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Kerr Acoustic K300

Passive Monitors
Published June 2018
By Phil Ward

This debut speaker from new UK company Kerr Acoustic combines a number of unusual design elements — to impressive effect!

Here at Sound On Sound we’re always on the lookout for less orthodox approaches to nearfield monitoring, so when we spotted the Kerr Acoustic K300 on a pro audio retailer’s web site, we had no option but to follow it up. Only then did we discover that the K300 is manufactured less than six miles away from the Sound On Sound offices on the outskirts of Cambridge. In other words, right under our noses.

Kerr Acoustic K300But what, you might ask, was the unorthodoxy of the K300 that grabbed our attention? Well, there are three elements to it. Firstly, the K300 is a passive monitor and, in an active dominated nearfield world, that’s of interest because it speaks of a design that swims against the tide. Secondly, in contrast to the AMT (air motion transformer) style folded-ribbon tweeters that are becoming increasingly popular among monitor designers, the K300’s ribbon tweeter sports a simple and extraordinarily light unfolded diaphragm. And lastly, the K300 uses neither a ported nor closed-box low-frequency system, but employs a folded, quarter-wave transmission line that’s similar to the technology employed by PMC in their very well-respected monitors.

But who are Kerr Acoustic and where did they spring from? Company founder Jes Kerr is a drummer and percussionist, and an Audio & Music Technology graduate from Anglia Ruskin University. Rather than forge a career as a studio engineer, Jes chose to follow his interest in speaker design to create a studio monitor business. There’s a family element to the business too, because Jes’ father Steve is a joiner by trade and responsible for the K300 enclosures. The current Kerr Acoustic range comprises the two-way K300 and the much larger main monitor, the three-way K100. Both are passive designs.

Box Clever

When, in a recent issue, I reviewed another monitor from a UK start-up, the OS Acoustics DB7, I commented that the DB7 enclosure was unusually deep. Well it seems I was wrong with the ‘unusually’ because the K300 is very similarly dimensioned. Consequently, the same warning applies: if you like what you read about the K300, make sure you have the space in your studio installation. In fact it’s not just the depth dimension of the K300 that’s similar to the DB7; its width and height are within a couple of centimetres too. Dimensions, however, are perhaps the only respect in which the DB7 and K300 are similar. In almost every other respect they are as different as chalk and cheese. The K300 enclosure is finished in a semi-matte dark grey paint, with other colours available to special order. It’s constructed with a mix of 24mm and 18mm Baltic birch ply and feels satisfyingly hewn. I’ll get on to a full description of the K300’s folded quarter-wave transmission-line low-frequency loading technology a little further down the page, but one of its undoubted side benefits is that the internal enclosure cabinetry required results in an extremely rigid structure.

Normally, with an active monitor, I’ll start with a description of the control and connection facilities on the rear panel, but of course with the K300, beyond a couple of 4mm socket/binding-post connection terminals and a switch that offers -2dB/0dB/+2dB options on the tweeter, there’s nothing else to talk about. For what it’s worth, the terminals are satisfyingly chunky and look as if they mean business, and the switch has a lovely precision-engineered feel. But it’s back around the front of the K300 that the interesting stuff resides, and call me an old stick in the mud, but that’s the way (aha aha) I like it.

Transmission Accomplished

I’ll start at the bottom. As I mentioned, the K300 sports a transmission line to load its bass/mid driver, and as this is the first time I’ve got to listen to such a thing in the context of an SOS review I’ll take a discursive paragraph or three to explain how a transmission line works and why it’s fundamentally different to the closed-box or port-loaded techniques we are all more used to.

The basic requirement behind all moving-coil-driver low-frequency systems is to deal with the acoustic radiation from the rear of the diaphragm. The problem with the rear radiation is that it is in opposite polarity to the front radiation, and so the front and rear radiations simply cancel out. This is why a driver with no enclosure plays no bass. The premise of a closed box is simply to ‘hide’ the rear radiation, but that comes with the price of potentially reducing the bandwidth of the system by raising its fundamental resonance. The rear radiation of the driver is also effectively discarded, and that seems a shame when acoustic radiation from a moving-coil driver is so inefficient anyway.

A ported (‘reflex’) enclosure attempts to deal with the driver’s rear radiation through using a resonant system to shift its phase 180 degrees over a narrow range of low frequencies, so that it reinforces the front radiation rather than cancelling it. This benefits both low-frequency bandwidth and efficiency but comes with time-domain downsides.

The transmission line approach to the problem is entirely different. But before I explain why, note that when I write about the rear radiation of the driver in the context of bass loading techniques, I’m mostly talking about low frequencies. However, if the bass driver happens to be a bass/mid driver, its rear radiation will extend right up to maybe 4kHz, so mid-range energy has to be dealt with too. In a closed-box system, polyester or wool wadding within the box will do reasonably well, but a similar technique is somewhat problematic in a ported box because wadding inside the box will tend to damp the port resonance. The reason this is relevant to transmission-line enclosures is that, in addition to managing the low-frequency rear radiation from the driver, the aim of a transmission line is to deal with all the energy that comes from the back surface of the diaphragm.

