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Trying to understand the logic of the 107 design

 
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idf
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Joined: 02 Sep 2009
Posts: 14

PostPosted: Wed Sep 09, 2009 12:39 pm    Post subject: Trying to understand the logic of the 107 design Reply with quote

Hey everyone,

I cant make sense of the KEF 107's design. I recently opened them up to redo the surrounds (what a difference, by the way!) and was surprised to see the configuration of the cabinet.

As many know, the 107 has 2 woofers firing in the same direction (downward) which are connected via a rod. The top woofer fires into a chamber from which you can see the back of the 2nd woofer, this chamber is connected to the chute that you see from the top of the speaker.

The second woofer has me puzzled though, since it fires into a sealed enclosure and is not connected to the chute. I'd always thought it was.

Logic would suggest that both woofers firing in the same direction in this configuration would cancel each other out.

Can anyone explain the design and why its made this way?

Thanks!
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speakerguru
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Joined: 18 Nov 2005
Posts: 958
Location: Green Hut, Tovil

PostPosted: Wed Sep 09, 2009 5:36 pm    Post subject: Re: Trying to understand the logic of the 107 design Reply with quote

idf wrote:
The second woofer has me puzzled though, since it fires into a sealed enclosure and is not connected to the chute. I'd always thought it was.

Logic would suggest that both woofers firing in the same direction in this configuration would cancel each other out.


The drivers are in phase acoustically. (They are both compressing the air in the centre cavity.) They are mounted out of phase mechanically and connected out of phase electrically. Thus two phase reversals resulting in in-phase acoustically and force cancelling mechanically to the box. Search on bandpass and coupled cavity for more info.

I'll try to dig out the first AES paper by KEF on the coupled cavity and get it on the site.
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Lee in Montreal
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Joined: 22 Aug 2009
Posts: 203

PostPosted: Fri Oct 09, 2009 11:56 pm    Post subject: Reply with quote

Hi IDF

Indeed a short interwebz research can help you a little understand the principles.

The first one is called "4th order bandpass enclosure" and the second is " Isobaric enclosure".
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Dyolf
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Joined: 08 Dec 2014
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Location: Norway

PostPosted: Sun Jan 04, 2015 3:28 pm    Post subject: Re: Trying to understand the logic of the 107 design Reply with quote

speakerguru wrote:
They are mounted out of phase mechanically and connected out of phase electrically. Thus two phase reversals resulting in in-phase acoustically and force cancelling mechanically to the box.


I think I get this part, but what is the advantage, if any, in mounting the woofers this way. Nearly any other search on the net for a "dual driver coupled cavity enclosure" depics the woofers firing facing the common cavity, ie. in phase both electrically and mechanically. I assume the rod would act similarly in and out of phase?
Steen
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SaSi
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Joined: 24 Aug 2008
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PostPosted: Sun Jan 04, 2015 4:49 pm    Post subject: Reply with quote

Because the drivers are mounted in the same orientation (i.e. not facing each other), they need to be connected in opposite electrical phase so that they are mechanically - and acoustically - in phase.

That means that when the top woofer retracts the cone upwards, into the top sealed cavity, the lower woofer extends the woofer, downwards, into the bottom sealed cavity.

Indeed the woofers could have been mounted facing each other or back to back (with the magnets in the middle). Orientation is mere cosmetics and as the drivers are inside the cabinet, of no consequence. Other factors become more important, such as:
1. Ease of assembly and replacement
2. Implementation of the connecting rod + fastening (to cancel out vibrations)
3. Implementation of effective volumes: The basket + magnet assembly takes up considerable space. Orientation here would affect the rear / front volumes of the coupled cavities.

I have modeled a pair of bass cabinets after the R107 (and used the original crossovers and heads). After some modeling and trials, I concluded that the closest and best match for that experiment was the SP1014 woofer. I have not linked them with rods and to achieve the target front/rear enclosure volumes, mounted the woofers with facing magnets. Makes it easier to mount/replace/seal with gaskets and has the magnets occupy space in the shared front cavity which I wanted to be small, leaving more space for the rear cavities which I needed to maximize - without altering the R107 enclosure outer dimensions.

