Focal ST6 Twin6

Focal’s premium ST6 series promises monitoring without compromise.

It’s only four months since I last looked at a new Focal monitor, the Alpha Twin Evo. It seems that Focal’s R&D department is on a roll, because the subject here is another new model, the ST6 Twin6. While the Alpha Twin Evo is very much part of Focal’s entry‑level active monitor range, the ST6 Twin6 aspires to a significantly higher level of performance, and consequently commands a significantly higher price.

The ST6 series, comprising the Twin6 and the smaller Solo6, effectively replaces the previous 6Be series, in which the ‘Be’ denoted the use of a beryllium tweeter diaphragm. And while the ST6 series has lost the ‘Be’ epithet, the beryllium tweeter diaphragm remains.

Like the Alpha Twin Evo, the ST6 Twin6 is arranged with dual 150mm‑diaphragm bass/mid drivers, one each side of the tweeter. A letterbox slot‑style reflex port is located to the side of each bass/mid driver. The ports include some internal and flare profiling that will help with airflow linearity and discourage any organ‑pipe resonance. The Twin6 is actually the same size as the Alpha Twin Evo to within a centimetre or two, yet it’s almost twice as heavy. The extra weight of the Twin6 arises primarily from its far more substantially built and rigid cabinet. The panels are thicker and there’s some more significant bracing inside — not least because, thanks to the electro‑acoustic format demanding that the two bass/mid drivers work independently, the cabinet needs to be divided internally into two separate volumes. The Twin6 also packs significantly more amplifier power than the Alpha Twin Evo, with its variable supply‑rail voltage ‘BASH’ technology amplifiers rated at 70 Watts for each bass/mid driver and 50 Watts for the tweeter. As far as signal inputs are concerned, the Twin6 offers just a single balanced XLR socket. Its internal signal chain is analogue throughout.

The bass/mid driver of the Twin6 is a new design and incorporates a couple of interesting proprietary Focal technologies: the Composite W Sandwich diaphragm, and Tuned Mass Damper surround. The Composite W Sandwich diaphragm comprises layers of a woven glass‑fibre composite with a structural foam inner core. Focal claim that the Composite W Sandwich has advantages over other composite diaphragm materials by providing a superior compromise between light weight, rigidity and damping. This may well of course be the case, but as with all drivers, success in performance terms is always more about the complete package than it is about one particular aspect of design or material choice. And as if to illustrate that, the second notable technology of the Twin6 bass/mid driver, the Tuned Mass Damper (TMD), while visually more subtle than the obviously unusual Composite W Sandwich, is perhaps just as significant.

TMD comprises a couple of strategically positioned radial ridges in the roll surround of the driver that control its resonant behaviour by damping energy in the 1‑3 kHz band. The radial rings are described as tuned mass dampers because they work by adding mass at a particular location in the surround to effectively set up anti‑resonant nodes that dissipate energy and stop it reflecting back into the diaphragm. Focal’s development of the TMD surround is a good example of the value of the kind of computer‑based dynamic system modelling that’s become available to speaker designers over recent decades. The problem of roll surrounds mis‑terminating diaphragm energy, and the resulting subjective signature, has been appreciated for many decades. However, before dynamic computer modelling came along, the only way to fix the issue was through trial and error, and often the generous application of flexible damping compounds, or through more heavily damped surround materials, all of which bring their own problems. Dynamic modelling of the surround behaviour will have enabled Focal to position the required masses at exactly the right locations in the surround and fix the problems, without all the potentially fruitless trial and error.

