Just a year and a half ago, B&W Loudspeakers unveiled yet another innovative idea evolved within its R&D laboratories to join such earlier innovations as the Keylar drive unit cone and the Matrix enclosure. They call this new form of construction the Prism system and it comprises a plastic moulded back panel having many of pyramid-shaped spikes projecting into the cabinet expressly to break up, and hence reduce the undesirable coloration from internal standing waves.

The first Prism loudspeaker was the DM302 (reviewed in April 1996), a budget bookshelf-sized model costing only £129.99 per pair. This latest manifestation of the Prism principle, the DM305 (£49.90), adds about 520mm to the height and £220 to the price. However, it should still be seen as a non-luxury loudspeaker in comparison with most systems of similar dimensions. It does not simply have a strong family resemblance to the earlier model, it looks uncannily as if the designers have taken a DM302 and stuck it on top of a box with a moulded fascia whose angled edges carry the DM302's profile down to floor level.

Of course there is more to the new model than this. Although the 26mm tweeter is the same wide ranging soft-dome unit as in the DM302, the woofer is entirely new. It is larger, at 165mm, and has a resin-impregnated homogeneous fibre (paper) coil with internal damping and a stiffened neck for improved stability. A novel feature, which enables the cone to emulate the smooth response of Kevlar cones, is a series of V-section embossed ribs described as 'wave spoilers'. These run out tangentially from the central former region almost to the cone edge and have the effect of inhibiting standing waves due to transmitted and reflected energy within the cone, including bell-mode or 'lampshade' resonance modes. The chassis is a high-rigidity casting.

The front baffle is of rigid moulded plastics and the two drive units are inset one above the other, secured from inside to present an unfussy appearance, with the tweeter recessed in a contoured convex plate to assist dispersion.

Reflex loading is employed for the bass/midrange driver but with an unusual configuration involving two linked volumes, both vented. The upper portion, which houses the drive units and the Prism back panel, with its array of inward projecting 'wedges' and six front-to-back braces, has a front facing letterbox shaped reflex port as in the DM302. An acoustic resistance partition leads through to the larger lower enclosure which has its own conventional cylindrical port set into the rear panel. Thus the total port area is quite large and 'chuffing' noises are subdued. In practice, the rear port is given most of the responsibility for lowering the effective bass roll-off frequency, but the front port is claimed to provide a better liaison with the radiation from the bass driver.

Crossover frequency is 3kHz and the two sections of the network are wired to separate pairs of deeply recessed terminals, as is usual these days to allow enthusiasts to opt for bi-wiring or bi-amplification. The nominal impedance is quoted as 8 ohms but this dips to a 4.2 ohms minimum which will not worry any normal amplifier and in fact may make the loudspeaker sound louder. Sensitivity is the same as for the DM302 at 91dB, promising realistic sound pressure levels from an amplifier rated within the recommended 25-120 watts per channel range, and of course the claimed frequency response goes a good deal deeper into the bass end of the spectrum.

Floor spikes are supplied which fit into bushes in the loudspeaker base and are recommended for most floor surfaces. The cabinets are finished in black ash vinyl with a cloth grille stretched over a contoured plastics frame.

Performance

Stuffing dusters into the rear vents (I'll try anything in the interests of science) robbed the loudspeakers of almost all of their warmth and bass extension. Similar degradation in quality was audible if the loudspeakers were placed too close to the rear wall, and so I soon settled for placement at least half a metre out into the room. Tweeter height is about ideal and so the main detail to be determined was whether to point the loudspeakers straight ahead or at an angle towards the listening position.

The former arrangement, which I suppose looks more disciplined, works well with over-bright loudspeakers as the listener is then marginally off-axis and extreme high frequencies are to some extent attenuated. However, I had already established to my satisfaction that the DM305 has a civilised top end, free of any serious treble edge. Therefore, although the tweeter design and its mounting plate combine to give a good measure of equable dispersion over a wide frontal angle, I angled-in the loudspeakers to put myself almost exactly on-axis.

As this is a moderately tall loudspeaker, I was surprised at its relatively low weight. Nevertheless I could hear bass notes that belied this lack of physical weight and decided that the extra structural stiffness and scattering of internal energy achieved by the Prism back panel and bracing was having something of the effect that other designs accomplish with thicker panels, sand-filling, lead shot and so on.

The overall tonal balance was well-judged if a little bland, but more detailed listening revealed minor shortcomings at both ends of the spectrum; hardly surprising perhaps since this loudspeaker is, after all, one of B&W's budget orientated efforts. The response has a tendency to be saucer-shaped with some recession in the midrange and a raised plateau at high frequencies. Resolution of detail and stereo spread were both fine at moderate listening levels but any attempt at window rattling brought on a rapid coarsening of texture.

As for bass, I have said that quite low-pitched sounds were clearly audible, but coverage of the lowest octaves was uneven. Organ pedal notes can be relied on to expose such irregularities, which do indeed have an association in some designers' minds with the use of reflex enclosures. Experiments with room placement did ameliorate this effect but could not cure it completely. Piano recordings always make a good test for residual cabinet resonances and I did detect a trace of coloration in the lower midrange when compared with much more expensive monitors from B&W and others.

I am inclined to sum up the DM305 as a worthy floor-stander that looks handsome, despite that modest black ash vinyl finish, and avoids the fiddle of marrying smaller loudspeakers to floor stands. However, it is a middleweight performer with a limited punch. The price is about right but its baby brother, the DM302 now looks like even better value for money at less than half the price.

John Borwick