of body I occasionally heard. The lower-frequency depression deepens for listening positions above the tweeter axis, which is 42" from the floor. Below the tweeter axis, the upper-midrange depression fills in, which is useful, considering that we have found the distance from the floor of the average seated listener's ears to be around 36", regardless of the listener's height.
Fig.4 Acapella High Violoncello II, anechoic response on tweeter axis at 50" with Midrange switches set to "0" and HF control to 8:00, averaged across 30° horizontal window and corrected for microphone response, with nearfield woofer response plotted below 300Hz.
The effects of the High Violoncello II's controls are shown in fig.5, normalized to the tweeter-axis response with the tweeter potentiometer at its minimum setting, so that only the changes in response can be seen. Setting the speaker's Midrange switches to "+1" gives the green trace in this graph; the speaker's output rises by up to 0.9dB between 1 and 3kHz. Conversely, setting the switches to "1" gives a 0.9dB-deep trough in the same region. The magenta trace in fig.5 shows what happens when the control is set to 9:00 rather than the recommended 8:00, the position used for the responses shown in figs.3 and 4. The entire region covered by the tweeter is elevated by 2dB. Similarly, rotating the control to 12:00 raises the tweeter level by another 2dB (red trace). I haven't shown the change with higher settings of this control; even with the moderate signal level used for these measurements, the MLS test signal, with its white spectrum, caused the tweeter unit's 400mA rear-panel fuse to blow.
Fig.5 Acapella High Violoncello II, effect of Midrange switches set to "+1" (green) and "1" (blue); effect of tweeter-level control set to 9:00 (magenta) and 12:00 (red); and effect of inserting 1k ohm series resistor in tweeter feed (black). All traces normalized to tweeter-axis response with Midrange switches set to "0" and tweeter-level control set to 8:00 (2dB/vertical div.).
Even at its factory-recommended level, the ionic tweeter is a little too sensitive compared with the Violoncello's midrange and woofer sections. Because the tweeter is self-powered, with a nominal 600 ohm input impedance, I could experiment with reducing its sensitivity by inserting a resistor in series with its input, without messing up the tuning of the crossover. The bottom, black trace in fig.5 shows the difference that a 1000 ohm resistor made to the on-axis response. The region covered by the tweeter has been reduced by 5dB or so, which suggests that a series resistor of around 600 ohms will give a flat overall response, at least on axis. Depending on the size of the listening room and the nature of its furnishings, however, a flat response may not be what is needed to give the most neutral balance, because of the restricted dispersion of the tweeter mentioned earlier.
The blue trace in fig.6 is the 1/6-octavesmoothed, spatially averaged response of the Acapella High Violoncello IIs in my room, as set up by Aaudio Imports' Brian Ackerman. He had set the Midrange switches to "0" and the tweeter-level controls to 8:00. (I perform this measurement by averaging twenty 1/6-octavesmoothed responses taken for each speaker individually in a rectangular grid measuring 36" by 18" and centered on the positions of my ears in my listening chair. I used an Earthworks omni microphone and a Metric Halo ULN-2 FireWire audio interface, in conjunction with SMUGSoftware's Fuzzmeasure 2.0 running on my Apple laptop.) Other than some minor peaks and dips in the midrange, the Acapella's balance is impressively even from 50Hz through 3kHz, though there is still not quite enough energy in the crossover region between the woofers and the midrange horn. The low frequencies are extended, if a little light in weight, but even with the treble horn's directional nature, there is too much energy in-room in this driver's passband.
Fig.6 Acapella High Violoncello II, spatially averaged, 1/6-octave response in JA's listening room as set up by importer (blue), and with series 600 ohm resistors in tweeter feeds (red).
As I said in the review, for the bulk of my auditioning I settled on setting the tweeter controls at their minimum, but the red trace in fig.6 shows the spatially averaged response of inserting 600 ohm series resistors in the tweeter feeds. This reduces the level above 10kHz by 4dB, but unfortunately leaves the mid-treble a little too high in level. As the individual drive-unit responses in fig.3 suggest, it looks as if the midrange unit is taken a little too high in frequency to achieve an optimal match to the treble horn. But in rooms smaller than mine (ca 26' by 15' by 7' 10"), where the tweeter level will be more difficult to optimize by experimenting with speaker toe-in, it might well be worth experimenting with a series resistor.
Turning to the time domain, the High Violoncello II's step response on the tweeter axis (fig.7) reveals that both the horn-loaded drivers are connected in inverted acoustic polarity, the woofers in positive polarity. The tweeter's output is the sharp down-up-down step, followed about 200µs later by the downward-going step of the midrange unit. The positive-going woofer outputs follow; the decay of each drive-unit's step smoothly blends with the rise of the step of the unit next lower in frequency, which suggests good integration of their outputs in the frequency domain on this axis. The cumulative spectral-decay plot on the tweeter axis (fig.8) is very clean, but with some low-level hash present in the region covered by the tweeter.
Fig.7 Acapella High Violoncello II, step response on tweeter axis at 50" (5ms time window, 30kHz bandwidth).
Fig.8 Acapella High Violoncello II, cumulative spectral-decay plot on tweeter axis at 50" (0.15ms risetime).
Other than its very high sensitivity, there is little sign in its measured performance that Acapella's High Violoncello II uses horn-loaded drive-units. Its sealed-box woofer alignment is tuned for clarity and definition rather than ultimate bass weight, which is presumably why it benefited from being used with the tubed Audio Research amplifier, which hung a bit more low-frequency meat on its bones.John Atkinson