Originally printed in the November 1978 issue of Gramophone.

In the ten years since John Bowers started designing loudspeakers, each design has shown a significant step forward, and the new DM7 is no exception. How successful his company's efforts have been since the introduction of the first linear phase loudspeaker, the DM6, will be seen in this review. When the team initiated the design of the DM7 some two years ago, they decided that the new loudspeaker should be compact, that cost should be secondary to performance and engineering excellence and that appearance should be acceptable for modern households.

Limiting the design to a three-unit system housed in a 40-litre enclosure using entirely new drive units, John Bowers aimed at satisfying the following targets. The free-field axial response should be held within ±2dB from 70Hz to 20kHz; the polar response should provide as wide a horizontal spread as possible and a restricted vertical spread; and within this listening window both frequency and phase should meet the closest tolerances. The research work initiated for the DM6 design had shown that inter-unit time delay was of considerable importance, and was to be used for the DM7 also. With the versatile computer facilities available, transient response should be investigated together with coloration in all its complex forms. Also, the computer should be programmed to assist with crossover filter design, and to formulate the weighting contour parameters.

Protracted measurements had shown that, when a high-frequency transducer is mounted within the cabinet, the usual hemispherical dome unit sets up standing waves which cause considerable irregularities in both the frequency response and phase characteristic. This indicated the desirability of mounting the HF transducer outside the cabinet, and that it should have a smoothly contoured design. With a target handling capacity of 100 watts RMS, the HF units normally available would suffer early destruction, so an improved design became necessary. One could increase the diameter of the speech coil to assist heat dissipation, but that degrades transient response owing to the higher mass. Alternatively one could improve the electro-mechanical transfer by using horn loading, but again this has the disadvantage of creating peaks and dips in the frequency response. A third method, finalised for the DM7, is to improve the efficiency of the unit. The TS26 spheroidal HF transducer uses a very light diaphragm of some 350 milligrams, in order to give a good transient response, and an extremely powerful centre-pole magnet which improves the acoustical output some 6dB over that of the bass/mid-range unit. Built into the crossover filter is an attenuator for the TS26 unit so that it gives a level response and also improves the power handling capacity by a factor of two. To protect the HF unit, a 0.5A fuse is mounted on the moulded terminal panel, together with a 2A fuse to protect the whole system.

Experience gained from the midrange unit in the DM6 has been used again in the DM7. The 145mm diameter diaphragm is constructed from the B&W matrix of aromatic polyarnid fibres suitably doped, which has been found to give a lower distortion figure than more conventional types. To combat mechanical distortion due to heat generation, it was necessary to use new materials for the speech coil and the rear centring device, the latter using the same material as the diaphragm. The chassis has six apertures allowing free movement of air at the rear of the centring device. Also the centre pole is hollow to relieve air pressure built up at the rear of the dust cover, all small points which improve linearity. The diaphragm is terminated with a high-loss roll surround giving a total excursion of approximately lOmm. A massive 140mm diameter by 17mm thick ceramic magnet is sandwiched between heavy plated steel discs giving a highly concentrated field in the speech coil gap, the size aiding heat dissipation.

The ultra-low-frequency passive radiator UBR220, is acoustically coupled to the bass/ mid-range unit, BM220, and is designed to augment the output below 100Hz. It uses the same chassis but the thick diaphragm, which has a flat front surface, is made from crosslinked polyethylene foam and is mechanically loaded to give a resonance frequency of 18Hz.

The 'Plastazote' diaphragm has the advantage of giving a low transfer of signal from the inside of the enclosure through the diaphragm material, a reduction of 10dB having been measured.

Mounted on the base of the enclosure is a printed circuit board carrying the components of the crossover. This consists of 13 elements.

The DM7's construction uses large ferrite and air cored inductors and precision polyester capacitors in a third order Butterworth circuit configuration. Sockets are mounted on the PCB for plug connections to the two drive units and the contour control.

