Curvi-BMR Loudspeakers Review
You don't see – or hear – loudspeakers like this every day, says David Price…
There's an old saying in Japanese: “The nail that sticks out gets hammered down.” Anyone who's ever lived in the Land of the Rising Sun will think it helpful for understanding Japanese culture, but it's also a pretty good metaphor for hi-fi loudspeaker design, too. We live in a world of groupthink, as every single two-way, bass-reflex ported box loudspeaker shows. There's nothing wrong with designing a speaker this way – if you're attempting to do a good jack-of-all-trades design – yet other approaches bring arguably superior results in some respects and inferior outcomes in others.
In terms of weirdness, the £6,000 Curvi-BMR you see here is off the scale. Designer Christopher Liauw tells me that he set out, “to produce an aesthetically beautiful design with an organic, human-like form, which is a radical departure from the standard rectangular box.” It certainly rides roughshod over conventional wisdom. It combines a radical cabinet shape with transmission line loading – and also uses the latest and largest balanced mode radiator driver available. Usually, one of these design facets would be enough to get me excited, but the combination of all three means I need a sit-down and a glass of water! What a breath of fresh air this is, from an industry outsider and former Manchester Metropolitan University academic – with a PhD in thermoplastic composites and a degree in polymer science.
The cabinet shape was registered back in 2006, and the first version of the Curvi originally ran a Jordan JX92S contro-flex full range aluminium drive unit; this was recently replaced by a latest-generation 128mm Balanced Mode Radiator, which has nearly twice the radiating area. BMRs are effectively a hybrid of other driver types, combining the properties of conventional pistonic speakers with those of flat-panel drivers that produce sound by vibrating. They have a wide bandwidth and excellent dispersion characteristics – up to a 180-degree arc, with the same dispersion in both vertical and horizontal planes. “The driver operates pistonically at low frequencies,” Chris explains, “but in the lower treble region, the diaphragm deforms in a manner similar to water after a stone is thrown into a puddle, as radial bending waves are formed. The maximum number of radial bending wavelengths (i.e. the number of modes) is a skilfully engineered parameter.”
Chris adds that the radial deformations of the BMR's diaphragm occurring at high frequencies are in contact with much more air than is the case with normal ribbons or dome tweeters. “This results in considerably improved treble dynamics and reduced distortion, giving a naturally fluid, fully resolved and extended treble which is entirely coherent with the rest of the spectrum”, he claims. “The lignin-cellulose based honeycomb laminate that comprises the BMR diaphragm is naturally stiff, low density and extremely well-damped – by virtue of both the nano and microstructures at a macromolecular level and its structural design. There's no interference with the designed–in quantised deformation behaviour that forms the basis of the BMR concept.”
Of course, using a single drive unit removes the need for frequency division and thus eliminates phase-related issues at a stroke. “The timing and impulse characteristics of the source material are preserved”, says Chris. “The BMR I have chosen operates in a quantised bending wave mode at high frequencies, ensuring a very fast impulse response with no overhang, and excellent off-axis coupling.” Obviously then, there's no crossover network in the Curvi-BMR – but it does have a baffle step compensation network. This is an all-pass three-element filter made with Mundorf components, including a 1.4 mm wire air-cored inductor. The board is compliantly mounted to minimise the effects of mechanical vibration, and Gekko internal wiring is used.
“Narrow cabinet designs that do not have a baffle step compensation network can suffer from an excessively forward presentation due to excess upper-midrange content being directed at the listener”, points out Chris. “Some designers get around this by incorporating resonance chambers into their cabinets, but these can lead to excessive thickening of the sound and over-hang at some frequencies. I believe electrical compensation is more controlled if implemented correctly. It won't compromise time-domain performance in the same way as high or low pass filters.”
