Kharma Midi-Grand Ceramique 1.0 loudspeaker Measurements

Sidebar 3: Measurements

At 87.5dB/2.83V/m, my estimate of the Kharma Midi-Grand Ceramique 1.0's voltage sensitivity was somewhat lower than the manufacturer's specification of 91dB. Partly this will be explained by the speaker's mid-treble frequency response (see later). Its impedance (fig.1) doesn't drop below 3.5 ohms and is almost always above 4 ohms, which, in combination with the generally mild phase angle, will reduce the stress on the partnering amplifier. As indicated by the saddle in the magnitude trace in fig.1, the tuning of the large rear-facing port lies at 23Hz, implying good bass extension.

Fig.1 Kharma Midi-Grand Ceramique 1.0, electrical impedance (solid) and phase (dashed). (2 ohms/vertical div.)

The traces in fig.1 are free from the small discontinuities that would hint at the presence of panel resonances. The bell-like ringing MF noted in his auditioning and that I heard for myself will probably be too high in frequency and in Q to have a subjective effect. However, investigating the behavior of the Kharma's enclosure with a simple accelerometer did reveal a very strong resonant mode at 387Hz (fig.2), which, all things being equal, should have had audible consequences.

Fig.2 Kharma Midi-Grand Ceramique 1.0, 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.)

Mikey Fremer had a problem with the Midi-Grand Ceramique's low frequencies, finding that "the bass lacked solidity, punch, and definition. Extension measured strong below 30Hz, but it didn't sound strong." Fig.3 shows what's happening below 1kHz with the Midi-Grand's midrange (red trace), woofer (blue), and port (green). The midrange appears to extend extraordinarily low in frequency, not crossing over to the woofer until 90Hz or so, and with fairly slow, 12dB/octave high- and low-pass filter slopes. The woofer's minimum-motion point coincides, as expected, with the peak in the port's output between 20Hz and 30Hz, and neither has any unexpected peaks in its output above its passband. However, with the level of the port's response scaled to the woofer's in proportion to their radiating diameters, the port's output does appear to be rather lower than is required to fully extend the speaker's response.

Fig.3 Kharma Midi-Grand Ceramique 1.0, acoustic crossover with nearfield responses of midrange unit (red), woofer (blue), port (green), and their complex sum (black).

The result is that the overall sum of the three drivers' outputs (black trace, which takes into account acoustic phase and the physical separation of the radiators) starts to roll off a little early—below 60Hz—though the rolloff slope is closer to 12dB/octave than to the 24dB/octave typical of a ported design. Note also that all the traces in fig.3 feature an approximate 3dB low-frequency boost due to the nearfield measurement technique. Overall, it appears that the Midi-Grand Ceramique's low-frequency tuning has been optimized for the speaker to be used close to the wall behind it. Moving the Kharma out into the room will therefore, as MF found, make it sound too lean.

This nearfield 3dB rise at low frequencies can also be seen in fig.4, which splices the complex sum of the individual drive-units (fig.3, black trace) to the farfield response averaged across a 30 degrees horizontal window on the tweeter axis. The Kharma's output is basically flat throughout the midrange and treble, broken only by a large suckout centered on 5kHz. As mentioned above, the lack of on-axis presence-region energy downrates the Midi's measured voltage sensitivity. In addition, considered by itself, such a suckout will render the Kharma's balance rather distant and uninvolving, with greater than usual soundstage depth. However, a speaker's on-axis anechoic response should never be considered by itself: in all but extremely large rooms, its dispersion will also have a strong influence on the perceived balance.

Fig.4 Kharma Midi-Grand Ceramique 1.0, anechoic response on tweeter axis at 50", averaged across 30 degrees horizontal window and corrected for microphone response, with the complex sum of the responses plotted below 300Hz.

Fig.5 shows the Midi-Grand's lateral radiation pattern. It's hard to interpret, but does show a good uniform spacing of the contour lines, which correlates with the stable, well-defined stereo imaging MF and I heard. Fig.6 simplifies the picture by subtracting the on-axis behavior from each of the off-axis responses. Again, the even spacing of the contour lines can be seen, along with a slight narrowing of the dispersion at the top of the midrange unit's passband. More important, the "horns" at 5.4kHz and 8kHz in the graph show that the large on-axis suckout seen in fig.4 fills in to the speaker's sides. This goes a long way toward explaining why the Midi-Grand didn't sound lifeless and sucked-out, though it should be noted that MF did find the cowbell on Little Feat's Waiting for Columbus album to sound as if it had "an absorptive coating that prevented it from 'popping."' But the larger the room, the more likely that the perceived balance will tend in that direction. This will therefore work against the possibility that the speaker will produce a more optimal bass balance in the bigger room, as Mikey conjectured.

Fig.5 Kharma Midi-Grand Ceramique 1.0, lateral response family at 50", from back to front: responses 90 degrees-5 degrees off axis, tweeter-axis response, responses 5 degrees-90 degrees off axis.

Fig.6 Kharma Midi-Grand Ceramique 1.0, 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.

These suckouts also fill in below the 43"-high tweeter axis (fig.7), but increase in severity if you sit higher than the tweeter. If you stand, you'll be afflicted with a very large suckout at the midrange/tweeter crossover frequency (2.3kHz).

Fig.7 Kharma Midi-Grand Ceramique 1.0, 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.

As MF noted, Kharma makes much in their literature of the phase coherence of their crossover design, but also as MF noted, the midrange unit is connected in inverted electrical and acoustic polarity to the woofer. This can be seen in the speaker's step response (fig.8), which also reveals the tweeter to be connected in positive polarity. What matters, however, is not the polarities as such but how they work with the inevitable acoustic phase shift associated with the crossover filters. In the case of the Kharma, the step response of each drive-unit smoothly hands over to that of the next, which correlates with the good integration between each unit already noted in the frequency domain and commented on by Mikey.

Fig.8 Kharma Midi-Grand Ceramique 1.0, step response on tweeter axis at 50" (5ms time window, 30kHz bandwidth).

Finally, the cumulative spectral-decay plot on the Midi-Grand Ceramique's tweeter axis (fig.9) is clean through much of the range shown, but a strong mode can be seen at 7.5kHz, midway between the two on-axis suckouts noted earlier. I would have thought, all things being equal, that this would add a slight steeliness or wiriness to the speaker's balance at high levels, but I didn't hear anything like this in my own auditioning.—John Atkinson

Fig.9 Kharma Midi-Grand Ceramique 1.0, cumulative spectral-decay plot at 50" (0.15ms risetime).

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