The enclosure is constructed from veneered particle board of high density and internally laminated to 12mm thick bituminous antiresonant panels, giving a total wall thickness of 25mm, a form of construction which is said to be better than using 50mm of reinforced concrete. To reduce cabinet wall resonance, the panels are braced together and lined with flameproof thick foam on all internal surfaces. In place of the conventional front panel carrying the drive units, a heavy moulded panel made from structural polystyrene foam is used. The grille support is also made from structural polystyrene foam, over which is stretched an acoustically transparent grille material. The top surface of the enclosure is covered with a brushed and anodised aluminium panel into which is recessed the four-position contour control and, near the front edge, the spheroidal HF unit. The latter is mounted on a neat circular moulding and a hemispherical protective grille is gripped between the moulding and the aluminium plate. On the rear panel is a flush moulding carrying the two fuse holders and DIN and 4mm sockets clearly marked for polarity. The fuse holders will accommodate both 20min and 32mm fuses. The 20mm type are supplied along with spare fuses and an Allen key. The enclosure is mounted on a 175mm anodised tube supported on a 5mm thick rectangular aluminium plate, brushed and anodised, the underside carrying four plastic glides. The instruction manual gives full details and photographs for correct assembly, covering electrical connections, the listening room, positioning of the loudspeakers, the functions of the contour control, the quality of ancillary equipment, a list of recommended gramophone records, fault-finding and service. The speaker is available in four different veneers or white satin, with, matching brown or black grille cloths, stand tube and control knob. Three additional grille colours, red, dark blue or orange will be available at extra cost.

How it performed An unusual opportunity enabled me to measure the performance of the DM7 speakers in the largest anechoic chamber in the UK. This is located at the Building Research Station at Garston, Watford and measures 14 metres long, 9.15m wide and 9.76m high between the tips of the 1.22m deep expanded polyurethane wedges. The cut-off frequency for the wedges is set at 70Hz. Frequency response measurements were made with the microphone at 1 and 2m and except for the lowest octave the curves are virtually identical—the differences at low frequencies being due to the chamber. One of the features of the DM7 design is the listening window and therefore frequency runs were made for horizontal and vertical dispersion at various angles with the microphone at 2m. The horizontal curves are virtually identical. Vertical dispersion was measured at +5°, —50 and —10° and again it will be seen that the curves are very similar. Figure 4 shows the effect of the four position contour control, allowing one to tailor the overall response of the loudspeaker to the listening room. Figure 5 shows second and third harmonic distortion, the curves being raised 20dB in order that they can be plotted on the 50dB graph paper. Finally, Fig. 6 shows the usual impedance curve which meets the manufacturer's specification.

My immediate reaction on listening to the DM7s was to the considerable improvement in stereo imagery, which I can only compare with the sound of stacked Quad electrostatic-speakers. Owing to the wide dispersion in the horizontal plane, seating positions are not so critical as with some other speakers and, with several listeners in a room, this is an important virtue. When seated in an armchair, one's ears are at approximately the same level as the spheroidal transducer and I usually listen at a distance of 3-4m. Standing up at this position made very little difference to the overall performance but, as one approached much closer to the speakers, one was conscious of a reduction in level of the highest frequencies. This is a considerable advantage in a low ceiling room, where reflections from the ceiling to the walls can cause muddled sound.

The overall sound quality is unusually open with excellent definition and integration. One is not conscious of the position of the speakers themselves and, considering their modest size, the bass response, whilst not quite so deep as the DM6s, adequately balances the middle and high frequency response. The DM7s have a more forward sound with almost complete freedom from coloration particularly in the critical midrange area. With solo instruments, such as piano, the sound is definitely located between the speakers and locked in position. Speech, and particularly male speech, has a naturalness sadly lacking in many loudspeakers and one could easily be deceived into believing that one is hearing a live conversation. The positioning of the speakers is less critical than normal in this case as the stand raises them sufficiently to eliminate unwanted floor-coupled resonances. With large-scale orchestral works, one has no difficulty in pinpointing the individual groups of instruments: with operatic records, the singers appear separate from the orchestra and remain in the correct perspective. The BBC transmissions of performances at Bayreuth produced a dramatic realism seldom heard before in which Fig. 4. Effect of contour control 

The terms "monitor" and "musicality" have become commonplace in loudspeaker descriptions. Neither will be found in the B&W literature, yet they could deservedly be used. Capable of very high volume with the minimum of coloration, they qualify for the monitor class of loudspeakers used by recording studios and broadcasters, whilst their musicality places them in the highest echelon of high quality reproduction. With careful use of the contour control, and with a small amount of extreme low frequency boost, the overall performance is the smoothest that I have experienced under normal domestic conditions. 

Manufacturer: B&W Loudspeakers Ltd., Meadow Road, Worthing, West Sussex, BN11 2RX. Recommended price: £99.78 per pair.

John Gilbert