Celestion System 6000 loudspeaker system Measurements

Sidebar 2: Measurements

A lab check on the controller (fig.1) provided proof of the somewhat different view that Celestion's designers have taken of system equalization. Taking the upper range or high-pass filter first, this shows a perfectly flat response with a crossover point at 100Hz, -6dB, falling almost precisely at a 24dB/octave rate, a 4th-order Butterworth. With no additional programmed extension, the lowest "70Hz" bass response setting is plotted lower in level than the high-frequency section, due to the high intrinsic sensitivity of the bass driver section, typically 88dB/W. This level offset gives a less than ideal graphical picture of the 100Hz crossover overlap. At -6dB, 100Hz, and in accordance with theory, the bass system also shows a rolloff approaching 24dB/octave, its rapidity beneficially muting the big cones in the midrange, where they might otherwise add coloration.

Fig.1 Celestion System 6000, high- and low-pass crossover responses, effect of extension control with level control set to "5" (5dB/vertical div.).

Note that no bass lift has been applied at all for the 70Hz limit. Indeed, given the -6dB level requirement to match the bass to the SL600, the subwoofer is, in fact, well under-driven, giving it a tremendous headroom on this and other similar settings down to 40Hz.

At extension settings below 70Hz, the bass boost begins to build. In the context of the 82dB/W SL600 sensitivity, this boost reaches +3dB at 20Hz for a 30Hz extension, and +4.5dB at 20Hz for a "20Hz" extension.

These are comparatively modest boost levels, allowing the use of a realistically sized bass amplifier. With lowering bass-cutoff frequency, the equalization slope approaches a maximum of 6dB/octave, while in theory the turnover point at nominally 100Hz should step back a little with each increment in extension. Fig.2 shows the range offered by both the bass-level and extension controls. For example, (a) corresponds to a full bass-level at 20Hz extension. The matching curve (b) for 70Hz indicates that the system will accommodate an upper-range unit with up to 10dB more sensitivity, or cope with similar sensitivity variations between the power amplifiers.

Fig.2 Celestion System 6000, high- and low-pass crossover responses, effect of level control with extension set to "20" and "70" (5dB/vertical div.).

Though the level control will go to zero, the "5" position was chosen to generate the second set of graphs. In fact, the level control range is, in my view, excessive, leading to cramped adjustment above "7." I would like to see a more open range—say from -12dB relative to the SL600 setting up to +6dB; thus catering for even the most unlikely amplifier/speaker combinations.

Distortion results for the controller lay in the -90dB region (0.003%), and at an 8V peak-peak output, HF intermodulation measured -99dB, almost 0.001%! The controller provides a voltage gain of 6dB with a 100k-ohm input impedance, and a low 10-ohm output impedance. A CD player with a volume control could be fed directly into the 6000 controller, without the need for a preamp at all. Electrical noise levels were very low; for example, -111dBA referenced to a IHF 0.5V output. Hum levels were better than 85dB down.

The Celestion forward responses at 1 meter (fig.3) relate to the axial responses shown in fig.4. As with the KEF, which has essentially similar dimensions and head geometry, the forward group of on- and off-axis responses is excellent. The well-behaved group of curves is, in large measure, responsible for the excellent stereo and neutral balance shown by both systems.

Fig.3 Celestion System 6000, responses on- and off-HF axis at 1m (5dB/vertical div.).

Fig.4 shows the third-octave axial response, noting the tapered room-matched bass rolloff, with good extension to 25Hz; the "rich," mildly downtilted response, and the slight lift in the treble around 5kHz (the "sibilance region"), are also apparent. Even under acoustically difficult semi-anechoic room conditions, a fine ±2.5dB was achieved in third-octave averaged mode from 40Hz to 16kHz.

Fig.4 Celestion System 6000, 1/3-octave response on HF axis at 1m (5dB/vertical div.).

Computer-averaged in-room, the directional woofer was tried in the straight-ahead position (fig.5), with full extension and a little bass cut, (ie, "level" set 1dB too low). The curve demonstrates good extension, but was muted, especially around 100Hz. Lifting the bass-level, however, was found to impair the upper-bass definition.

Fig.5 Celestion System 6000, spatially averaged, 1/3-octave, in-room response with woofer straight ahead, extension set to 20Hz, and level set to -1dB (5dB/vertical div.).

Celestion recommends a position with the 600 angled in by 30 degrees and the woofer axes angled in by 15 degrees. A setting of 7.1 was determined for the controller level. This resulted in the curve shown in fig.6, a trace with an extraordinarily even low-frequency response, and a subjective sound to match. At times I preferred a touch more bass "weight," achieved finally in fig.7 by increasing the bass level to 7.25.

Fig.6 Celestion System 6000, spatially averaged, 1/3-octave, in-room response with woofer angled in by 15 degrees, extension set to 20Hz, and level set to 7.1dB (5dB/vertical div.).

Fig.7 Celestion System 6000, spatially averaged, 1/3-octave, in-room response with woofer angled in by 15 degrees, extension set to 20Hz, and level set to 7.25dB (5dB/vertical div.).

As these curves show, the 6000 integrates very well with my room acoustic. The directional bass response does seem to produce a uniform, clean, transient quality, in my listening area at least. In terms of room matching, this speaker was one of the best I have tested at my location. I look forward with interest to the results from other reviewers in different rooms.

Concerning the other test parameters, the System 6000 does have a low system sensitivity of 82.5dB/W, but this represents quite a uniform and fairly kind 8-ohm load impedance, both for the woofer and head sections. The only warning concerns the SL600 load impedance at near ultrasonic frequencies, where it becomes predominantly capacitative at around 4.5 ohms. One or two amplifiers with a poor stability margin might be uncomfortable with it.

With a 200W peak program as an acceptable maximum input and 75W as a sensible minimum, the 6000 will deliver maximum sound levels of 101dBA in typical rooms measuring 80m3—comfortable (3.5dB louder than the Duetta II), but neither deafening nor earthshaking.—Martin Colloms

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