Another radical aspect of the Curvi-BMR is the cabinet - it's made of multiple layers of birch plywood, giving a surprisingly light but highly self-damping structure for the BMR to work within. “This laminate is less dense than synthetic composite materials and metals seen in some other cabinets. It has more lossy vibration propagation, transverse to the interfaces between the cellulose fibres and lignin matrix in the wood structure”, Chris explains.
“I have varied the wall thickness of the curved profile, so it's at its thickest close to the drive unit and thinnest at the port end. This spreads any residual vibrational energy over a wide frequency range and ensures that decay rates are faster. Without very strong damping and careful design geometry, metals have a tendency to ring – their generally perfect elastic behaviour means that they are essentially non-lossy, their relatively high-density results in high energy storage and long decay times.” He adds that “nature is still the best materials scientist. In many respects, synthetic composites are now only just beginning to approach the performance and complexity of natural composite materials.”
Inside the cabinet, you'll find Herdwick wool and polyurethane foam damping to keep it a light but relatively non-resonant structure. A massy slate plinth is attached to the cabinet via maple connector blocks at the bottom, to provide stability. The finish is really quite something, and the design is topped off by a viscoelastically attached bezel that hides the drive unit mounting hardware and comes in a variety of finishes. The standard cabinet finish is semi-matt, hand-rubbed acrylic lacquer.
The 1,000x230x450mm, 24kg cabinet is a proper transmission line design, so there are none of the phase issues that bass reflex ports can introduce, yet the speaker does end up more efficient than an infinite baffle design. “I wanted to maximise bass extension and the perceived speed of reproduced bass notes, together with minimised distortion”, he says. “This system lets the drive unit operate unrestricted by the elasticity of air in a closed cabinet, allowing transients to form and breathe naturally. The curved transmission line I've used eliminates reflection of rear-radiated mid-range and treble content though the BMR diaphragm, and I think gives even bass reproduction with no bloating at certain frequencies.”
The measured performance of this loudspeaker makes interesting reading. The designer claims it has a frequency response of 60Hz to 20kHz at -6dB points. I'd like to see it at -3dB points because the speaker lacks low bass and doesn't quite have the shimmering treble you'd get from some price rivals. Sensitivity is a claimed 85dB/1w/1m, again something that seems a little on the generous side to my ears; you need a chunky solid-state amplifier to get serious sound levels out of it. That said, I found it can deliver modest levels in smallish rooms with a lowish powered valve amp. Chris says the speaker drops down to a 5 ohm load at 20Hz, although is 9.5 ohms at 1kHz; this makes it a tougher thing than some to drive, but not impossible. Recommended amplifier power is said to be 30 to 150W.
This is a very interesting loudspeaker, one that may elicit quite extreme reactions in a listener. On first audition, some might find it rather underwhelming. It's the polar opposite of – say – a big JBL or Klipsch since its treble doesn't come out and bite you, nor does its bass flap your flares or put cracks in your listening room walls. The midband at first appears soft and delicate, but not especially revealing. However, the more you listen, the more you realise that it has none of the 'character' of conventional loudspeakers – there's no 'boom-tizz' or shouty midband that knocks you back on your sofa. Rather, this is a truly even sounding loudspeaker that requires the listener to unlearn their preconceptions about what good speakers 'sound like'. The Curvi-BMR doesn't 'sound like' much else around if anything…
I am ashamed to admit that as soon as I get a new speaker in to review, my first instinct is to try to position it in the great hi-fi scheme of things. I initially found the Curvi-BMR very difficult to pigeonhole – indeed the only 'signal lock' I could get on my audiophile radar was that of the Quad ESL-63 electrostatic. Like this classic, it has a self-effacing presentation with an open, even and neutral sound. There's also an enjoyably sweet treble and an unexpectedly taut and extended – if not prodigious – bass. Its overall tonality is subtly dark and velvety in texture, which is nothing like most speakers around today.
