DALI Helicon 400 Mk.2 loudspeaker Measurements
Given that the ribbon supertweeter should be quite directional in its top-octave passband, I measured the Helicon 400 Mk.2 primarily on that axis. The speaker offered a higher-than-average voltage sensitivity, at an estimated 90dB(B)/2.83V/m, though its impedance was fairly low overall, ranging mostly between 4 and 6 ohms (fig.1), with a minimum magnitude of 3.8 ohms at 150Hz. The electrical phase angle is also usefully low, with perhaps the worst-case condition at 100Hz, where a magnitude of 4.1 ohms is accompanied by a capacitive phase angle of –25°. Overall, the DALI speaker should not be a difficult load for the partnering amplifier to drive.
Fig.1 DALI Helicon 400 Mk.2, electrical impedance (solid) and phase (dashed). (2 ohms/vertical div.)
A small discontinuity is just evident between 400 and 500Hz in the impedance traces. Investigating the cabinet's vibrational behavior did uncover a fairly strong resonant mode at 450Hz as well as a couple lower in level (fig.2). The audibility of such modes depends on the area of the affected panel and decreases with increasing frequency, so it's possible that this behavior is benign. However, I do note that Michael noted some sort of overhang in the midrange, which might be the result of this panel behavior.
Fig.2 DALI Helicon 400 Mk.2, cumulative spectral-decay plot calculated from the output of an accelerometer fastened to the center of the sidewall 12" from the base (MLS driving voltage to speaker, 7.55V; measurement bandwidth, 2kHz).
Examined in the nearfield, the rear-facing ports measured identically; however, the lower woofer rolled off gently above 700Hz or so, lying 4dB beneath the level of the upper woofer at 1kHz. The saddle centered on 32Hz in the impedance-magnitude trace suggests that this is the tuning frequency of the ports, and indeed, the summed output of the woofers has a sharply defined minimum-motion notch at this frequency (fig.3, black trace), and the ports' response peaks broadly in this region (fig.3, red). Of greater significance in this graph, however, is the large peak just below 900Hz in the ports' output. This, too, might have contributed to Michael's feeling that the 400 Mk.2 had a "'post-event' additive artifact" evident on solo piano, where there will be limited masking, even though the ports point away from the listener.
Fig.3 DALI Helicon 400 Mk.2, acoustic crossover on ribbon axis, corrected for microphone response, with farfield responses of tweeters (blue) and woofers (black), with the summed nearfield responses of ports (red) and woofers (black) plotted in the ratios of their radiating diameters.
The twin tweeters (fig.3, blue) roll in with what looks like a third-order slope, and the crossover point is closer to 2.7kHz than the specified 3kHz. The woofers' rollout above this frequency is rather uneven, with a couple of low-level peaks evident between 5 and 10kHz. The tweeters' output on-axis is about 3dB too high in level in the top audio octave—Michael did find that the speaker "runs a bit hot on top." Fig.4 shows the DALI's overall farfield response averaged across a 30° horizontal window on the ribbon-tweeter axis. The response is still a little excessive in the top octave, but there is also a slight peak evident in the transition between the midrange and treble. This probably aided the speaker's presentation of details, but might also have contributed to the slightly bright balance, considering the slight lack of energy in the frequency region beneath it. This balance again correlates with Michael's listening impressions. The sum of the woofer and port outputs peaks up by 3dB or so in the midbass; this is almost entirely due to the nearfield measurement technique. The speaker actually offers good low-frequency extension, considering its relatively small footprint.
Fig.4 DALI Helicon 400 Mk.2, anechoic response on ribbon axis at 50", averaged across 30° horizontal window and corrected for microphone response, with the complex sum of the nearfield responses plotted below 300Hz.
The use of woofers with a relatively small diameter means that the Helicon 400 has wide, even dispersion in the lateral plane (fig.5), with no off-axis flare at the bottom of the tweeter's passband. (The off-axis ridge centered on 7kHz is actually due to an on-axis response notch filling in to the speaker's sides.) This well-controlled off-axis behavior correlates with accurate stable stereo imaging. In the vertical plane (fig.6), the ribbon tweeter is not nearly as directional as I had conjectured. A sharply defined suckout develops in the crossover region for listening axes above the ribbon tweeter. As Michael observed, this speaker is best listened to sitting down.
Fig.5 DALI Helicon 400 Mk.2, 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 DALI Helicon 400 Mk.2, vertical response family at 50", normalized to response on tweeter axis, from back to front: differences in response 15–5° above axis, reference response, differences in response 5–15° below axis.
Fig.7 is the DALI's spatially averaged response in Michael's listening room, taken in a vertical rectangular grid centered on the position of his ears. This technique smooths out the contribution of room resonances and generally gives a result that correlates with the perceived balance. As Michael noted, the middle of the midrange is a bit suppressed. While the lows are well extended to below 30Hz, an excess of midbass energy is apparent. The treble rolls off in the top octave due to the increased absorptivity of his room's furnishings in this region. I suspect the speaker's brightness was due more to the slight plateau at the top of the midrange than to the top-octave peak in the on-axis response, which will tend more to add "air" and spaciousness to the sound.
Fig.7 DALI Helicon 400 Mk.2, spatially averaged, 1/6-octave response in MF's listening room.
In the time domain, the 400 Mk.2's step response on the ribbon axis (fig.8) indicates that all four drive-units are connected with positive acoustic polarity. The cumulative spectral-decay plot (fig.9) is generally clean, especially at lower frequencies. However, there is some low-level mid-treble hash evident, this associated with the peaks noted in the woofers' rolloff above the crossover. Some delayed energy is also associated with the small response peaks between 1 and 2kHz noted earlier.
Fig.8 DALI Helicon 400 Mk.2, step response on ribbon axis at 50" (5ms time window, 30kHz bandwidth).
Fig.9 DALI Helicon 400 Mk.2, cumulative spectral-decay plot at 50" (0.15ms risetime).
Its measured performance reveals some areas of strength, but also that the Helicon 400 Mk.2's balance will require careful matching with system and room to get the best from it.—John Atkinson