Diagram 1: The K300’s folded, quarter-wave transmission line.Diagram 1: The K300’s folded, quarter-wave transmission line.There are fundamentally two components to transmission-line rear-radiation energy management, both founded on the idea of partitions within the enclosure creating a folded labyrinth down which the energy is progressively dissipated by a thick lining of wool, foam or wadding. Diagram 1 illustrates a folded transmission-line labyrinth (for clarity’s sake, I’ve not attempted to show the lining) based on the design of the K300.

The first component of energy management is the wadding that progressively dissipates the mid-range energy before it reaches the mouth of the labyrinth. The wadding also damps the inherent organ-pipe resonance of the labyrinth. The second energy management component effectively arises because, to dissipate the low-frequency energy entirely, a labyrinth needs to be far longer than can be folded up and put in a box. All is not lost, however, because, along with dissipating mid-range energy, the labyrinth wadding can slow the propagation of low frequencies such that the energy that exits the mouth will be in phase with the front radiation of the driver over just the right portion of bandwidth to extend the cutoff point of the system. The slowing of the rear radiation through the labyrinth doesn’t, however, imply a port loading-style increase in low-frequency latency: firstly the latency is much shorter, being comparable to closed-box loading, and secondly it’s not associated with either a high-Q resonance or any significant bulk air movement (so there’s no noisy turbulence or aerodynamic compression).

As you might imagine from reading that description of transmission lines, successfully implementing one is far from straightforward. Although there are mathematical modelling tools around, designing a transmission line is a somewhat experimental, trial-and-error kind of process that’s very different from the filter-theory-based simulation techniques that work so well for closed-box or ported systems. And that’s probably one good reason why transmission-line systems have become relatively rare across both hi-fi and pro speaker design — despite the fact that, as PMC and now Kerr have shown, the technique can be made to work extremely well. A second reason is, of course, the cost of complicated cabinetry.

Driving Force

In all that description of the transmission-line labyrinth I’ve referred to the bass driver numerous times without ever describing the one fitted to the K300. The K300’s driver is actually a bit of a classic among speaker geeks. It’s manufactured in Denmark by Scanspeak (www.scan-speak.dk), who are long-established high-performance driver specialists, held in particularly high regard by speaker designers across the world. The particular driver used in the K300, the 18W/4531G01, has been in Scanspeak’s range for a decade or two and is relatively conventional in terms of its technology (the cut and re-glued paper diaphragm is its most notable feature), but it’s a seriously high-performance driver nonetheless. I’ve personal experience of the 18W over the years and have always appreciated its notably uncoloured and natural character, combined with its low levels of distortion and substantial magnet system and voice coil.

Tweeter

Before I move on to writing about the performance of the K300, transmission-line bass loading is not its only unusual feature, for it also sports a ‘true’ ribbon tweeter. Ribbon tweeters of the folded AMT (Air Motion Transformer) type are not, these days, that unusual in nearfield monitors, however there are very few that incorporate pure, unfolded ribbons.

The K300’s ribbon tweeter is unusual in that, unlike the more common Air Motion Transformer design, the ribbon is left unpleated, and also forms its own conductive element, making it significantly lighter, at the expense of a much lower impedance (the latter is remedied with the use of a transformer).The K300’s ribbon tweeter is unusual in that, unlike the more common Air Motion Transformer design, the ribbon is left unpleated, and also forms its own conductive element, making it significantly lighter, at the expense of a much lower impedance (the latter is remedied with the use of a transformer).In describing them as folded and unfolded I’ve explained the fundamental element of the difference between the two species, but there is a little more to it. A folded AMT-style ribbon tweeter will have its electrical conductors embedded into, or sometime etched onto, its folded diaphragm. This means a significant length of conductor can be used, which results in an input impedance that amplifiers can drive directly, but it also results in a diaphragm of relatively high mass (although still typically significantly lower than that of a dome tweeter).

Conversely, in an unfolded ribbon tweeter, the diaphragm and conductor are one and the same thing — usually a thin sliver of aluminium — so while the moving mass is far less, the impedance is way too low for direct amplifier drive. A transformer is therefore required. The K300’s ribbon tweeter is manufactured by Fountek in China and incorporates an integrated impedance-matching transformer to bump its inherent 0.02Ω impedance to a more usable 8Ω. There’s no denying the unfolded ribbon’s incredibly low moving mass (0.027g on the K300 tweeter) and the simple purity of its design, and in terms of subjective performance there’s nothing else quite like a well-sorted pure ribbon.

A further argument, however, for the folded-ribbon diaphragm of AMT-style tweeters is that they achieve greater effective diaphragm area without increasing the diaphragm’s linear dimensions and compromising high-frequency dispersion. It’s another reason why unfolded ribbon tweeters, despite their undoubted qualities, are the exception rather than the norm. By way of illustration, the K300 ribbon diaphragm is specified as 12mm wide by 60mm long (by 0.015mm thick!), so its high-frequency dispersion horizontally and vertically ought to be significantly different. To test this, I took some FuzzMeasure curves of the K300 and, as expected, its high-frequency response varied with microphone position rather more on the vertical axis than on the horizontal.

