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Celestion SL6 Speakers

Article from Home & Studio Recording, July 1984

The SL6 is on the small side for a 'serious' speaker, costs around £250 per pair, and has been well regarded in the domestic Hi-Fi scene for around 2 years - that's a long time in such a fickle business. Put succinctly, the design philosophy of the SL6 is along the lines 'OK, we know a 2 way speaker of small dimensions has its limitations. But let's take the strong points, and refine and polish them until they outshine the weak areas'. The means to this was via some intensive research on speaker cone behaviour, using optical interferometry, as opposed to the intuitive, cut-and-try methods by which many of the best speakers have arrived.

The inference here is that when manufacturers boast about amazing 'scientific' gizmos used to develop a product, there's often something badly wrong. Musicians are, of course, above average at sensing hype - one of my colleagues won't even consider a product if the much abused word 'scientific' appears in the advertising rubric! But the SL6 is one instance where sensible premises ('The first step towards producing an accurate loudspeaker is to secure the simple and controlled behaviour of each driver'), and high technology research really do seem to have paid dividends.

Cone Break-Up

As remarked in previous HSR speaker reviews, many of the dominant errors in sound arise when the cone breaks up. No! This has nothing to do with actual damage to the drive unit; it's jargon for the cone's misbehaviour when it ceases to act as a piston.

Let's try an analogy: Grab a large book and waft it up and down. It's stiff, and behaves like a piston, not bending or vibrating, despite imparting violent vibration to the surrounding air. This is the ideal behaviour. But imagine doing this at 10 kHz (that's 10,000 times a second) - it's simply not possible. However, 10 Hz (10 handshakes a second) is feasible if you're fit and healthy. But not with a big book - it's too tiring. So rest, then pick up a thick sheet of paper, and wave it up and down at 10 Hz. Of course, it waggles uncontrollably - it has break-up.

Did you also get the paper to develop a regular harmonic motion? This is a break-up mode - at which point the cone vibrates powerfully and spuriously over most of its surface. The result of cone break-up is colouration concentrated on a series of spot frequencies, and break-up is one of the principle obstacles to attaining a flat frequency response in speakers.

Apart from demonstrating the nature of cone break-up, our paper demo also shows us why the problem arises in the first place. Like you and I, the drive unit is not infinitely energetic, and if it's to reproduce sound at 10 kHz (say), with any sort of liveliness, the cone must be lightweight. And that, in general, means thin materials which are more prone to break-up.

Preventing cone break-up isn't easy, because you can't see it directly, and meaningful vibration measurements over the whole area of a cone-shaped surface aren't really feasible. And because you can't easily measure it, there's little in the way of data which can be turned into useful, predictive models. Although cone break-up is all too visible in frequency response plots, these don't reveal how best to eliminate it.


Celestion's work therefore was initially concentrated on techniques to make breakup modes visible in a meaningful sense, in the real-time. Using optical interferometry (a case of shining light at the cone and deriving beat frequencies) Celestion were able to view the cone's behaviour on a VDU. It's all rather like seeing an illuminated cross-section of a speaker cone in operation under a microscope, with the flexure frozen by a strobe light.

Using this tool, Celestion went on to improve the design of their cone, noting the weak areas - the joints at the dust cap and around the edge, the coil terminations, and the shape of the cone. In case you didn't know, the raison d'etre of the cone shape is that it's the best way to stiffen flexible materials like paper; and Celestion went on to seek out the profile which offered the best rigidity and behaviour at high frequencies.

The results of all this experimentation are clearly visible. If you examine the SL6's 6½" bass/mid driver, you'll notice that there's no sign of voice coil terminations - these are kept well away from the cone's surface. Also, the dust cap is concave, and like the surround, the mating between the surfaces is very smooth, rigid and lacking in discontinuities. Put simply, the cone area overall is more integrated, and it doesn't display the fact that it's really just a collection of bits (cone, surround, spider, dustcap and voice coil) all glued together, as readily as most drive-units. Another point about the bass/mid driver is that it operates in the ordinary 'infinite baffle' mode, in an airtight enclosure; Thiele loading has been eschewed.