To Be Or Not To Be

The Twin6 employs a 38mm‑diameter beryllium tweeter diaphragm. The use of beryllium to manufacture driver diaphragms dates back to the mid‑1970s when Yamaha introduced the groundbreaking NS‑1000 (you can read about the NS1000 at www.soundonsound.com/reviews/golden-gear-yamaha-ns-1000-monitors). Beryllium is rare, expensive, not easy to work with, and its dust has very significant health issues (the Twin6 monitors ship with a ‘Beryllium: Precautions For Use’ document), but it’s particularly attractive to loudspeaker engineers thanks to its combination of low density and high rigidity. In comparison to aluminium, which is often used for tweeter diaphragms, beryllium is between three and four times as stiff but around two‑thirds as dense. The result of that almost unbeatable combination of light weight and immense rigidity is that a tweeter diaphragm can be made so that, rather than breaking up into a jumble of discrete resonant regions, it continues to move as a whole to frequencies well above the limits of human audibility. Such full‑band ‘pistonic’ diaphragm behaviour has been the Holy Grail of tweeter design for, well, as long as tweeters have been designed.

However, as I have written before, designing a high‑performance tweeter is about much more than simply employing a pistonic diaphragm material. For a start, there’s the behaviour of the diaphragm surround (which can contribute a surprisingly large proportion of the total radiating area), and the distortion contribution of the motor system (magnet and voice coil) to consider. The deeper you dig into tweeter design, the more intricate and interconnected become all the fine details. The diaphragm material is, of course, a big deal, especially when it’s beryllium, but it’s just one element of a complex whole. That this is the case is not only witnessed by the very high performance that can be obtained from soft‑fabric dome tweeters, but also the performance of Focal’s own non‑beryllium units.

Switching It Up

Like the Alpha Evo Twin, the Twin6 is not a D’Appolito design, where both bass/mid drivers cover the same frequency band in parallel right up to the point where the tweeter takes over. It is instead a ‘2.5‑way’ system, where the two bass/mid drivers work in parallel at low frequencies, with one then slowly attenuated as bass becomes midrange.

This kind of driver arrangement results in asymmetric horizontal dispersion (with the monitor in landscape orientation) through the upper midrange band so, like the Alpha Twin Evo, the ST6 Twin6 is equipped with a rear‑panel switch that defines which of the two bass/mid drivers extends up to the tweeter crossover and which is rolled off early. According to the user manual I was sent, the switch should assign midrange duties to the inner drivers when the Twin6 is used in landscape orientation. This makes sense, as assigning midrange duties to the outer drivers would likely put a nearfield listener significantly off the perpendicular axis. My review pair of Twin6 monitors appeared to work contrary to the user manual and assigned midrange to the outer drivers when the switch was in the position corresponding to the monitor’s location (left monitor = switch in ‘left’ position, right monitor = ‘right’ position). When I brought this up with Focal, they responded firstly to say that the user manual is at fault and has temporarily been withdrawn. Secondly, Focal say their guidance on midrange driver assignment has evolved to be less prescriptive and to accept that some rooms and monitor installations may be better suited to apparently contrary driver assignment. As a result, the assignment switch labelling on the back of the Twin6 will be revised to make clear that the switches denote which of the drivers is the mid driver, rather than which speaker is the left or right.

Unlike the Alpha Twin Evo, which is suitable for landscape use only, the Twin6 is able to be mounted in portrait orientation. In my monitoring setup, at least, the lower bass/midrange driver was best assigned to midrange duties in portrait mode, because otherwise the midrange driver would be located significantly above head height.

As I’ve mentioned before, I feel Focal’s decision to run with a 2.5‑way mid‑tweeter‑mid architecture, rather than full‑fat D’Appolito, is debatable. Focal say the decision to reject the D’Appolito format was down to the significantly narrower midrange dispersion that would result; on the other hand, however, the dispersion would be symmetrical and not require a rear‑panel driver assignment switch. It would also make the Twin6 better suited to a landscape‑orientation centre‑channel role in multi‑channel monitoring systems; as designed, its asymmetric horizontal dispersion perhaps makes the Twin6 less than ideal for that application. Ultimately, however, the 2.5‑way architecture is a perfectly valid choice.

It’s as if the monitor is making every effort not to stamp any character on, or dull any detail of, the input signal. It’s all there, presented with expansive yet sharply focused stereo imagery and a natural, neutral tonality.

