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Barefoot MM45

Active Three-way Monitor
Published November 2015
By Phil Ward

Barefoot MM45

Barefoot’s newest monitor may be their most affordable to date, but the company’s meticulous design ethos is evident throughout.

Look back at my previous Barefoot review for the magazine (www.soundonsound.com/sos/jun14/articles/barefoot-mm35.htm) and you’ll see that I’m a fan. It’s not just that I liked how the MM35 Gen 2 performed, I admired its design philosophy too. There is, to my mind, a clear electroacoustic logic to the way the MM35 came to be the way it is. The only thing I didn’t like (or rather, couldn’t afford) was the price. So, here we have a less expensive Barefoot: the MM45. Despite the bigger model number, it’s actually a smaller and less ambitious product than the MM35. It’s still electroacoustically intriguing though, and it borrows both the MM35’s design philosophies and some of its features, which is all potentially good news. The potential bad news, however, is that despite its less eye-watering price, I still can’t afford a pair.

Reductio!

The design logic that typifies the Barefoot approach continues with the MM45 and, that being the case, the solution to its different set of design requirements is a different electroacoustic architecture. Those different design requirements of the MM35 were, presumably, reduced cost and reduced physical dimensions, and it’s interesting to see how they are expressed in the way the monitor has turned out. The first and most obvious change from the MM35 is that the twin, side-mounted bass drivers are gone — replaced by a single, forward-facing bass driver. My guess is that this one change is responsible for the most significant chunk of the cost and bulk saving.

Another significant cost saving might have been found by making the MM45 a two-way system, with just a bass/mid driver and a high-frequency driver. The forward-facing bass driver would certainly make such a decision acoustically viable. Barefoot’s designers chose, however, to stick with the three-way architecture that characterises all their monitors, and in my opinion that’s a very positive sign because the electroacoustic demands on a driver that aspires to reproduce relatively low bass in a sealed enclosure tend to make it less capable at mid-range frequencies. The same is, of course, true of drivers optimised to reproduce the mid-range: they’re not always great at playing bass (see the box for a little more on this).

Having settled on a three-way architecture with conventionally front-facing drivers for the MM45, Barefoot could easily have followed convention yet further and employed a single 100mm to 150mm mid-range driver. A quick glance at the photographs, however, and you’ll see that’s not how things panned out. The MM45 has not one but two mid-range drivers. These relatively small (65mm diameter) drivers are spaced around 110mm apart and located between the 200mm bass driver and the 30mm ring-radiator high-frequency driver. Both the bass and mid-range units incorporate aluminium diaphragms, the bass diaphragm being capable of a remarkable ±13mm linear displacement.

Twin Perks

The decision to employ two small mid-range drivers rather than a single larger one is, especially in the context of making cost savings, a fascinating one. Barefoot must believe that two mid-range drivers bring some significant benefits. So what might they be?

The first benefit that springs to mind is that small mid-range drivers are generally able to reach higher frequencies before their diaphragms become chaotically resonant (a phenomenon speaker designers call ‘break-up’) and directional. Higher reach from the mid-range unit takes some of the load off the low end of the tweeter, which can be significant in terms of high-frequency distortion, power handling and thermal compression. And as if to illustrate my point, the Barefoot MM35, with its single 160mm mid-range driver, hands over to the tweeter at 3kHz, whereas in the MM45 the crossover is at 3.6kHz.

Staying with the thermal compression issue, twin mid-range drivers will definitely have benefits. A moving-coil driver typically dissipates over 95 percent of its input power as heat, and as the wire of its voice coil gets hot, its electrical resistance increases and the acoustic output of the driver reduces. This is clearly not a good thing. With two mid-range drivers working in parallel, the same amount of heat is dissipated by two voice-coils so, all other things being equal, the thermal compression effect is halved.

The vertical alignment of the two mid-range drivers endows the MM45 with a  tighter dispersion in the vertical plane than in the horizontal.The vertical alignment of the two mid-range drivers endows the MM45 with a tighter dispersion in the vertical plane than in the horizontal.A third potential advantage of the twin mid-range solution on the MM45 is something that comes about thanks to the physical arrangement of all its drivers. The arrangement that the two small mid-range drivers makes possible means that the composite array of all four drivers can be mounted closer together, to the general benefit of dispersion consistency and control. Furthermore, twin drivers also mean that the monitor’s vertical dispersion (when it is mounted as recommended in landscape orientation) through the mid-band becomes tighter. The tighter dispersion comes about thanks to the two driver’s physical spacing in relation to the frequency range they reproduce. It’s fundamentally the same phenomenon that is sometimes employed to great effect in line-source PA systems, although modern such systems take the idea further by introducing subtle relative delays to some of the drivers in the line, in order effectively to make the dispersion of the line ‘steerable’ (by which I mean it can be manipulated to favour a particular direction).