Diagram 2: The K300’s frequency response, measured in front of the tweeter on axis (red trace), 10 degrees to the side (orange), and 10 degrees above (green) and below (blue) the tweeter.Diagram 2: The K300’s frequency response, measured in front of the tweeter on axis (red trace), 10 degrees to the side (orange), and 10 degrees above (green) and below (blue) the tweeter.The FuzzMeasure curves are shown in Diagram 2 and perhaps need a little explanation. The red curve shows the K300’s frequency response from 200Hz to 20kHz, with the measuring microphone located on the centre line of the front panel and aimed at the tweeter. The orange curve shows the same arrangement with the K300 rotated 10 degrees horizontally on its stand. The change in frequency response is pretty minor (the 10-degree curve is actually a little smoother than the axial curve, probably because the angle suppresses symmetrical diffraction effects). The green and blue curves illustrate the change in response as the microphone is moved respectively upwards and downwards by a linear distance that would result in a 10-degree angular change. As is pretty clear, the change in response, vertically, at high frequencies is greater than that horizontally, and not only does the tweeter level change, the above-axis response begins to display a mid-range ‘suck-out’ as the microphone moves further off axis of the bass/mid driver and the relative path lengths from the drivers to the microphone change.

Listening In

A couple more observations on the Diagram 2 curves are that the K300 displays an emphasised upper-high-frequency balance and a recessed presence band (say, 1.5kHz to 6kHz), and those observations bring me finally to describing what I heard when I placed the K300s on the wall brackets either side of my DAW. I mentioned the high-frequency emphasis and recessed presence band because that’s what I was initially most conscious of. The K300’s warm and relaxed nature in the mid-range brings to mind BBC monitors of old, but its upper-high-frequency emphasis in that context seems slightly incongruous. I quickly settled on the -2dB setting on the rear-panel high-frequency level switch, but even then I felt the K300 was a little bright. I suspect the recessed presence band leaves the high frequencies sounding a little exposed.

However, I think that one of the remarkable aspects of monitors and tonal balance is that if a less than ‘flat’ frequency response is not accompanied by cabinet panel colorations, driver distortion or resonance, and as long as it’s not really extreme, quite soon it becomes academic. We become used to it. And this is the case with the K300: the performance of the drivers and cabinet, and the fundamental quality with which everything is put together, shines through to create a really effective monitor.

Starting at the bottom end, the K300 transmission-line bass is a great success. In common with the low-frequency performance of numerous PMC monitors, it pulls off the trick of sounding full-bodied, with a real sense of impact, dynamics and extended bandwidth, but without any hint of sluggishness, overhang or pitch ambiguity. Moving further up the band, the mid-range speaks with exactly the kind of natural, uncoloured quality I’d expect from the Scanspeak driver. Despite its use of an apparently low-tech paper diaphragm, there’s neither any apparent lack of detail or clarity, or any narrow-band emphasis. Finally, the Fountek ribbon tweeter is natural and delicate, and effortlessly reveals fine details without emphasis or sibilance. It’s the kind of high-frequency performance that makes you wonder if dome tweeters were ever really a good idea.

From all that you’ll gather that I really like the K300, and I do, it’s a really fine monitor, but there’s just a couple of small caveats to throw into the mix. Firstly, as a result, perhaps, of its mid-shy balance, I found the K300 stereo imaging to be very marginally short on focus. Perhaps the K300 would image with a more explicit nature if it was a little more ‘pushy’ in the upper mid-range. And secondly, there’s no denying that the narrow vertical dispersion of the ribbon tweeter is audible in that the high-frequency balance changes with listening height. It may well be that when used in a concentrated, head-clamped, nearfield mixing session such a quirk won’t be too troubling, but it’s there and I found I needed to be aware of it. I actually ended up using the K300s upside-down in my installation because that suited the geometry of my workspace a little better. Upside-down installation also seemed to sharpen the stereo imaging a little.

I’ve concentrated perhaps a little too much on the K300’s mild tonal quirks, but to some extent that’s because the rest of the design, in terms of its time-domain performance, lack of resonant coloration or any sign of other disturbing phenomena is so satisfying. The K300 is one of those monitors that fundamentally and unambiguously does the job required. It’s a genuinely accomplished first effort from Kerr Acoustic.  

Alternatives

Passive monitors seem to be making a comeback and there’s no lack of great options around the same price as the K300: try the Amphion One18 and ATC SCM20PSL Pro, in particular.

About The Author: Phil Ward’s loudspeaker career began in 1982 when he joined UK hi-fi company Mordaunt-Short in a junior design role. After leaving Mordaunt-Short in 1987 for a spell in audio PR, Phil joined Canon as Design Manager for the Japanese multinational’s range of consumer and custom install speakers, and then Naim Audio as speaker design and project manager. Since 2001 Phil has worked as a freelance consultant and writer across both the pro and consumer audio sectors. Phil plays electric and double bass and has recorded, produced and mixed numerous bands and artists. Phil's blog can be found at http://musicandmiscellany.com   

Published June 2018