The tweeter went through the same development programme. Here, an ultra-thin aluminium dome with a compound radius was found to behave most like a pure piston, and by careful tuning, the main break-up mode was placed just above 20kHz, out of harms way.

The dome fabrication also incorporates the voice coil former. More often, the two are glued together, but here, they've been made from a single piece of metal, which obviously limits the extent to which mechanical disparities can occur. Another remarkable feature is the tweeter's voice coil diameter, which is equal to the bass/mid driver's, and imbues the tweeter with a massive 50 watt power handling capacity (PHC), apparently on continuous sine waves. But remember, this healthy PHC means little if you drive your amp into clipping!


In a word, the SL6's exterior is exquisite. Designed by Allen Bothroyd, it combines a rugged utilitarian appearance with aesthetic finesse. Or at least, that's my own reaction. But we can be fairly certain that the SL6s will look like they mean business, whatever the surrounding decor and support equipment.

The snap-on grille is attractive enough for it to be left on, but whilst Celestion recommend its removal for serious listening you may feel that hearing two drive units as one speaker is easier without distracting visual clues. In fact, the SL6 is one instance where the auditory sensation of integration overrides the visual perception of two drivers. And it's bigger sister speaker, the model SL600, doesn't even bother with a grille.

Both the drivers are secured with Allen-head (hex) bolts. Being of rugged, high-tensile steel, these can be tightened down hard, so the absence of locking washers is no disgrace. Again, both drivers are surrounded by fluted alloy sections, which at first sight are purely decorative. These are intended to overcome diffraction effects, ie. perturbations when the edge of the emergent sound wave interacts with the cabinet's front edge.

Inside, all connections are soldered, which is par for the audiophile course, if not exactly convenient when you need to replace a driver in a hurry. Failing this, it's a good move if you distrust connectors. The crossover builds on the signs of meticulous design found so far. For instance, it elegantly screws down directly onto the input terminals with eyelets on the PCB, to ensure a positive electrical connection. These fixings have the vital lockwashers, by the way!

The two capacitors in series with the tweeter are polypropylene types, which is a worthwhile feature when so much effort has gone into developing a tweeter free from break-up. Just to remind you, polypropylene capacitors can enhance bite, attack and transparency in the treble, especially on complex percussive sounds. Both crossovers have -12dB/octave sections, not an ideal choice on paper, but one that occasionally works well in practice. In fact, Celestion have staggered the cross-over points for best integration between the drivers.

Aside from the crossover per se, there's a filter with a steep slope which 'takes out' the response peak just above 20kHz, where the tweeter's first break-up mode lies. In addition, a series filter deals with the main resonance below the 2-3 kHz crossover point. This mitigates in part for the -12dB/octave slope, which would otherwise be too shallow to make the resonance inaudible.

Around the back, the 4mm terminals/binding posts are of above average quality, and will accept '14 gauge' wires. In practice, this makes it easier than usual (but still awkward) to thread stranded wires through the terminals, the problem being in the recess. As these speakers are designed to be used well away from walls, the recess is totally unnecessary, and a lot of tiresome fiddling, interspersed with cries of 'got it!' could be avoided if the terminals were mounted flush. As the recess also, is the only part spoiling the overall aesthetic refinement, Celestion would do well to flush mount the terminals on these grounds alone.

Celestion SL6 frequency response plot.

The Sound

The SL6's response is not as flat as some domestic Hi-Fi speakers in their class, but what colouration there is does no harm. Nor do Celestion try to hide anything. Indeed, quite the reverse; with every pair of SL6 there comes a pair of individually traced response curves.