I consider the experiment a success and the tightness and clarity of low frequencies coming out of these enclosures (without a KUBE but with some help from a parametric equalizer) is phenomenal. Comparing them with my pair of R105s, I must admit that the bandpass enclosure model is a much more efficient one compared to the sealed enclosure.

And I'm happy I found some good use for my SP1014s, as they are not particularly good in bass extension in either sealed or vented enclosures.
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speakerguru
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Joined: 18 Nov 2005
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PostPosted: Mon Jan 05, 2015 9:08 pm    Post subject: Re: Trying to understand the logic of the 107 design Reply with quote

speakerguru wrote:

The drivers are in phase acoustically. (They are both compressing the air in the centre cavity.) They are mounted out of phase mechanically and connected out of phase electrically. Thus two phase reversals resulting in in-phase acoustically and force cancelling mechanically to the box. .


In case this is proving too succinct I will expand/paraphrase.

The drivers are in phase acoustically. (They both compress the air in the centre cavity.) They are mounted out of phase mechanically and connected out of phase electrically. Thus two phase reversals results in both drivers being in-phase acoustically in regard to their orientation in the outer ported cavity. The reaction force on the magnets, being equal and opposite, cancel by means of the coupling rod. The mechanical force transmission from driver chassis/magnet to the box is thereby reduced.
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vistisen
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Joined: 01 Sep 2006
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Location: Denmark

PostPosted: Sat Jan 10, 2015 7:32 pm    Post subject: Reply with quote

I love my 107's still. Nothing I've heard makes me want to change them, But almost every HiFi shop talks about them being "good for their time", but that there have been major advances in design, materials and so on. But is this true? or are they just cheesed off that they cannot persuade me to buy something? I've heard speakers that seem to reproduce a clearer vocal ( but canít make large scale classic music sound real). Some seem to have better imaging, but to me they are 'shouting' at me. Am I a fuddy duddy who is not willing to except the truth? or are they still after so many years, one of the best speaker designs at the price ever? (He asks not very objectively Rolling Eyes )

I still want to find a pair of 107/2s, and I'd love to hear a set og Maidstones (109s)
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proffski
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Joined: 22 Aug 2003
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PostPosted: Sat Jan 10, 2015 8:53 pm    Post subject: Reply with quote

"But canít make large scale classic music sound real".

Totally agree, I listened to some new loudspeakers today with a friend which he bought.
They could not help SHOUTING at me, it may impress some but not me.
Sadly there were no Spendor or Harbeth there to compare.
These I feel still have that lovely "British Loudspeaker Sound" signature to this day...
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SaSi
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Joined: 24 Aug 2008
Posts: 223

PostPosted: Sun Jan 11, 2015 10:41 am    Post subject: Reply with quote

vistisen wrote:
I love my 107's still. Nothing I've heard makes me want to change them, But almost every HiFi shop talks about them being "good for their time", but that there have been major advances in design, materials and so on. But is this true? or are they just cheesed off that they cannot persuade me to buy something? I've heard speakers that seem to reproduce a clearer vocal ( but canít make large scale classic music sound real). Some seem to have better imaging, but to me they are 'shouting' at me. Am I a fuddy duddy who is not willing to except the truth? or are they still after so many years, one of the best speaker designs at the price ever? (He asks not very objectively Rolling Eyes )

I still want to find a pair of 107/2s, and I'd love to hear a set og Maidstones (109s)


I'm sure that a pair of Blades would knock you off and perhaps make you think about replacing your R107s. But I would not expect any $3-4K system to compare favorably.

Of course that would be true if:
1. The tweeters haven't had their ferrofluid dried out
2. The woofer foams are ok (properly replaced, correctly centered)
3. The capacitors haven't drifted too much from their original values causing the response to suffer.
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vistisen
Intermediate Contributor 50+


Joined: 01 Sep 2006
Posts: 57
Location: Denmark

PostPosted: Fri Jan 16, 2015 3:33 pm    Post subject: Reply with quote

SaSi wrote:


I'm sure that a pair of Blades would knock you off and perhaps make you think about replacing your R107s. But I would not expect any $3-4K system to compare favorably.

Of course that would be true if:
1. The tweeters haven't had their ferrofluid dried out
2. The woofer foams are ok (properly replaced, correctly centered)
3. The capacitors haven't drifted too much from their original values causing the response to suffer.


So where would I go to get these things checked?
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