Celestion SL600 Speaker System Review

The Celestion SL600 is a near-twin to the company’s SL6, a speaker whose introduction a couple of years ago aroused considerable favorable comment in the audio world. Although the two speakers use the same two drivers and crossover networks, and although they are almost exactly the same size, the SL600 costs nearly twice as much as the SL6. Why is this? Because the SL600, in design and execution, is truly a “high-technology” loudspeaker system.

For example, the cabinet material is really exotic: an aluminum honeycomb sandwich structure called “Aerolam,” which was originally developed for aircraft floors. Aerolam is characterized by very light weight and high stiffness, both desirable characteristics in a loudspeaker cabinet. Such properties work to eliminate the resonances unavoidable in wooden cabinets, resonances that Celestion engineers believed imparted some slight coloration to the sound of the SL6 (and, for that matter, to any speaker using conventional wooden cabinet constructions).

Aerolam’s honeycomb sandwich, like any structural material, does have resonances, but because of its high stiffness-to-weight ratio these occur at much higher frequencies than in a wooden cabinet and are less likely to be excited by the woofer in its frequency range. The inside of the SL600 cabinet is also treated with acoustically absorbent material to damp most of the remaining cabinet-wall resonances, which lie in the range of 1 to 3 kHz.

The driver design is also unique. For one thing, the aluminum tweeter dome is integral with its voice-coil former, eliminating any possible decoupling effects from conventional glue joints and providing efficient conduction of heat from the voice coil. The woofer cone, made of Kobex PVC plastic, is molded with an integral plastic center cap, replacing the usual glued-on dustcap and providing a single point of attachment for the voice-coil former. One benefit of this unconventional driver design is said to be very high power handling ability. The SL600 is rated for use with up to 100 watts of program material and may be used safely with amplifiers rated at 100 watts or even more if reasonable care is exercised in the choice of playback level.

The exterior of the cabinet is finished with Nextel, a dark gray paint with a smooth but velvet-like texture. The drivers are mounted flush with the metal front board of the cabinet, and no grille is used, though the tweeter is protected from prying fingers by a metal screen. Mating banana plugs are furnished for the connection jacks, located in the rear of the cabinet, and a pair of wooden stands are supplied with each pair of speakers, supporting them 18 inches above the floor. The hollow upright posts of the stands are meant to be filled with sand to damp their mechanical resonances.

The SL600 is a compact speaker measuring 14-1/2 x 8 x 9-1/4 inches and weighing only 11-1/4 pounds. No level balancing or response adjustments are needed or provided. Price: $1,500 per pair, including stands.

Lab Tests

Our quasi-anechoic frequency-response measurements, made with an IQS FFT analysis system, agreed quite well with the swept-sine-wave frequency-response curves furnished by Celestion for the test units. However, our measurement showed an increased output in the mid and upper bass. At a 1-meter distance (with on its stand) the output varied only 7 dB overall from about 3,000 to 22,000 Hz. Through the most audible part of that range (up to 11,000 Hz) the total variation was only 3 dB.

A notch at the 2,300-Hz crossover frequency was visible in our measurement (and, at a reduced amplitude, in the Celestion curves). Other variations, including a small ± 1-dB ripple in the 3,000- to 6,000-Hz range, and the tweeter resonance at 20,400 Hz were also present in our measurements exactly as they were in the Celestion curves. The tweeter resonance, incidentally, is not damped in the speaker driver, but is equalized out by a notch filter that forms a part of the crossover network. The filter’s effectiveness can be seen in the small residual rise in output at the tweeter resonance frequency (only about 2 dB).

Our averaged room-response measurements conformed reasonably well to the anechoic measurements, though narrow-band effects such as the crossover notch were greatly reduced in amplitude. Our close-miked woofer-only response curve showed a maximum output between 100 and 150 Hz, falling off about 6 dB from there to 1,500 Hz and at a 12-dB-per-octave rate below the woofer resonance at 65 Hz. We used the FFT data to aid us in splicing this curve to the averaged room-response curve, resulting in an overall 8-dB variation from 500 Hz to more than 20,000 Hz and a further rise to a maximum of +4 dB at 120 Hz. The speaker sensitivity was moderately low, though slightly better than rated.

Our bass distortion measurement was made with a drive level corresponding to the midrange 90-dB SPL level, or a constant drive of 5.7 volts. It was most impressive to see (and hear) how the diminutive woofer delivered a solid bass output with less than 1 per cent harmonic distortion down to about 75 Hz, and a gradual rise to only 7 per cent distortion at 30 Hz. The group delay varied only about 0.2 milliseconds between 3,000 and 20,000 Hz. The tweeter’s dispersion, despite its relatively large diameter, was quite good. The response curves measured on axis and 30 degrees off axis began to diverge above 7,000 Hz, but the difference between them did not exceed 6 dB up to about 15,000 Hz.

We checked the peak power handling ability of the speaker (an important consideration, given the small size of its drivers) with a tone-burst signal of one cycle “on” and 128 cycles “off.” The test frequencies were 100 Hz, 1,000 Hz, and 10,000 Hz. The drive level was increased until definite waveform distortion was visible either in the acoustic output of the speaker or in the amplifier output signal. The corresponding drive level, expressed in RMS volts (not “peak,” which gives a doubled power figure), was used with the actual measured impedance at that frequency to compute the power input. Since the impedance is not necessarily a pure resistance, this is not a true power, but it indicates how much drive level the speaker can handle. At the three test frequencies, the measured limits were 200 watts, 140 watts, and 430 watts. But only during the 100-Hz test was acoustic distortion observed, the other measurements being limited by amplifier clipping.

Comments

On first hearing the Celestion SL600 speakers, and before making any measurements, we were aware of their smoothness, mingled with a surprising sense of warmth. That warmth is one of the most unexpected qualities of this speaker and stems probably from the slight elevation in upper bass response. It even sometimes tends to sound heavy, especially when reproducing male voices. The generally downward sloping (with increasing frequency) overall room-response curve gave it a soft quality, not really lacking in highs but completely without the shrillness that mars the sound of many small speakers.

The imaging qualities of these speakers have received much favorable comment, particularly in the British audio press. Although at times we heard a quality of depth that seemed less obvious with other speakers, this effect was not apparent with most program material. To us an additional octave or so of low bass would be preferable to the possibly superior imaging qualities of the SL600, but that is a purely personal reaction, and everybody will have his own ideas on the matter.

We were quite impressed by how well the SL600’s handled very high power inputs. They performed well during our tone-burst measurements, but we were even more convinced by the way that they could be played about as loud as we could tolerate (with frequent peaks of 350 watts or more) yet give almost no sense of strain or compression. Nevertheless, in view of their cost, we were reluctant to push these speakers to their utmost, preferring not to risk their destruction!

Attempting to judge the rigidity of the aluminum honeycomb cabinet by the usual knuckle-rapping test produces an effect much like that of rapping a concrete block. Certainly we have never felt such an inert, solid-seeming enclosure with any other speaker in our experience. It does not feel in the least like metal, and it is definitely not like wood; solid concrete or stone are the materials suggested by this test—until one picks it up (an 11-pound concrete block?).

Celestion claims significant audible advantages for the SL600 compared with its sibling, the SL6. Since we did not have the SL6 on hand for comparison, we cannot comment on that. Judged by itself, however, the SL600 is obviously a very fine speaker, with a host of virtues and few faults.

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