B & W DM2 Monitor Loudspeaker Price: /J59-90 + tax, each. Manufacturer: Bowers & Wilkins Electronics Ltd., Littlehampton Road, Worthing, Sussex.

ALTHOUGH the firm of B & W Electronics is only six years old, its maturity of design philosophy is remarkable and all stems from the outstanding enthusiasm of a small team of engineers and designers headed by John Bowers. He is ably supported by Dennis Ward and Malcolm King, who joined the team a few months ago after gaining high academic honours at Southampton University. Specialising in the field of acoustics, he was responsible for the design of the acoustic line used in the DM2.

The specification Table reveals only a thumbnail sketch of the intriguing approach to the design of a new system and it is worth amplifying the details. Some 18 months ago John Bowers, having designed several very successful large loudspeakers, recognised that many enthusiasts' listening rooms would not easily accommodate large enclosures, or they wished to fit them into bookshelf structures. Further, although the power output from modern stereo amplifiers was rising, he felt that this was no excuse for reducing the conversion efficiency of the loudspeaker. It should have a medium sensitivity so that it could be used with lesser power amplifiers. Another target was to keep any inherent distortion down to a low figure but at a higher acoustical output than loudspeakers of similar size and cost.

Mid-range frequency distortion and linearity is a problem every loudspeaker designer meets and, as B & W designed their own bass-midrange unit, considerable research was instituted to improve these parameters. This has resulted in a unit using a heavy ceramic magnet mounted on a diecast aluminium non-resonant chassis. The diaphragm is formed from Bextrene. It has a central 54mm dome, and the whole of the front surface is treated with a synthetic black dope. The outer surround is a formed roll type giving considerable freedom of movement. The speech coil is 38mm in diameter and wound on an aluminium former which assists heat dissipation, reduces the dynamic mass and applies electrical damping.

To cover the lower high frequencies, the designers chose the Rola-Celestion HF 1300 unit which is used in the BBC Type LS3/6 monitor speakers. However it was decided that, if one wishes to extend the high frequency response to above 20 kHz, it is desirable to use a separate super-tweeter: a 25mm dome type unit is used, and this extends the frequency range beyond 25 kHz.

Design of enclosure

Having finalised the individual units, the next task was to design a medium size cabinet which would produce a full bass response, whilst still maintaining reasonably good sensitivity. There are several possible approaches from completely sealed cabinets to horn loading, but it was decided to investigate the possible ways of rear loading the bass unit with an acoustic line. In the ideal case, one loads the rear of the diaphragm with an infinitely long expanding duct, covered with an absorbing material, so that by the end of the duct all the sound is absorbed. It was at this stage of development that Malcolm King joined the company and developed a rear loading system using an eighth wave folded tapered line which terminates at the bottom of the front baffle board, and is approximately the same area as the bass unit.

If you visited the Sonex exhibition, you probably saw the elegant demonstration of the effectiveness of this system. A conventional cabinet had one side replaced with a heavy Perspex sheet, and a variable low frequency signal was applied, while viewing by a stroboscopic lamp. The movement of the combination acoustic loading could be viewed as a gentle motion reaching a peak at the bass resonance of the system. The loading is so effective that it limits the normal high amplitude movement of the diaphragm at its natural resonance and extends the low frequency response.

To achieve this, three different materials are used in the acoustic line. The three units are mounted on a one-inch thick, dense baffle board together with the crossover and filter network. The layout of the units is unconventional, with the bass unit offset from the

Fig. I. Diagrams illustrating the Left/Right arrangement of the high frequency units (from B & W leaflet) centre line and mounted at the top of the cabinet (if it is used vertically). Below this are the two high frequency units and, in order to achieve the best stereophonic results, their positions are reversed in the left and tight hand enclosures. The baffle board is 60mm shorter than the cabinet, thus giving the required termination of the acoustic line. Behind the baffle board is the acoustic loading material and the folded tapered duct by which the rear of the bass unit is isolated from the two high frequency units.

The crossover and filter unit is one of the most complex I have seen in a modestly priced enclosure. It consists of four air cored inductors, two ferrite core inductors, ten precision polyester dielectric capacitors and four wirewound power resistors all mounted on a substantial printed circuit board. The circuit follows the well-known Butterworth third order configuration which gives a stop band attenuation of 18 dB per octave with phase and impedance correction for the higher frequency bands.

The cabinet is constructed from 18mm thick blockboard veneered in alternative finishes, and is internally braced with the tapered duet. It is acoustically dead and, even under the highest power conditions, the surfaces of the cabinet are free from resonance. The front grille is removable and covered with an acoustically open weave material backed with a thin black foam which makes the grille optically opaque. The terminals at the rear are mounted in a deep plastic moulding, phased red and black, and thus the enclosure can be positioned flush against a wall.

Alternative mounting accessories are available. There is a black, floor-standing squaresectionsteel plinth mounted on free running castors. Alternatively there is a chromium plated central column supported on four legs, or wall mounting brackets specially designed to support the weight of the enclosure (23 Kg).

Calibration curves

Accompanying each DM2 is a calibration curve plotted in the B & W anechoic chamber using Bruel & Kjaer automatic recording equipment. The frequency response quoted by the designers is ± 2 dB from 300 Hz to 6 kHz and within ± 3 dB between 65 Hz and 20 kHz, and the curves meet this specification. The frequency and polar curves were plotted for this review in the anechoic chamber at The Polytechnic of North London and, as this is rather larger than the B & W chamber, they show an extended bass response (see Fig. 3).

