The Celestion SL-600, first of the luxury compact monitors
Earlier, I wrote about the ProAc Tablette mini-monitor. This was a tiny speaker that used generally tried and true technology to produce a fine miniature loudspeaker. A bit later, Celestion came up with a completely different approach to the small monitor, the SL-600. The difference was the technology that was brought to bear. The acquisition of Laser Interferometry equipment allowed Celestion’s designers to examine the action of drivers and what was discovered was that most speaker drivers operated in “break-up mode” through significant parts of their ranges where different parts of the diaphragm were moving independently rather the ideal of “pistonic action”, that is, the driver moving as one piece.
This resulted in development of all new drivers for the SL-600 (and its little brother, the SL-6). The woofer looked fairly conventional, but it did benefit from the new measurement technology. But the really obvious thing was the tweeter. It sported a copper (!) dome and did indeed act much more pistonic than typical domes of the day, at least up to about 19 Khz, where its fundamental resonance resulted in a large peak. In practice, a notch filter was introduced into the crossover to remove this. The copper dome also had another effect. Since it was relatively heavy, it lowered the sensitivity of the tweeter. Enough that, had the woofer sensitivity been lowered to match, the system would have been almost impractically inefficient. So what was finally accepted was a tweeter that was around 2 db down from the level of the woofer. Keep this in mind for later.
For the premium SL-600, there were more tricks up Celestion’s sleeve, mainly involving the cabinet. It was always known that resonances in speaker cabinets had an effect on the sound, but around this time speaker designers started to seriously attack this problem. Within a few years, the Wilson WATT would come out which used methacrylic material to deaden the cabinets and cabinets generally became thicker, heavier and better braced.
Well, Celestion took a different tack. Unlike the SL-6, which employed a more normal MDF cabinet, they decided to design a LIGHTER yet still stiff cabinet using a metal honeycomb material called Aerolam, commonly used in airplanes. The idea was that the material would not store and re-release energy and what resonance they had would be high enough in frequency to be out of the most critical range. They also employed mounting plates to attach the drivers to the front baffle, further stiffening the cabinet. These plates were serrated vertically with small channels to break up sound diffusion off their surface. All in all, it must be said that they started this design off with a clean sheet of paper. So how does this speaker sound today and how successful was its design in retrospect?
One thing the SL-600 was always celebrated for was its disappearing act and disappear it does. The image is vertically challenged as are most small speakers (though not as much as the original ProAc Tablettes), but, beyond that, the images are quite holographic. Mainly, I suspect, because the boxes radiate almost no sound from themselves to confuse the imaging issue. These cabinets also do not have serious resonances that color the sound of the drivers in tonal terms, either. And this results in a clarity through the midrange that is good even by current standards.
But there is another factor, controversial at the time, less so today, that factors into the sound of this speaker. That speakers should not measure dead flat in the treble is a commonly held view today, but back in the ’80’s this was not so much the case. Remember the less sensitive tweeter we talked about earlier? Well this resulted in a response tilted down somewhat from the mids into the treble. So you had a speaker that sounded less bright and more velvety than the norm of the day due to this tilt (and truthfully, by the lack of break-up artifacts in the Celestion tweeter that most tweeters added that contributed spurious treble energy to most conventional speakers). Many listeners of the day rejected the SL-600 for this characteristic, feeling they were slow and dark. Ironically, that “problem” with the tweeter resulted in a speaker more in keeping (in my view) with a natural tonal balance on the largest variety of recordings.
Of course, there are problems here, the worst of which involves the bass/midbass. It’s extended enough for the cabinet size, but sounds a bit vague, as though the woofer runs out of low-distortion steam as you go into the mid-bass. This has the effect of making a discontinuity in the sound, instruments whose fundamentals are higher sound very clear while the lower voiced instruments can sound less so. This is not a world-shattering effect, but it’s there and the louder you play the SL-600, the more this effect can be noted.
Due to the small size of the drivers and the cabinet, dynamics are good rather than great. Here it’s hard to single out the SL-600, most small speakers suffer from this. But some newer designs its size do somewhat better in this area. There is also a bit of a sense of hollowness to the sound, I suspect from the residual cabinet resonances. This is not a severe effect, fortunately. And there is not quite the silken beauty of the best of today’s drivers here.
The intent of the SL-600 at the time was a no-holds-barred assault on the ideal small monitor. It retailed for a high price for its time, but, especially in the context of the mid ’80’s, it was pretty much a successful design. And if you aren’t too troubled by its modest failings, this is still excellent performance in many ways for a small monitor speaker even by today’s standards. Of course, if you are willing to go for the throat, you can certainly outdo the SL-600, but not for near their price on the current used market…