If you have a look at the Kef Concord from the late 1960s, about half of the B139 seems to be discreetly concealed:
Why would Kef do this?
What effect would it have on the sound output?
I am not really interested in the qualities of the speaker as a whole, I am aware that the B139 is only effective up to about 800Hz and that the T15 isn't really likely to cope adequately from there on up. I just can't understand the logic behind hiding half of the bass driver; particularly since in relation to this speaker, Kef say
Kef link
To understand the answer to this question you have to understand the state of the audio scene in the fifties and sixties, especially the British scene. Although a child, I was in the thick of that scene up to my neck and may be deeper.
Gilbert Briggs, a highly accomplished pianist, founded Wharfedale in 1933 at Bradford Yorkshire. As far as audio was concerned, he was a highly talented amateur, in the best sense of the word. He set about trying to improve the lamentable state of the loudspeaker. Paul Voight was also blazing a trail with full range horns and his company,
Lowther, survives to this day.
After the war, Gilbert Briggs and Peter Walker of Quad, both very close personal friends and accomplished musicians, seriously set about improving the state of the art. Peter developed amplifiers and the electrostatic loudspeaker. Gilbert Briggs set about improving the moving coil loudspeaker. His senior engineer was Raymond Cooke. Others in the game were Tannoy with their dual concentric range, and Goodman's. The chief engineer at Goodmans was the very famous Ted Jordan. These firms really were the original forces behind high fidelity loudspeaker along with Lowther in the fifties.
The problem with the moving coil speaker was a lack of any decent midrange and HF drivers for multi way systems. Paper cones were the rule and drivers had a rough response.
Raymond Cooke with Gibert Brigg's blessing and encouragement went on to found KEF in 1961 at Tovil Maidstone, Kent on the banks of the River Medway, 15 miles from where I grew up. He bought an old iron foundry. Going through the rubble on the site, he found an old casting stamped "Kent Engineering Foundry." The name KEF was born.
Raymond Cooke's main intent was to get away from paper cones and develop synthetic materials. These included Polystyrene, Aluminized Mylar, the Bextrene and then Polypropylene. All these materials were used by KEF first. Raymond also encouraged Thiel and Small and were the first to design speakers using T/S parameters.
The bass KEF B 139 was dead right from the start and remains one of the best bass drivers of all time. It is highly sought after on eBay and high prices justifiably paid.
The bigger problem was designing mid and HF drivers to match, of which by far the biggest problem (and still is) was designing the midrange drivers.
So in that era pushing bass drivers too far up in frequency was common.
For the B139 this was a serious problem as the polystyrene starts to resonate at 1000 Hz and violently at 1200 Hz. In my view the B139 needs to be 24 db down by 800 Hz. I never cross them over above 400 Hz.
So in the KEF concord a very crude attempt was made to suppress this excess energy due to resonance. This is nothing as sophisticated as a phase plug!
For KEF to properly utilize their B139 required the development of the B110 Bextrene Bass/Mid.
The KEF concerto was the result.
Now one thing to understand about the British Audio scene in the fifties and sixties, was the very strong DIY market. To my memory it seems every one was at it.
This was strongly encouraged by Gilbert Briggs in a number of excellent publications, beautifully and amusingly written. They are classics and a wonderful read for his humor alone.
Raymond Cooke followed in his footsteps. He was a keen proponent and encourager of DIY and while he was at the helm had brisk sales of raw drivers. He had his KEF constructor pamphlets. I think it is fair to say that they would have held to view, that if it wasn't a DIY effort it probably was not the best of speakers. They positively encouraged purchasers to mix and match drivers from different manufacturers and experiment with enclosure and crossover designs. Raymond Cooke was much more interested in making good drivers than complete loudspeakers.
I think at that time at my tender age, if I had even suggested owning a manufactured speaker, Raymond Cooke would have expressed his disapproval vigorously.
Now another development occurred in that Ted Jordan developed the Jordan Watts driver along with the accountant from Goodman's, Leslie Watts. I have discussed this driver that appeared in 1959 at length. This was a speaker that ultimately, I ended up getting deeply involved with, but that's a long story. Anyhow I still regard that driver as the best full range driver I have come across. Fuzz a member here has one of these drivers I sent him for a center channel and says it sounds excellent. I sent Afterlife 2 a set of speakers with this 4" aluminum full range driver after he was wiped out by Hurricane Sandy.
The Jordan Watts modular full range loudspeaker.
So immediately I set about mating this driver with the KEF B 139 and topping it out with just a first order network to the tweeter.
This is before the Concerto came on the scene.
In my first level system I have a modern version of the Concerto, using the B139, as a tribute to Raymond Cooke and his achievements.
Another facet of those days, which modern MBA and marketing types would not understand was the degree of cooperation and collaberation between competitors.
BBC engineering had a big part to play in the development of loudspeakers.
In the design and construction of the legendary TL monitor speakers at the BBC, Raymond Cooke, Peter Walker, Jim Rogers and members from the TL development team from Radford were all involved. And by the way, not all of that team were Radford emplyees. Dr Bailey for instance was from Decca. It is very hard in these money grubbing times of optimistic specs and flashy marketers to get the feel of the ethos of that era, but that is the way it was.
Multiptiple firms were also involved in the development of the BBC small monitor LS3/5A. That is why so many firms produced versions of that classic. You just could not see that happening today.
If you click on my signture you will see my dual triamped TL loudspeakers, which I regard as the direct descendants of those BBC TL monitors, which be the way were decomissioned less than 10 years ago, I think around 2005.
So it is important to have a perspective on the times, and understand what was known and unknown, and how a crude move like partially covering a driver to muffle a resonace could be done without shame.
As the seventies dawned, this wonderful "Golden Age" was swept away by the onslaught of Japanese mass manufacture.