The main problem area is in the low midrange, which is slightly muddy, and certainly congested. The harmonics on some of the bass notes are indistinct because of this, on bass drums especially. This is principally a result of the cabinet resonances we looked at earlier (rather than the rise in response around 200-500Hz, evident on the supplied graph). Incidentally, the SL6's sister, the SL600 overcomes the cabinet defects, but costs around twice as much.

Surprisingly, the upper-bass defects do not detract from the low bass response, which is unusually accurate and detailed. To be honest, most UK speaker manufacturers have difficulties with their Thiele alignments, particularly with regard to attaining adequate power handling on Rock material below 130Hz. So Celestion's choice of 'infinite baffle' loading has a lot going for it, and is, in part, responsible for the superb bass articulation.

Another factor is the unusual smoothness of the bass roll-off, which is almost exactly -9dB/octave, and looks like it's been taken from a textbook! Note also that roll-off begins early, around 100Hz. The absence of the usual bumps and humps undoubtedly assists our hearing mechanism, as it sets about equalising the deficiency; the smoothness of the curve means it can do this more readily, and accurately.

In effect, the ear synthesises the fundamentals from the (accurately reproduced) harmonics, so you will still 'hear' low bass on the SL6s, even if there's no great physical sensation. Indeed, the low bass is surprisingly solid. And there are hidden advantages: The 'minimalist' bass of the SL6 doesn't aggravate resonant modes in the room, so the bass stays clean, making it particularly easy to analyse the sound of individual bass instruments, even deep, synthetic bass.

In the mid range, the sound again is detailed, highly revealing of recording techniques, and slightly 'bitey'; there's certainly no lack of articulation or incisiveness on vocals, but the slight suckout at 2-5kHz obviates any 'nastiness'. The top-end sound is silky, and the rising response above 15kHz has no apparent ill-effect. Better still, the tweeter never sounds disembodied, even close up - it's integration with the mid driver being very good indeed.

Powerful Stereo Image

This leads us to stereo imaging, which is the SL6's keynote. For our tests, they were sited in the middle of the room, about 5 feet apart, and 4 feet from the listening position, to correspond with a medium sized mixing console. Also, for the best image, we found it best to set the height of the enclosures so that the tweeters were off-axis, slightly above the ears - around 9" higher say. This appears to be against all the rules, but it suggests that with the SL6s, it's more important to have the mid/bass driver on axis to (ie. level with) the ears, than the tweeters.

I will now describe stereo imaging qualities typical of electrostatic or horn-loaded monitors, like my own Turbosound TMS4s:

1) The speakers 'disappear' - or at least, aren't perceived as the sound source.

2) The central image focuses tangibly close to the forehead; in some instances, you may feel it foreclose on the space occupied by your own head, vaguely similar to listening on headphones, but far more spacious, and 'real' (for want of a suitable adjective).

3) The boundaries of the room, as perceived by the ears, are altered radically, according to the recording techniques.

4) The soundstage covers about a 270 to 300 degree solid angle; sound can come from behind your shoulder, from above, or even below.

To many readers, these qualities will sound bizarre, and experiencing real stereo for the first time can be 'startling', 'disconcerting' or 'amazing' - these are quotes taken from listeners and reproduced in the SL6's manual, under a section entitled 'What you will hear'.

The SL6s possess all the qualities enumerated above, if not quite to the same extent as more exotic phase-aligned, constant directivity speakers. Despite this, they can be regarded as an essential tool if you want to fully exploit the stereo picture, and need to hear a potent, well-focused 3D image, or simply as a means of exploring the less well known realms of the stereo medium. You will, for example discover that only around 10% of rock records possess credible stereo ('Is this something to do with the monitors used in the recording?' we ask ourselves!), and that the most powerful or uncanny imaging is associated with specific musicians, producers and cutting rooms.


