When Arthur Bailey first described the transmission line loudspeaker enclosure in the pages of Wireless World in 1965, and then again in 1972, there seemed every prospect that this new alternative to the familiar sealed box or reflex cabinet would come to enjoy widespread use. Yet it never did. Instead transmission line loading has remained a relative rarity, associated with a handful of speaker makers in particular: IMF in the early years and PMC more recently, although B&W has made perhaps the cleverest use of it in the form of its inverted horn Nautilus tubes, which had their ultimate expression in the snail-shaped speaker of that name. Although the transmission line is often classified as a form of bass loading, its modus operandi actually affects a much wider frequency range, as B&W’s use of it confirms.
   Bailey’s idea was to provide a long, absorbent-filled duct behind the drive unit to dissipate its rear radiation without reflection or enclosed air resonance. For this to be the case the line must be some metres in length to be effective at low frequencies. To accommodate this, conventional transmission line enclosures are built rather like folded horns, although the cross-sectional area generally reduces somewhat along the line rather than expanding. But folding the line using internal cabinet partitions introduces the possibility of reflection and resonance, making a curved line a better prospect. At high frequencies the line can be shorter and thus straight, as in B&W’s Nautilus tubes.
   With the notable exception of the eye-wateringly expensive Nautilus itself, few transmission line speakers have deployed a curved line. And none in my recollection has looked as extraordinary or as elegant in doing so as the Curvi Model 1.

FORMED AROUND FUNCTION
It’s not just its shape but the fact that it’s formed from CNC-routed plywood sheets that makes the Curvi’s cabinet such a novelty. It won’t appeal to everyone but, as an old art editor colleague of mine used to say, love it loathe it you can’t ignore it. Moreover, it isn’t merely an aesthetic gesture as the plywood construction is primarily chosen to quell structural resonances.
   The pictures may suggest that the Model 1 is larger than it actually is, principally because its single full-range driver is not, as you might suppose, an 8in unit but Jordan’s compact JX92S, which has an effective radiating area of 78.5cm2 – equivalent to an effective cone diameter of only 100mm (4in).
   In fact the Model 1 measures just 970x165x450mm (hwd) overall and, according to my bathroom scales, weighs a modest 17kg – although that’s sufficient to incorporate a line of 2.4m in length.
   If the Curvi’s sinuous transmission line were full of graduated absorbent in the conventional way then this length should be sufficient to absorb effectively down to around 40Hz. But the Model 1 is unconventional in that only a small portion of its length contains absorbent, in the form of hollow-fibre polyester rather than traditional long-hair wool (because, says designer Chris Liauw, this prevents moth attack without the use of chemically unpleasant repellents). As a result, output from the open end of the transmission line is not restricted to low bass frequencies. In fact my measurements show that the line output is less than 10dB below that of the driver up to about 500Hz, above which it begins to tail off.
   So over music’s most energetic frequency range, the Curvi 1 delivers from its port an only mildly attenuated signal delayed by about 7 milliseconds. This ought to have undesirable consequences for sound quality but doesn’t appear to in practice, probably because the port output arrives at the listener’s ears after the floor reflection and is perceptually fused in the same way.

IT'S IN THE AIR
No, the Curvi’s obvious problems – which are apparent within seconds of beginning to listen to it – originate elsewhere. First is an on-axis response that free-field and diffraction-corrected near-field measurements show declining at about 3dB per octave below 1kHz, and second at the other end of the frequency range where the Jordan driver, despite its controlled diaphragm flexure, has poorly maintained off-axis output.
   These manifest themselves in the listening experience as, respectively, a shortfall in dynamic weight and tonal warmth, and a lack of ‘air’. The former can be fixed, to some degree, by electronic compensation, as I demonstrated by inserting a Behringer DEQ1024 digital graphic equaliser into my system – but few users will want to take such drastic remedial action. It further stresses the small Jordan driver at low frequencies for one thing. Passive EQ within the Curvi would help – it already incorporates an inductor-resistor network to compensate for the ‘baffle step’ – but this would reduce sensitivity to impractical levels. The denuded HF off-axis response can only be ameliorated by sitting close to the speakers so that the contribution of the room’s diffuse field is minimised.

A TOUCH OF MAGIC
But even if you do sit close and angle the speakers so that their axes cross either a little in front of or a little behind the listening position to flatten off the treble, these problems persist. The Model 1 works best on small-scale musical works but, without tonal correction, that tailing off in output below 1kHz remains manifest.
   Ella Fitzgerald’s voice was denuded of its seductive warmth and Paul Smith’s piano accompaniment missed body and dynamic scale [Verve 839 838-2]. On Leonhardt and Kuijken’s Mozart Sonatas for Violin and Piano [Seon SBK 62953] – a tough test for even the most tonally well-balanced of systems – the sound was much too upper-mid forward, with little body to either violin or fortepiano. And yet, particularly with tonal correction applied, some of the magical coherence I heard in the Jordan-equipped Aurousal A1 Mk2 was there to be enjoyed. So this speaker has potential – but right now hides its light under a bushel of tonal aberration.

VERDICT
After my positive reaction to the Aurousal A1 Mk2, I’d hoped that
the Curvi Model 1 would elevate the full-range driver experience to a new level. But this loudspeaker is too fundamentally flawed. Unless you deploy tonal correction and can forgive its innate lack of treble sparkle, this novel and attractive design currently does not deliver £4k sound quality.