EQ Options

The Twin6 offers 150Hz and 4.5kHz shelf EQ options, which max out at ±3dB in both cases, as well as a LMF (low‑mid frequency) ‘Desk EQ’ option, which offers ±3dB of gain around 160Hz. All the EQ functions are accessed using recessed trim‑pot style knobs that require a small screwdriver for adjustment. It’s an arrangement that makes inadvertent adjustment unlikely, which is good, but the trim pots are small and setting them identically across the pair of monitors is a little challenging. It’s unlikely to be something that can be done easily once the monitors are in their working positions unless they are mounted a significant distance from the back wall. The final rear‑panel switch provides low‑frequency roll‑off options for when the Twin6 is used with a subwoofer.

The rear panel hosts controls for adjusting the HF, LF and Desk EQs, as well as designating which woofer is dedicated to reproducing the midrange.

There is one further switchable feature of the Twin6, which Focal call Focus mode. It’s engaged or disengaged through rear‑panel jack sockets and a footswitch. In Focus mode, the Twin6 tweeter and the bass‑only driver are disabled, leaving just the midrange driver working. The midrange driver’s low‑pass crossover filtering is also disabled and some EQ is applied to roll off its low‑frequency bandwidth early. Focal suggest that Focus mode enables the Twin6 to operate as two styles of monitor: the full‑fat Twin6, and something possibly representative of single‑driver systems with limited bandwidth such as televisions, computer speakers, car speakers, or media docking stations.

Although my likes and working practices may differ from others’, Twin6 Focus mode for me falls into the “solution looking for a problem” category. I say that for two reasons. Firstly, to my way of thinking, the fundamental role of a monitor is to provide a consistent high‑performance reference that enables informed decisions to be made when recording, mixing or mastering. Those decisions not only concern the artistic and creative quality of the work, but also concern how the work is likely to be perceived when reproduced on other playback systems in other environments. It’s part of the skill set of an audio engineer to be able to use the tools in the studio to manage the ‘translation’ of audio material when it’s released out into the wild. If that’s the case, then what is the role for a monitor that’s intentionally handicapped? To go back to the tool analogy, the Twin6 is a very sharp, high‑quality tool, so why would you intentionally want to use a blunted version?

Secondly, even working in a bandwidth‑restricted mode with no tweeter, the Twin6 bass/mid driver is probably in some respects far better than (or certainly unrepresentative of) any full‑range driver fitted to a TV or radio. So what, in Focus mode, is the Twin6 actually trying to impersonate? If any secondary reference is appropriate in studios these days it probably ought to be a pair of earbuds, because that’s what most music is consumed through. Monitoring via just a Twin6 bass/midrange driver to me seems somewhat meaningless, because it’s really not going to sound like much else out there in the real world. As I alluded to at the top of this paragraph, however, there are numerous different styles of working in a studio; for some, Focus mode might perform a useful secondary monitoring role, but I suspect that for me, it would result in doubt and confusion.

Measuring Up

Diagram 1: The ST6 Twin6’s on‑axis frequency response, in ‘flat’ mode (red trace), and with Desk EQ set to minimum (blue) and maximum (green).

In order to investigate the Twin6’s performance, I fired up FuzzMeasure and made a few measurements. I’ve recently begun using a new larger measurement space that enables better low‑frequency accuracy, so rather than cutting the response curves off at 200Hz, as has been my habit previously, I’ve included the full audio band down to 20Hz. However, the curves still aren’t accurate below 100Hz, so this region should be disregarded.

Diagram 2: The ST6 Twin6 in flat mode (red), and with the HF and LF EQ controls set to maximum cut (blue) and boost (orange).

Diagram 1 illustrates the Twin6 axial frequency response along with its ‘Desk’ EQ set at maximum and minimum. There’s no particular surprises to report. The axial response fits generally within ±2dB limits and the desk EQ behaves as specified. I think I might have gone with a slightly higher Q on the EQ, however. Diagram 2 again shows the axial response overlaid with EQ curves. This time it’s the LF and HF EQ options set to maximum and minimum. Again, this is all as expected.