Indeed, you can distil this speaker's sound down to three standout characteristics – a highly open and fluid midband, excellent stereo imaging and accurate bass. Feed the Curvi-BMR with a crisp, clean recording like Kraftwerk's Tour de France, and you're instantly aware of what it does well. This speaker has an almost translucent feel that gives an insightful reading of the recording; it digs deep into the mix and conveys everything going on accurately. It rendered the track's synthesiser sounds really cleanly, without focusing on one particular part of the frequency spectrum. There was a lovely, delicate timbre to those classic synthesiser voices, one with no apparent edge or grit. You could criticise it for sounding just a little matter-of-fact, but this is because it's unusual to hear this recording rendered so precisely. It doesn't garnish the sound with any additional flavouring, to make it artificially spicy.
This speaker is wonderfully liquid too, delivering a naturally shuffling, rhythmic feel to whatever music you care to play. There's an innate sense of eagerness to the music, so the Curvi-BMR never feels like it's dragging its feet. This was a treat to hear on soaring soul songs like Bobby Womack's Across 110th Street. This mid-seventies soul classic has a wonderfully deep groove that shows you how accomplished the session players were, but it can get lost through lesser loudspeakers. Despite not adding any artificial tonal edge to the proceedings, this speaker ably captured the moody feel. It doesn't sound overtly fast, just enjoyably fluid – I suspect this is due in part to the lack of shouty treble, which some of its price rivals use to liven things up. Importantly, all of the sound arrived at the listener at exactly the same time; bass, midband and treble.
Another delight of the Curvi-BMR is its soundstaging. It's not physically large in the great scheme of floorstanding speakers – so you don't get a vast, cathedral-like sound that's big enough to dominate your local town hall. Yet within its obvious physical constraints, this speaker does very well indeed. Put on a clean rock recording like REM's Fall On Me, and you're instantly invited into the recorded acoustic. It's not the sort of speaker than blasts the sound out at you; instead, I found myself peering into the recording, through a modestly sized but optically perfect lens. I loved the accuracy with which all the various strands of the mix were placed in space. Stage depth was also surprisingly good, the soundstage falling a good long way back.
The Curvi-BMR's handling of frequency extremes is interesting. Its treble lacks the bite of your average dome tweeter, so can initially appear soft and lacking in energy. At the same time, however, it's well able to signpost the timbral differences between hi-hat cymbals and tambourines, for example. On well-recorded pop like Haircut 100's Fantastic Day, this speaker gave a silky rendition to cymbals, yet didn't sound too sickly sweet. This speaker lacks grunt at the bottom end but has a gentle and linear roll-off, so I was still hearing a surprising amount of low bass guitar notes coming through. When I cued up a thumping piece of techno like Nookie's Give a Little Love, I hear the speaker missing out on the very bottom notes of the bass synth, but it did catch the ball and run from the mid-bass upwards. What low end you do hear is beautifully taut and tuneful, and totally in time with the rest of the music.
The Curvi-BMR is a quirky but brilliantly executed design, then. Within its own particular constraints, it's hard to beat, and the result is a compact floorstander that has surprisingly extended and controlled bass, an open and communicative midband and a smooth but insightful treble. Its only faults are down to its size; it doesn't sound 'big' and doesn't thump out nightclub-style sound pressure levels with ease. Yet within its own operating parameters, I found it charming, fun and highly able – a highly endearing music playback partner.
Designer Chris Liauw says that Balanced Mode Radiator drivers are, “a healthily disruptive, game-changing technology” – and in the Curvi-BMR he's proved what's possible. It's still very much a speaker that you'll love or hate, yet is intrinsically very well balanced and highly capable in its own way. If you're looking for something that challenges design groupthink, or just want to hear music in a straightforward yet engaging manner, this is an essential audition.
For more information, visit Curvi-Hifi.
David started his career in 1993 writing for Hi-Fi World and went on to edit the magazine for nearly a decade. He was then made Editor of Hi-Fi Choice and continued to freelance for it and Hi-Fi News until becoming StereoNET’s Editor-in-Chief.
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