Volume 4 No. 1
Curvi Hifi Model 1 V2
Curvi by name as Martin Colloms analyzes the strengths and weaknesses.
Review By Martin Colloms

  Now in its second version and with some UK availability, Chris Liauw has created this elegant tapered line loudspeaker using a full range 5in Jordan X92S driver. Following some years of experiment with several enclosure, drive unit and crossover technologies, the single driver approach came out the winner.

The enclosure of this second version of his Curvi  Model 1 follows a specified tapered, folded curve that transmission line loads the driver. It incorporates electrical equalisation of the inevitable diffraction step, comprising a series inductor largely bypassed by a high quality resistor, to provide a mild 3dB or so of equalisation. For this second version the line loading has also been revised, from a loose tangle stuffing to a more open style, using side-wall absorption by polyurethane foam pads.

Employing nine laminations of laser cut and progressively profiled birch ply, the enclosure is built up and bonded using a polymer adhesive with chosen damping properties, to help make a strong and clean sounding structure. No panel areas are alike and this feature plus the progressive tapering of the line results in a largely non-resonant structure.

The speaker stands 1m high and sits on a 30mm thick slate base with a footprint just 16.5cm wide but 45cm deep. Polished spikes allow for good floor coupling and accessory studs are provided for polished floors. Electrical input requires 4mm plugs to the high quality multi-contact captive sockets. Specified at 50 to 200W power matching, some warnings given concerning how loudly it will play, while a wide range frequency response is quoted, with -6dB points at a low 35Hz and a high 25kHz. It weighs 24kg and is rated as a 6ohm amplifier load.

 

Sound Quality
While some might regard the Krell 402/Audio Research REF5 combination as overkill, it did help explore the limits of this single driver approach in coherence, integrity of timing and harmonic purity. At the same time the Naim CDX-2 /SUPERNAIT pairing was found to be ample in power and quality for the main review work.

Placement proved relatively uncritical and positions close to the standard were used – a good sign, indicating a smooth response and good overall frequency balance. The main adjustment was to ‘toe out’ by about 10 degrees from the ‘point at the listener’ axis, which helped balance the upper treble: a known feature of this full range driver, this axis also gave the best frequency measurement. Both Yter and DNM Precision speaker cables proved a good match. Listening began with small forces: acapella vocals, harpsichord and classical piano gave a crisp ‘open’ sound free from significant enclosure sound, well balanced and quite natural, neither ‘plastic’ nor ‘wooden’ in character. While realistic piano loudness was not possible, it could play loudly enough for most requirements in medium sized rooms.

I moved on to some wide band but relatively kind popular material (Eleanor McEvoy and Leftfield at rather moderate sound levels). At about animated conversation level the quite extended, agile and uniform bass was untypical of a small full range driver system. Unfortunately that was as far as it went. As soon as I turned up the volume, even to medium levels (just a few watts), the driver was moving quite a lot and the bass line started losing definition and power. Any louder and the mid began to blur and compress. Every design has one limitation or another and for this Curvi it is power and volume. The bass line overloads early, first doubling the lowest frequency signals to the second harmonic, and then adding progressive compression and loss of clarity thereafter. In small two-way systems such behaviour is partly offset by the undistorted output from a separate tweeter, but the Curvi’s single driver approach cannot benefit from this. Furthermore, small boxes have more air spring control behind the driver aiding power handling.

Coloration is quite low, though a mild ‘shhsy’ quality is audible on sibilants and related treble sounds, which may be due to residual upper range resonant modes of the alloy cone. While I enjoyed the pin-point focus and overall timing coherence of the concept, the thrill factor which can be engendered by the explosive and expressive dynamics of high efficiency full range designs is absent. I’m no headbanger, but the Curvi simply will not play loud enough in my largish room.

 

Lab Report
Sensitivity was low, if on spec. at 82.7dB/watt, with a rated 6ohm impedance and a 5 ohm minimum. (For reference, 96dB is loud, 106dB very loud, and to achieve the latter a Curvi pair would need a 200W input.) Unfortunately a music related pink noise test signal at more than a few watts overloads the driver at low frequencies due to excessive excursion, as the voice coil moves out of the working magnetic field. Thus 2W at 35Hz results in 8% measured distortion, which is the effective power limit. Just to hear 35Hz in a good room you need some 88dB spl from each speaker, where it has already reached its power/frequency limit.

Correlating well with the listening, at 40 Hz 4W input was tolerated but with audible levels of 8% second and 10% third harmonic distortions. As expected, at progressively higher frequencies the power handling improves because cone excursion is reduced. Thus 10W at 200Hz gives 93dB spl with a creditable 0.5% second and 0.1% third harmonic.

Well voiced for a single diaphragm, the Curvi has a surprisingly uniform frequency response on its ideal ‘10 degree axis’, +/-2.5dB 45 Hz to 18kHz, which is much smoother than a Lowther, for example. Offaxis the cone geometry interacting with the radiation angle gave rather greater variation than usual, but the single source also showed its strengths in general tidiness, and a good showing on the room averaged response. However, note the greater than usual loss of treble power above 5kHz, due the larger than usual ‘effective size’ of the treble driver; the room sound may consequently lack some ‘air’.

The waterfall decay plot shows a clean first arrival signal that is also linear phase as single driver theory predicts. After a couple of milliseconds some lower level hash is present in the treble, showing residual modal activity. The bass line is relatively undamped, and an anti-resonance is revealed by a deep notch in the nearfield driver output at 110Hz. While the line exit has a power maximum at a low 25Hz, it also peaks again at 180Hz and shows significant output right up to 500Hz at a level only 5dB below its main 27Hz signal.

 

Conclusion
At low and moderate sound levels the Curvi has arguably the most extended bass and uniform sound of all single full range driver speaker systems. Chamber music and piano fare very well, demonstrating the focused images, good timing and clarity that distinguish this type of speaker. However, on wideband material it will not play even moderately loud.

There are oblique hints to this behaviour in the instruction manual, and my lab report provides unequivocal corroboration. The amplifier load is fairly kind and the frequency responses are good, but sensitivity is very low and power handling poor.

The costly construction is well crafted and delivers what is arguably the most aesthetically pleasing example of the compact folded line genre. It’s certainly a design that ought to be auditioned, provided the power and loudness limitations are not considered barriers.

 

 

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