Diagram 3: The red trace again shows the on‑axis response in full‑range mode. The blue and green traces show the response measured 30 degrees towards the LF driver side, and 30 degrees towards the midrange driver side. Note the asymmetry.

Diagram 3 shows the axial response curve again, but now overlaid with curves taken 30 degrees off the perpendicular axis to the left and right (with the monitor in landscape orientation). The asymmetric nature of the Twin6’s horizontal dispersion is revealed, with the curve showing the more severe discontinuity (green) measured towards the side of the monitor reproducing bass only.

Finally, Diagram 4 shows the Twin6’s axial frequency response one last time, along with the axial response in single‑driver Focus mode. Also shown, in green, is Focus mode measured at 30 degrees off the perpendicular axis. The restricted bandwidth at both ends of the spectrum generated by Focus mode is clearly apparent, as is the narrow high‑frequency dispersion of the 150mm driver: from 6kHz upwards the 30‑degree off‑axis output hovers around 10dB down on the axial.

Diagram 4: The red and blue traces show the on‑axis responses of the Twin6 in full‑range and Focus modes, respectively. The green trace shows the 30‑degrees off‑axis response in Focus mode.

So, while the Twin6 is perhaps a little unusual thanks to its 2.5‑way, rather than D’appolito, format, and its Focus mode, it’s also a very well‑made monitor with apparently extremely high‑quality drivers, and a clean set of acoustic measurements that offer nothing to raise an eyebrow, so it ought to sound good. And it does. It sounds extremely good.

I’ve been fortunate in recent months to review a couple of very high‑performance monitors, in the shape of the PSI A21M and PMC6, and to my ears the ST6 Twin6 falls into a similar performance category. There’s an immediate sense of accuracy and reliability to its overall tonal balance, and a great sense of ‘hear‑through’ clarity, especially in the vital vocal midrange band. The Twin6 does voices extremely well. It’s as if the monitor is making every effort not to stamp any character on, or dull any detail of, the input signal. It’s all there, presented with expansive yet sharply focused stereo imagery and a natural, neutral tonality.

In my studio room, with its relatively large desk and adjacent back wall, the rear‑panel EQ came in useful to fine‑tune the tonal balance. Just a dB or two cut using both the Desk and low‑frequency shelf EQs did the trick. The high‑frequency balance of the Twin6 worked untouched for me straight out of the box; however, something I’ve come to appreciate over the years is that when a tweeter performs to a genuinely high standard, as does that of the Twin6, I’m rather less sensitive to its level. It seems that the cleaner and more accurate a tweeter is, the less it matters how loud it is. The Twin6 tweeter falls into this category. It’s one of those tweeters that you hardly notice. You hear lots of clearly detailed high‑frequency content, of course — it just doesn’t seem to be coming from a tweeter because it doesn’t carry a tweeter signature along with the music. Maybe the beryllium dome is responsible, or maybe the Twin6 tweeter is simply a very well engineered and designed driver. Most likely it’s a bit of both.

The bass also works well. It’s extended in bandwidth, without being over‑emphasised, and dynamic in nature; it starts and stops well. I think I’m beginning to identify a trend in reflex‑loaded monitors for designers and manufacturers to work a bit harder on port design. Ports are becoming a bit more complex than simple unflared tubes, and I think this is reflected in their performance, specifically in terms of better linearity, less port compression and reduced tendency for midrange organ‑pipe resonance. I didn’t include a close‑mic port measurement among the published diagrams, but I checked the Twin6 ports for organ‑pipe resonance and they delivered an almost perfect result. The Twin6 ports are also tuned to a relatively low frequency (around 38Hz), which will help keep their peak group delay and overhang away from the busy parts of the music.

Finally, I did try the Twin6's Focus mode on a few occasions. It does what it says on the tin and, at the flick of a switch, turns a very fine monitor into a tonally distorted and bandwidth‑limited one. If I was lucky enough to own a pair of Twin6, I’m pretty certain my use of Focus mode would be very occasional. Your mileage may, of course, vary.