For any given array of multiple drivers, including that of the MM45, the narrowing effect becomes more pronounced as frequency rises, so let’s look for a moment at the highest frequency of the MM45 mid-range drivers. At the MM45 mid/high crossover frequency of 3.6kHz, the wavelength of sound is around 9.5cm. This means that at locations in space vertically off the central axis of the two mid-range drivers, where the difference in acoustic path length to a specific location equals 4.25cm (half a wavelength), the output of the drivers will be 180 degrees out of phase and will destructively interfere.

Barefoot MM45The results of the destructive interference between the two MM45 drivers can be clearly seen in its published frequency response curves. At 30 degrees vertically off-axis there’s a broad 8dB suck-out centred around 3kHz. By 6kHz (at which frequency the tweeter has fully taken over), the 30-degree off-axis output returns to near parity with the on-axis output, before decaying again as the tweeter itself becomes directional. Depending on the frequency and the off-axis angle, the vertical dispersion from the twin drivers will vary as their outputs go in and out of phase. The technical term is ‘lobed’, and with a pink noise signal it’s relatively easy to hear the vertical dispersion lobes on the MM45 — its subjective mid-range tonal balance changes with head height. In contrast with its vertical dispersion, the MM45’s horizontal dispersion shows a different, more conventional trend: dispersion simply reduces as frequency rises from around 2kHz upwards (although, firstly, I’d expect horizontal dispersion to begin to lobe further off axis, and secondly I’d expect it to be asymmetric).

Time To Reflect

If you’ve read any of my previous ramblings on monitors you’ll probably be prepared to hear that if an electroacoustic technique, like the use of twin mid-range drivers, brings some advantages, it will invariably bring some disadvantages too. So while the narrow, and sometimes steerable, vertical dispersion of line-source arrays in PA systems is demonstrably a good thing, for speakers used in studio or domestic environments I think the jury is out.

A fundamental difference between the nearfield environment and the PA environment is the delay time of the early reflections (those from back wall, side walls, floor and ceiling). In the PA environment the early reflections are usually late enough that the brain doesn’t combine them with the direct sound — we’ve all experienced that live-sound phenomenon where the first reflection is heard as a distinct source. In a studio environment, however, the early reflections arrive at the ear only very shortly after the direct sound, and the brain will tend to combine them, so that the perceived sound is a composite of the two (or more) sources. If the tonal characteristic of the direct and reflected sound are similar, or at least don’t diverge wildly, this integration of direct and reflected sound is relatively benign. There’s good psychoacoustic evidence, however, that the brain can be confused by early reflections that don’t bear a tonal resemblance to the direct sound, and will perceive the speaker as coloured as a result.

So, getting back to the MM45 and its vertical dispersion, when located at a pretty typically nearfield listening distance of between one and two metres, and with a floor or desk at the distance that floors and desks typically tend to be, the first reflection from either is very likely to be imprinted with the MM45‘s mid-range suck-out. This would, as I mentioned above, normally be considered a Bad Thing... but part of me wonders if, in this very specific nearfield application, a suppressed first mid-range reflection from the desk might not be a good thing? Perhaps Barefoot believe this to be the case?

Business As Usual

Having described the unconventional twin-driver aspect of the MM45, it’s maybe time to describe its more conventional elements. First off, as with the MM35, the M45 is a little bigger and heavier in reality than I imagined it would be. It is still a perfectly viable nearfield monitor, but its 17kg weight means some pretty major-league mounting brackets or stands are needed. The rounded-edge cabinet is finished in a black textured paint, and while the speaker engineer in me appreciates that the component budget has mostly been spent on the parts that actually make the noise, the aesthete in me would prefer a less functional and cost-effective cabinet finish. The MM45 is, after all, an expensive product.

Along with the vast majority of active nearfield monitors, the rear panel of the MM45 incorporates its amplifier heatsink and connection panel, while also carrying sensitivity and equalisation adjustment control. The EQ and sensitivity controls are not knobs but detented pots that require a small screwdriver for adjustment, and in my case, reading glasses to be certain of their setting. I’d have much preferred something a little more helpful.

Barefoot’s decision to include a range of preset EQ options on the MM45 is one I’m not really sure about. The MM35 ships with Barefoot’s MEME monitor modelling technology, which, at the turn of a wired remote knob, ‘imprints’ on the speaker some of the dynamic, distortion and tonal characteristics of a range of monitor types. The MM45 EQ option is both far less sophisticated, being simple tonal adjustment, and far less accessible, in that it is on the back of the cabinet and requires a screwdriver. The back-panel photo shows the EQ options, and while I can appreciate them offering interesting comparisons to the ‘flat’ response, or providing some compensation for, say, a dull-sounding room, I suspect most people, after experimenting a little and finding the EQ to behave much as expected, will leave it set flat and rarely use it again.

