The KEF R107 Cabinet Thread

SimplySound

Kent Engineering & Fondry
Subscriber
There's many speakers out there with some pretty complex cabinets. I'm thinking transmission lines and the such. The KEF R107 was revolutionary in it's day for the fact that it delved into the world of "Coubled Cavity Design" to extend the Bass Range of a speaker. The cabinet to support this is a work of engineering in it's own right.

I thought it would be interesting to have a thread dedicated to the Cabinet Design of this particular speaker and it will give me a place to help people like SaSi who might want to contemplate building their own DIY cabinet.

Hopefully others will consider documenting their cabinets when they have them torn apart and can take measurements. This thread is a work in progress so come back often to see if anything has been updated.
 
The Design

Coupled Cavity Bass System is a form of Bass Loading. There are a number of other techniques like Transmission Line (TL) Loading and Bass Reflex that are used to improve the Bass Response of a speaker. KEF's implementation of Coupled Cavity (CC) was revolutionary at a time that most manufacturers were focusing on Bass Reflex and TL.

Here's some excerpts from a Chicago Tribune Article on the KEF 107....

"Laurie Fincham, chief design engineer at KEF, spent a decade developing the technology that elevates the 107 to one of the world`s best speakers. The most visible--or, in reality, invisible--of his innovations is the twin coupled-cavity bass loading premiered in the KEF 104.2. Two 10-inch woofers, acting in opposition but linked by a force-canceling rod, suppress unwanted vibration and coloration.

But you`ll never see these woofers because they fire upwards within the cabinet and radiate from the top of the enclosure right below the separate midrange and tweeter "head". This design reaps dual rewards. The woofers no longer interact with the floor, thus avoiding reflections that color the sound. The bass source is now as close to the midrange and tweeter as possible, resulting in more coherent sound.

The 107 can hammer home at 112 decibels when powered by a good amplifier (a full symphony orchestra playing a fortissimo hits 112 dB). KEF delivers this extra bass from an active electronic equalization box (K-UBE) for KEF Universal Bass Equalizer. This box allows precise control of bass response to room acoustics, source material and listening preference while providing that extra energy in the low bass region."


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The Construction

All comments are associated with the picture below them. All measurements are in "mm" and are approximate. I will do my best to cross check and sanity check the stack up, but there may be some minor variations. Width is from side to side in the cabinet, Depth is from front to back of the cabinet and Height is up and down in the cabinet... ALWAYS!

This is a KEF R107 Cabinet with the "Top Hat". The cabinet itself measures 330W X 450D x 790H. The Top Hat is about 350H.

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So let's talk about the outer cabinet construction. The cabinets are made out of 23mm thick MDF and finished with a Walnut Veneer (Black Ash and Rosewood were an option). The front joint construction has a 10mm quarter round in the corner with a fairly complex joint behind them. The backs appear to be a mitre joint.

I believe everything is glued and tacked prior to the veneer being applied, but what is interesting is the veneer is wrapped up onto the ends of the cabinet sides. So perhaps the cabinet sides are pre-veneered, machined and then assembled. That doesn't make sense to me, but the pictures don't lie. There are four cabinet divisions that span from the top to bottom of the cabinet and again, I would think they would be tacked in from
the outside prior to the veneer being applied. We'll discuss these partitions in more detail later.

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In talking about the complexity of this cabinet design it is probably best to start at the bottom. This image shows the base of the cabinet. The Base Cover is made out of 23mm thick MDF and protrudes from the bottom by about 5mm. The outside external edge has about a 5mm bevel for esthetics. It is held in place by 10 screws that screw into pressed in threaded fasterners attached to the Base Plate. The Base Cover measures 405D X 285W X 23T.

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The fasteners in the corners of the backside of the Base Cover (same type as are in the Base Plate) are used to attach the feet. Also note that there is a relief all around the outside for a gasket! The depth of the relief is less than the gasket thickness to create an air tight seal.

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This image shows the Base Plate which is the lowest of the four divisions within the cabinet. This is used to support the mounting of the Base Cover. This is made out of 18mm thick MDF, which all of the dividers are. So this would be 285W X 405D X 18T. The inner opening in it is 245W X 365D (ie; the web is 20mm thick) with a 25mm Radius on the inner corner.

