What is the effect of hiding the B139 in the Kef Concord? - diyAudio
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Old 22nd June 2013, 12:55 PM   #1
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Default What is the effect of hiding the B139 in the Kef Concord?

If you have a look at the Kef Concord from the late 1960s, the B139 seems to be half hidden behind the baffle board:

Click the image to open in full size.

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 above that. I just can't understand the logic behind hiding half of the bass driver; particularly since in relation to this speaker, Kef say
Quote:
Many otherwise fine loudspeakers at the time were ruined by their grille fabrics, which upset the frequency response ...
Kef LINK
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Old 22nd June 2013, 02:09 PM   #2
DavidL is offline DavidL  United States
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Some sort of an attempt at using the baffle board as a phase plug to correct for frequency peaks or droops? According to this link audio-talk :: View topic - Kef Concord loudspeakers. "The Concord had B139 XOed to T15 at 1kHz, The B139 has a nasty resonance right near there and even using it as high as 400 Hz is pushing it. To get the tweeter to go that low its extension is not all that good. "

Factory data sheet here. TL Links
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Old 22nd June 2013, 04:21 PM   #3
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It's a phase plug. Nexo uses them to this day:

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Old 22nd June 2013, 04:25 PM   #4
Zero D is offline Zero D  United Kingdom
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Apart from what surv1v0r noted, i think there "might" be other reasons, and/or too.

1 - That device appears similar to what's used in some Pro line arrays.

2 - Richard Small of Thiele/Small fame, in one of his papers stated that, obstructions can either increase/decrease output, depending on type.

So a couple more ideas to consider

If anybody knows exactly why KEF did that, i'm sure we'ld like to know
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Old 22nd June 2013, 04:30 PM   #5
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More from Nexo:

"In line arrays, the physical diameter of cone drivers would appear to make it impossible to achieve interference-free, close coupling of wavesources at the frequencies necessary to crossover with HF drivers. NEXO’s revolutionary Directivity Phase Device causes an 8 inch driver, for instance, to behave as twin 4 inch drivers, with two acoustical centres spaced 5 inches apart, cleverly extending the upper frequency limit for line source coupling between adjacent woofers."
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Old 22nd June 2013, 05:12 PM   #6
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Possible that in the Kef baffle, the twin eyeshaped openings create two new acousstic centers, then facilitate the crossover add on point to the tweeter? , it is , one B139 becomes = two B110's ?
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Old 22nd June 2013, 10:58 PM   #7
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Quote:
Originally Posted by Mosquito View Post
Possible that in the Kef baffle, the twin eyeshaped openings create two new acousstic centers, then facilitate the crossover add on point to the tweeter? , it is , one B139 becomes = two B110's ?
I must admit that that had crossed my mind but as you perhaps suggest, why waste half of a B139 when it would seem much more logical to use all of two B110s?
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Old 23rd June 2013, 12:08 AM   #8
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Quote:
Originally Posted by surv1v0r View Post
I must admit that that had crossed my mind but as you perhaps suggest, why waste half of a B139 when it would seem much more logical to use all of two B110s?
When you mask off the diaphragm it doesn't 'waste' any of the speaker. Output is determined by displacement*, and displacement doesn't change when you mask off the cone of a driver. So you basically get the output of an eight, with the directivity of a couple of fives.

* exception to this rule would be when you're limited by power handling. But generally one eight will have power handling which is comparable to a couple of fives, unless the voice coil on the eight isn't any bigger than on the fives.
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Old 23rd June 2013, 12:59 AM   #9
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In those days voice coil overhang was proportional to cone area. Maybe they were loading the driver to lower Fb?
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Old 23rd June 2013, 02:00 AM   #10
bjorno is offline bjorno  Sweden
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Hi,

FYI: IME (40 Years ago) If detaching the restrictive covering on the Driver front-side: Frequencies around ~1 kHz did not sound as good as when mounted. The Speaker acted like an T-W-W system with halving the C-C distance between the Drivers for a much better XO summing than if no area splitting is in use.

I used those speakers placed horizontally(One of KEF:s recommendations) with the tweeters at the inside of a ~isosceles triangle.

The sweet-spot could accommodate 2 listeners at most with unsurpassed phantom focus if compared to a vertical set-up. My room at that time was very absorptive and walls acting very diffusing on reflexes.

b
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