MM45 connections include just a mains power socket and a single balanced XLR input. It’s interesting that Barefoot clearly feel the cost and potential audio degradation of A-D and subsequent D-A conversion is outweighed by the benefits of handling signal processing (primarily the three-way driver filtering) in the digital domain.

The MM45’s power amplification is just as generous as on the MM35, with Hypex Class-D modules rated at 250 Watts for the bass section and 180 Watts each for the mid-range and high-frequency sections. There is no shortage of power or volume available from the MM45, and thanks to the genuinely high-quality drivers there’s never any sense of strain or audible distortion when things get loud. I’d have no worries in terms of volume level or power handling if, for example, I was setting up drum mics with just a pair of MM45s for monitoring.

Hearing Test

So how does the MM45 perform? Well, first things first: while subjective acoustic memory is said to be unreliable, to my ears the MM45 is unmistakably cut from the same cloth as the MM35 that I admired so much. It demonstrates a similar sense of unflustered precision all the way from very low bass to as high as I can hear. It also delivers a similar sense of extreme clarity, so that subtle details right at down in the depths of a mix become easily audible — and it does so with a satisfyingly neutral tonal balance. There’s a sense too that the MM45 doesn’t just reproduce a mix; it presents all its elements individually — both in terms of the ‘space’ around each element and the pretty sharp and focused stereo imaging. Of course, having a mix pulled apart like this is not always a pleasant experience, but in putting emphasis on making the elements of a mix integrate, the MM45 should work really well as a transfer standard — which perhaps is the number one priority of any mix monitor.

On a less philosophical note, I’ve mentioned this phenomenon in a few reviews of three-way monitors, but I’m convinced that there are genuine benefits of a format that enables bass to be reproduced by drivers engineered specifically to generate low frequencies. The 200mm MM45 bass driver is not that much bigger than would typically be a bass/mid driver in a two-way system, yet the dynamic, controlled, deep and unambiguously pitched bass it is capable of is in a different league. The closed-box loading of the MM45 helps, of course, but the seriously engineered bass driver makes a huge difference too, I think.

And what of any audible pros or cons of the twin mid-range units and their lobed dispersion? I can’t honestly say that I really heard anything in the MM45 that convinced me that lobed vertical mid-range dispersion is either of benefit or a problem (although any speaker’s dispersion characteristic is partly responsible for its subjective characteristics). However, right at the end of the review process I tried the simple experiment of using the MM45s in portrait orientation (taking care not to listen too far off axis), which of course completely changes the way in which their dispersion characteristics interface with the listening environment. And while I can’t entirely put my finger on what the difference was, I preferred it. The sound was, to my ears, a little sharper, with a more finely defined stereo image. Maybe the dispersion in portrait mode suits my room a little better, or maybe it just pressed my subjective ‘like’ button a little more effectively. Either way, however, the bottom line is still that the MM45 is undoubtedly a hugely capable and classy active nearfield monitor. If it’s on your financial radar, you should undoubtedly hear it.

Alternatives

Also worthy of consideration are the Genelec 8351, Unity Audio’s Super Rock 2, the Focal SM9 and ATC SCM20ASL Pro.

Why Three-ways Are Better Than Two

The two-way speaker format that became the dominant architecture for compact high-performance speakers around the 1970s almost defines the art of engineering compromise. I say this because, thanks to a confluence of mechanical and acoustic factors (the density of air, the weight/strength ratio of viable diaphragm materials, and so on), the two-way format, in truth, only just works. For example, if you want a driver to be able to move enough air that it can reproduce low frequencies at a realistic volume level, its diaphragm needs to be relatively large, very rigid, and able to move a considerable distance very freely.

The trouble is that if you also want that diaphragm to reproduce mid-range frequencies, it needs a degree of damped flexibility so that the unavoidable mechanical resonances are reasonably well controlled and so that, to improve dispersion, its radiation area (assuming it is driven at the centre) tends to reduce with frequency. Similarly, for a conventional moving-coil tweeter to have acceptable dispersion at high frequencies, its diaphragm must be dimensionally relatively small; say, less than 30mm diameter. Yet to have acceptable efficiency and power handling at lower frequencies, where it can integrate well with a bass/mid driver, its radiating area needs to be maximised. The conventional two-way format is like a couple reaching for a handshake when they’re just a bit too far away. History, obviously, recounts that thousands of successful two-way speakers have been designed, but it remains the case that a three-way architecture can potentially sweep away a whole set of electro-acoustic problems.

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 November 2015