This is inset into the cabinet by 18mm, so when the 23mm cover is mounted, as I pointed out before, it protudes by about 5mm. Also note the reliefs for the threaded fasteners for the feet.

The mounting holes are 130mm on center in X&Y. On the X axis the first hole starts 70mm across from the inside edge. In the Y axis the first hole is at 80mm up from the inside edge.

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OK, let's talk about foam. The KEF R107, being the speaker it is, has some pretty sophisticated dampening placed inside the cabinets. And you can rest assured a lot of development went into where the foam is placed and why. It is a standard open cell form of foam kind of like these, but it is placed and cut with some precision and care.

The picture below shows the foam that has been cut right to the opening in the Base Plate (notice the black pen outline). It measures 250W X 390D X 50T. The base plate is 18mm thick, so this piece sticks down into the cavity and is meant to compress a bit.

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Removing this reveals a less densely blown foam that fits to the measurements of the inside of the cabinet. So it is about 285W X 405D X 100T.

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This image shows the difference between the densities of foam and the sizes.

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Looking inside the lower bass cavity, there is foam that has been placed around the speaker to give it room to extend and generate sound. This Foam Pocket is created by a strip of 25mm thick and 45mm height foam that extends 280mm into the enclosure from the back panel. In other words, the strip is probably ~850mm Long but spans an area 285mm wide (width of the internal cabinet space) and 280mm deep (from the rear panel to where it interfaces with the foam block), and again is 25mm thick all around at a height of 45mm.

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It is important to note that this "lower bass cavity" (285W X 405D X 190H) is fully enclosed and does not vent. The rectangular piece of foam that creates the support along with the strip described above is 285W X 125D X 45T.

The opening for the speaker is a hole 240mm Diameter that is centered across the width and 20mm in from the rear panel of the speaker. As mentioned above, all dividing panels are 18mm thick.

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hi SimplySound,
I remove my woofers from the cabinet for refoaming them; there are 2 foam fixed on one side of the aluminum rod who connect the two drivers; do you know what kind of glue I need to use for gluing the foam to the lower driver?
 

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I don't see the central enclosure the woofers share before venting to the outside. it looks like the lower driver loads the upper driver.
 
hi SimplySound,
I remove my woofers from the cabinet for refoaming them; there are 2 foam fixed on one side of the aluminum rod who connect the two drivers; do you know what kind of glue I need to use for gluing the foam to the lower driver?

I never did fully finish this thread. I might get motivated to keep going now that Jeff has jangled my chain about rebuilding my R107's.

Bilbel, for glue do a Google search for "Spray Tacky Glue"... I'd use something like this.

Let us know what you end up using and how it works out. :thmbsp:
 
It was an ash laminate with a black stain. Don't think they would have black stained walnut.... too expensive for that!

The thing is I'm not sure how thick the laminate is and how deep the stain may have penetrated. The issue will be how deep you have to sand to get the stain out. I would maybe suggest a stripper for stain to see if it will lift it before you sand. Thing is you might want to try it in the center of the cabinet to see how well it works before you go at the whole cabinet.

Just my thoughts. Good luck and let us know how it goes!

PS: I do need to finish this off. I have all the measurements on a sheet somewhere, and I have a pair sitting in the basement waiting for repair.
 
The surround should be glued to the cone and to the frame. That allows the Cone to move but be seealed. The one most people get wrong is there is the inner diaphram which should be glued to the cone and the inner part of the core. This seals the driver completely. Many people find they didn't replace the inner diaphram and they will have leakage of air when the driver moves reducing the Bass output.

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To get the speakers out, you have to take out the bolt that goes through the magnet of the upper speaker and captures the center rod. The woofers are "coupled" together by this rod so you need to separate them.

The tricky part I found is getting the nuts off the frame attachments (screws). KEF didn't "capture" those screws on the back side and the nuts are Nylocks. So you have to try to use a pair of needle nose to get the nuts off the screws and get the lower driver out. Then you can work on the upper driver from both sides. Sort of a PITA!
 
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Reviving this superb post from 5 years ago.
Thanks for your excellent and precise description as well as for sharing the dimensions of the bottom cavity and external dimensions.