Regular readers will know that speakers reviewed in HSR are frequently criticised in respect of their sensitivity and/or power handling. The reason is that high SPLs - and also clarity at high levels - are both pretty essential when we come to scrutinise the music. The SL6s point in a different direction though: their sensitivity is at an all time low, at -82dB @ 1 watt @ 1 metre. So, in plain English, 100 watts on the SL6 is as loud as 10 watts on a Tannoy Stratford or EV Sentry, or 500mW (½ watt) on a Turbosound TSM4 module!

But oddly, this doesn't seem to matter in the least. To begin with, the SL6s work well close up, so you can make the most of their limited SPL capabilities (it's about 104dB at maximum) by sitting close up; even at a distance of two feet, the sound integration remains excellent.

Secondly, the SL6s sound loud, and in any event, the sound is sufficiently detailed for you to hear all you need without high SPLs. Indeed as you approach the maximum SPL, the revelation of slight shortcomings in recording techniques is ruthless, and you may hear more than you want for your peace of mind. In short, the SL6s could send you into a frenzy of remixing! Lastly, the tweeter's power handling capacity is excellent.

For our tests, we initially used a Turner B502 power amplifier, rated at 190 watts into 8 ohms. Later, we switched to the Yamaha PC2002M. This has peak power metering, so we could establish the drive level. Both these amplifiers were partnered with a mainframe version of AMP-01, an analogue control unit, which is virtually direct-coupled (ie. no bass roll-off at all), so the speakers see wholly unfettered low-bass energy. As a result, cone bottoming usually defines the maximum SPL, and the maximum drive energy in the signal.

For example, with Funk material (Brothers Johnson, say) the SL6's limit was around 100 watts, whereas levels up to 300W could be accommodated without any obvious, audible compression - or other stress - on material which emphasises strings and vocals as opposed to bass synths! So as usual, provided you heed warning signals, namely the sharp sound of the cone bottoming, these speakers will mate happily with amplifiers in the 200W to 300W region.


The SL6s score highly on the grounds of their stereo imaging properties alone. They're also sympathetic to all forms of rock music. Indeed, it wasn't possible to find any musical style which the SL6s didn't get along well with. They make ideal monitors if your musical output is stylistically across-the-board, and if you're serious about working in the stereo medium, and are seeking clarity and detail (especially in the low bass, the mid and top end; you'll have to have another set of monitors for sussing out the low midrange - that's the only snag!), rather than the physical and emotional manifestations of low bass at high SPLs.

In any event, the all-powerful stereo imaging helps explain why the sound emanating from the SL6s is so much 'bigger' and subjectively louder than you'd expect from such a small box. In their manual, Celestion take the hitherto unprecedented step of describing not only 'what you will hear', but also listing records which will display the SL6s capabilities to the full. But failing this, perhaps try 'Let's start the dance III', on the Bohannon Drive album. If you survive the experience, who knows, we might see a flood of albums and tapes with stereo being taken more seriously than hitherto; it's high time it was.

The Celestion SL6 speakers retail at £279.00 including VAT (per pair).

Further information from Celestion International Ltd, (Contact Details).

Celestion SL6: Shortform Specification

Power handling capacity 120 watts (300 watts)*1
Relative LF response -3dB @ 75Hz, -6dB @ 60Hz
Sensitivity 82dB SPL @ 1 watt @ 1 metre
Impedance 8 ohms*2
Dimensions 370x200x255 mm (HxWxD) (14½"x8"x10")

*1: Figure in brackets is the short-term programme power.
*2: There are no significant impedance dips.

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Goldline Music 10 Real Time Spectrum Analyser

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Uher CR160

Publisher: Home & Studio Recording - Music Maker Publications (UK), Future Publishing.

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Home & Studio Recording - Jul 1984

Donated & scanned by: Mike Gorman

Gear in this article:

Monitors/Speakers > Celestion > SL6

Gear Tags:

Hi-Fi Speakers

Review by Ben Duncan

Previous article in this issue:

> Goldline Music 10 Real Time ...

Next article in this issue:

> Uher CR160

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