For the benefit of the user, B & W have also produced an excellent instructional manual which will enable one to enjoy maximum SPECIFICATION AND TEST RESULTS B & W DM2 MONITOR LOUDSPEAKER 1. Type 2. Drive unit diameters (in.)

3. Crossover frequencies (Hz) 4. Frequency Response (Hz 5. Power handling capacity (Watts) 6. Impedance (ohms) 7. Dimensions (in.)

8. Finish

Makers Specification

Three unit Monitor with patented eighth wave rear acoustic line

Lower HF unit, Celes H -tion F1IOO

Bass-midrange 150mm Super tweeter 25mm dome type

Third order Butterworth crossover filters with 18dB/octave attenuation 20-25,000 up to 60

Nominal 8 ohms 254 x 13 x 13 Weight 28Kg

Teak, walnut or satin white

Test Result

See Fig. 1 agreed See Fig. 3

Fig. 2. Polar response at 100, 1,000 and 10,000 Hz benefit from the enclosures. As nearly everyone these days uses loudspeakers under stereo conditions, B & W supply matched pairs with different polar response curves. In order to identify the units, the carton of the left hand speaker is printed in blue and the right hand speaker in red, with the B & W insignia also on the left and right when facing the speakers. The manual has chapters on the effect on performance of the listening room, its furnishing and the positioning of the enclosures. Whilst one may not have any control over the floor structure, the ideal would be a ground floor with a solid concrete base; a suspended floor on joists can lead to accented bass radiation.

To improve the polar response, the left hand speaker has the super-tweeter mounted to the left of the high frequency unit, and the right hand speaker has the super tweeter to the right.

Fig. 4. Plot of impedance against frequency

OHMS 20 15 10

S 20 II!!I I!!EiUII• iiIIilIuuIlNr4 50 100 200 500 1K 2K 5K 10K 20K

FREQUENCY lHzl

This effect will be seen in the polar response curve (Fig. 2). Normally one would expect the dip on the 1 kHz curve to appear in line with the 1800 axis, and the bunch of bananas at 10 kHz to be symmetrical about the 180° axis. From the diagram it will be seen that the 10 kHz distribution is biassed anti-clockwise for the right hand speaker, thus giving a wider stereo image at high frequencies compared with two identical enclosures. Although not shown, the left speaker possessed a clockwise bias for the highest frequencies.

A further page in the manual specifies the type and gauge of cable to use between the amplifier and the loudspeakers, together with a paragraph on phasing. Another entry emphasises the importance of the quality of other parts of the reproducing chain for, whilst more modest speakers will hide slight distortion say from an amplifier or pickup cartridge, the analytical performance of the DM2 Monitor speaker may show up faults which might be wrongly blamed on the speaker.

Finally there are two pages of typical frequency, distortion, impedance and polar distribution curves.

How they performed

It so happened that the pair of DM2 Monitor speakers arrived at my home] ust before one of our 'Sounds in Retrospect' panels met to analyse a number of recently released records. The panel's immediate reaction was one of praise for the smoothness of response, power handling capacity and freedom from distortion of any kind. Subsequently I have used the speakers for nearly a month and directly compared them with four other pairs of highly favoured enclosures. When placed in accordance with the designers' recommendation, there is obviously a wider stereo image than obtains with a pair of identical speakers. As an experiment, the DM2 speakers were purposely reversed and immediately the stereo stage contracted towards the centre. One can visualise a condition where this might be of advantage for, if it is necessary to place the enclosures snugly in the corners of a wide room, a high frequency spread towards the centre might help to fill in the 'hole in the middle'.

Tests with sirsewave gliding tones failed to show up any audible resonances either of the units or of the cabinet. As the cabinets were raised on their floor stands, the high frequency units are roughly at ear level when one is seated in an armchair, which enables one to enjoy the widest frequency coverage and least absorption from furniture, carpets etc. So good are the crossover filters and network, that it is very difficult to hear the transition from one unit to the next, and the frequency response curve does not give a clue.

I have enjoyed listening to a wide range of music and speech at varying volume levels and I was struck by the extension in the lower bass register due undoubtedly to the acoustical line loading. The Polytechnic plotted frequency response curve (Fig. 3) shows an unusually high level below 100 Hz and, if compared with previously published curves in this journal, it will be seen that the bass is exceptionally well maintained down to 50 Hz and then smoothly falls away. The fall-off below 70 Hz is due to the finite dimensions of the anechoic chamber but, as all speakers are measured under identical conditions, the graph of the DM2 shows a positive improvement over competitive models.

The designers' target of producing a midrange free from colouration has been reached for, on pianoforte music, there is a clarity and openness seldom experienced from modestly priced speakers. Speech is free from chestiness and sibilant sounds do not excite resonances in the high frequencies. A test with a pink noise generator produced the sound of gently hissing steam, without emphasising any particular part of the sound spectrum. Thus the DM2 Monitor speakers passed all the tests with outstanding results and, even when operated at a sound level far too loud for comfortable listening, there was nosuspicion of overloading and distress.

In the past, I have always commented on the excellence of John Bowers' designs and his constant endeavour to recreate a musical sound nearer and nearer to the original studio performance. The DM2 Monitor truly lives up to this aim and, considering that its price is lets than half that of the DM70 electrostatic-dynamic combination and that it is considerably smaller, it is a very close approach to the performance of its senior partner. I have no reservations in placing this speaker in the top echelon of modern high quality systems.

JOHN GILaERT.