ProAc’s brochures—and its website—carry the boast: ‘Quality Speakers—Proudly Made in England.’ The brochures and website could also claim that not only are the speakers still made in England, but the company is still owned by an Englishman. The sad truth is that these days, very few other famous British speaker brands could make either of those claims. Most so-called ‘British’ speakers are actually made in Europe, India or China, and most of the ‘British’ speaker companies still building speakers in Great Britain are owned by people of other nationalities: Chinese, Indians, Canadians… even Australians!
The Englishman who runs ProAc is none other than the famous Stewart Tyler, who also designs the speakers. Tyler founded the company that would become ProAc in 1973, but when he started, the company was called Celef. According to David Amy, who joined ProAc in 1987, Tyler called the company Celef because for two or three years prior to starting it, he had been building speakers on a small scale in his own home, using bass/midrange drivers he purchased from Celestion, and a KEF tweeter. (Interestingly, there’s an Australian connection here too, because in the very early days of Celef, Tyler hired an Australian electrical engineer, ‘Bluey’ Sutcliffe, to help him out, but he left after a short while and returned to Tasmania.)
Tyler decided to drop Celef as a brand name in 1980, replacing it with ProAc, which is an abbreviation of ‘Professional Acoustics’. The impetus for the change was, curiously enough, because Celef speakers were so popular—particularly in the UK—that in order to meet the increasing demand, Tyler was going to have to add extra employees, or stop making his speakers by hand, neither of which he wanted to do. He decided that if he reduced the size of his range and also concentrated only on building more expensive speakers he could keep the company the same size, and continue making hand-crafted speakers. However, since his new speakers would be more costly, the brand ‘Professional Acoustics’ or ProAc, would be more appropriate. Thus it was that Celef became ProAc. In all the years since, ProAc has never made a ‘budget’ speaker, and Tyler is actually on record as saying that he’s proud of that fact. ‘The ProAc range is, as they say, reassuringly expensive,’ he says. ‘We do not make cheap loudspeakers and certainly do not produce cheap sounds. Happily, many thousands of audiophiles worldwide agree with our philosophy.’
The Response D Two uses a 165mm bass/midrange driver made specifically for ProAc by SEAS. The cone is made from woven glass fibre and it’s driven by an ‘Excel’ magnet system, with its distinctive copper phase plug. According to ProAc, although this driver is new, with a revised coil and magnet, it uses the same cone technology as the bass/midrange driver in the Response D15. ProAc ‘specs’ the driver diameter at 165mm, but this actually is the distance between the mounting holes—a measurement that’s often used by driver manufacturers. The overall diameter is a little larger: 197mm. However, it’s the Thiele/Small diameter that’s the important specification, and this is much smaller—133mm. This would normally give an effective cone area (Sd) of 139cm², but because in this design there’s no dustcap, Sd calculates out as being just fractionally smaller: 134cm².
The ‘Excel’ magnet system addresses a problem that affects all moving coil loudspeakers, which is that the coil—like any coil of wire—is an inductor. And, when that coil is sitting around the pole-piece (as all of them do in a moving-coil loudspeaker!) it’s an iron-cored inductor. If the voice coil didn’t move, the inductance of the coil would stay the same and you could incorporate the appropriate compensation for the inductance in the crossover network. However, in a moving coil speaker, the coil moves, and as it does, its inductance changes as a direct result of the movement. The problem with this effect is that one of the most important factors in determining the upper roll-off frequency of the bass driver is its inductance. The value of inductance also affects the relative acoustic delay between the bass/midrange driver and the tweeter. So as the inductance varies, so too will the roll-off frequency and acoustic delay. When does this happen? Basically, whenever you can see the cone moving with the naked eye, the inductance will be so far in error that the speaker will be producing intermodulation distortion and frequency modulation distortion. (Speaker designers refer to the overall problem as ‘inductance modulation’). Generally speaking, inductance modulation is most likely to occur whenever the speaker is reproducing deep bass, and most particularly when the volume is also high. Generally, audiophiles perceive distortions caused by inductance modulation as an overall ‘blurriness’ to the sound quality.
In ‘Excel’ drivers, such as used in the ProAc Response D Two, the pole-piece is plated with copper. Doing this ‘shorts-out’ the eddy currents induced within the magnet structure by the voice coil and results in a far-lower inductance, which means that although inductance modulation still occurs, the inductance of the voice coil is so low that the variances in inductance are consequently smaller, so there’s less inductance modulation. SEAS is not the only driver manufacturer to have addressed this problem: Dynaudio uses a similar system in its drivers, so too does Scan-Speak. Most recently, Dali Loudspeakers has patented a new approach to solving the problem, which it calls a ‘Linear Force Motor System.’
The Excel bass/midrange driver crosses to a 25mm silk dome tweeter (exactly the same one used in other models in ProAc’s range) which has a claimed h.f. extension to 30kHz. Unusually, Tyler does not use ferro-fluid to cool the tweeter: it’s air-cooled. Also unusually, the tweeters are mirror-imaged on the front baffles. This is clear evidence of hand-assembly, because large manufacturers just won’t go to the trouble (nor will they wear the added expense!) of producing mirror-imaged pairs. But from an owner’s point of view, mirror-imaging gives enormous additional scope when positioning the speakers, because if you have to put your speakers fairly close together, positioning the speakers so the tweeters are to the ‘outsides’ of the baffles will give you a perfect stereo image at the listening position. If necessity means you have to put the speakers further apart than is usually ideal, you simply swap the speakers around so the tweeters are on the ‘insides’ of the baffles and once again, you will have a perfect stereo image.