1- Bottom cavity : 405 Depth x 285 Width x 190 Height
2- Overal dimensions : 451 Depth x 331 Width x 790 Height
3- MDF thickness : 23mm everywhere except the top and bottom "flange" that holds the top and bottom panels.

Providing the above measurements are correct, I was able to redraw the entire cabinet. Assuming that top and bottom cavities have the same volume. I have to figure out if volume corrections were inserted to compensate for the volume of the bass crossover and upper speaker body though.

For those interested in my thought process, I started with the bottom cavity internal dimensions and positioning of the cut out for the B250. These are temporary calculations.

- 405 Depth x 285 Width x 190 Height = 21.93 Litre volume
- Cut out 20mm from the back wall

The upper cavity shall have the same volume as the bottom cavity, and the upper cut out shall be aligned with the bottom cut out and centered on x and y axis. I deducted the following measurements:
- 280 Depth x 285 Width x 275 Height = 21.945 Litre volume - I rounded up the dimensions, hence the 0,07% difference in volume :) . I didn't yet take in account the volume of the crossover or the woofer. Currently digging for that info.)

Total height being 790mm, I could calculate the height of the center cavity at ( 790 - 4x23 - 275 - 190) = 233
233mm is providing the top cavity panel sits flush with the top of the cabinet. Which it doesn't as it sits recessed perhaps 10mm below, with a panel covered with fabric sitting on top.
Can somebody tell me how high is the recess? Then I will lower the upper cavity by this amount, and will reduce the center cavity's height by that same amount.

I estimate the top recess being 8mm high, with the upper edges being 10mm thick as 10mm quarter rounds are used on the corners. Therefore the center cavity's height shall be 225mm and no longer 233mm.

Then, from the total depth of the cabinet of 450mm, I could deduct the dimension of the bass duct at 101mm

Am I correct in my assessments? :)

I can publish a drawing for those interested in producing their own cab. Currently reworking a set of Kef 104/2 that had a very sorry life, flooded in a wet basement.

My current working set-up is old JBL pro stuff. Which I made.

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Your post brings back memories. I based a rebuilt of cabinets on this thread and other info I found in different places. I still recall the "magic" number 19lt net for the two enclosures. The upper one needs to be slightly bigger than the lower one to accommodate the crossover which is mounted on the top cover of the top enclosure.
Can't give you dimensions of the original cabinets as I never had one close to inspect.

I built my pair of kinda R107 having the two crossovers and mid/hi pods in mint condition and nothing else. After long experimentation (fun) I settled for B200 SP1014 for the bass cabinets (to stay in the family). The end result looks and sounds fabulous.

I used 20mm plywood and it was lined internally with 3mm bitumen sheets all over. Highly recommended.
 
Hello

if you used B200 sp1014 woofers, then no wonder you went with 19L cavities. That drivers is specified for 20 to 30 litre

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But the KEF 107/2 used the larger B250 250mm sp1215 woofer which was specified differently than the B200 sp1014. Hence the larger cavities. That being said, the B200 you used must sound very good with Fs 25Hz.
 
Great informative thread thank you!
I have that pair with Raal ribbon and was totally unaware that I had to position head units in a way so that the Raal ribbon is at 90 degrees to the floor/cabinet. So the head would need to tillt forward down and stay in that position, which they wont do as they are designed to go left/right and thats it.
So what quality option do I have ? Should I machine new ängled" aluminium part for the head or for the base? Or should I just fix it in place with sand bags? ( might be hard to accurately position them that way} ....
I had another idea but I totally forgot what it was about. Ah yes...3D printing a gasket in a shape of a piece of cheese :) for the ribbon so only ribbon is angled down to 90 deg. ...in that case midrange would be firing as usual, so slightly upwards and ribbon would be firing straight into the room. I dont know how that would sound. I have seen speakers like that but thay are designed like that from the start. So thats the option 2.
Option 1 being both drivers firing into the room at 90 deg. So both options are changing direction of one driver as compared to the original.
crossover is slightly changed to accomodate ribbon and speaker was voiced while head was like in option 1 with both drivers at 90d (head tilted forward) so maybe thats the best approach but i need to come up with a solution to keep the head secure in that position.

Any ideas fellas ?
 

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