Quite possibly my last speakers

[rowuk]

Faital HF146/250Hz horn
2x Fane Collosus 12" Neodymium
MiniDSP HD for development
RH307 Super Amp
RH84 Revision 2 Amp

2.5 system with the lower woofer cut off above 120 Hz. Horn is crossed over at 400 Hz
Still to come is the Karlsson K-Cube tweeter:

If there is interest, I have pictures of the build. The side walls are curved 10mm particle board with cork sandwiched between them.
Testing commences this weekend.

[simon]

More photos please!

Can you describe how you laminated the particle board (chipboard?) and cork. Which glue did you use and were the fixings visible used to sandwich it together, or perhaps some kind of vacuum former?

[Dave the bass]

More photos please!

Not 'arf! Those Sandals are amazing!

[Paul Barker]

In aww of you on so many levels. My best speaker so far only one on the right. The hermit who gave it to me cant yet pluck up the courage to unearth the second. But hay overall sound better with imballanced speakers than with an inferior pair.



My level of orgasiation and tidyness reflects my internal brain salad surgary.

[rowuk]

I started by cutting and milling the edges of the front out of 1" Bamboo multiplex. Then I glued and screwed the frame slats as shown below. The glue is a standard PVA white glue from Ponal (White bottle in the background). It is not the express drying version.

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Then I glued and screwed the inside chipboard layer to the frame. 10mm bends easily enough. I screwed this from the outside in - countersinking the screws.

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I placed some round braces at calculated positions and screwed them also from the outside of the inside layer.

Then I applied glue with a roller to both sides of the cork. I immediately layed the outer layer of chipboard on top of the cork and with many clamps, pressed it against the inside layer. I then screwed the outside to the inside from the inside through the inner chipboard and cork. That takes us to the laminated picture. There are no screws from the outside of the laminate inwards. I hope that this explanation makes sense.

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The bass reflex port takes into account a discovery that I made a while back. They work "better" when positioned about 1/3rd the way of the cabinet height - just like when we build "modern" transmissionlines, we put the driver about 1/3rd down the line for smoother response.

The bottom woofer is crossed over at around 120 Hz The horns around 400Hz. The tweeter will be crossed over around 8K. I had a ukrainian guy wind me a coreless output transformer for the tweeter.

More photos please!

Can you describe how you laminated the particle board (chipboard?) and cork. Which glue did you use and were the fixings visible used to sandwich it together, or perhaps some kind of vacuum former?

[Daniel Quinn]

Excellant !

I love idiosyncratic builds .

[rowuk]

Here is the top that was made by gluing the off cuts and making a lot of sawdust.

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Here is a picture from the side and back without the drivers.

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The finish on the side is a dispersion paint from Alpina. It is called "Sturm-Optik Anthrazit" here in Germany.I put a layer of primer down to seal the chipboard first.

The midrange horn can be adjusted for phase alignment.

[rowuk]

The inside is 120 liters big. The Fane woofers have a Vas of 56 each. The horn is mounted so that the tweeter mentioned above will be at ear height.

[simon]

Very well done rowuk, thanks for sharing.

So do you think the constrained layer damping type approach of the chipboard and cork makes a difference?

[rowuk]

Short answer: Yes, cork makes a substantial difference, is cheap and works well with curved sides and white PVA glue. No special tools were required. Jigsaw, drill, hand belt sander, router, knife. Oh yes, also an industrial vacuum cleaner.

Long answer: I have experimented with sand filled, lead sheets, Bitumen, silicone caulk and cork as a damping layer. Corks greatest damping effect in this configuration is between 80 and 150 Hz where many cabinets that I have built were mildly resonant in spite of bracing. Chipboard is really not great by itself, with the curving under pressure the resonant frequency going up, the cork "sandwich" brings it back down. Cork does not help MDF at all. My no compromise solution would have been sand filled, but the challenges with simple hand tools and my 67 year old skeleton are simply too great.

I do plan to test the cabinet with an accellerometer to find out exactly how well the cork is doing (and if I need more bracing).

I currently have no damping foam inside the cabinet. That may change, but initial listening has not yet shown any need. Still the speakers are probably months away from being fine tuned.

Robin

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Very well done rowuk, thanks for sharing.

So do you think the constrained layer damping type approach of the chipboard and cork makes a difference?

[simon]

That's some good experience there. It prompts a few questions, but I suppose the one I would ask is do you think sand is best regardless of wood type, or just with chipboard?

For a smallish open baffle from say 200Hz upwards what would you say is the best combination? Perhaps 6mm mdf or 5.5mm ply with a 10mm? sand filled layer between? Interested in your thoughts on this.

[rowuk]

I don’t think that anything is generally “best”. If my goal is to reduce the contribution of the box material, that can be accomplished by raising the material resonance above the range where the bass driver plays. This makes it easy to damp (B&W matrix or my braced curved walls).
We could also lower the resonance by using sand or making the walls from concrete or stone but then we have a lot of other challenges. Sand settles with time, potentially leaving voids. We must design the possibility to add sand later. We must also achieve a critical mass to make sand effective. A 6mm thick layer of sand on panels the size of my speakers, is far less effective than 6mm of cork. I have not yet tried a sand/epoxy mix but the thought is appealing. It could be the solution to sand shifting.

In my view, the number one thing that bothers me about many open baffle (and closed box) designs is the lack of proper bracing/damping. Vibration/flexing are real. Chipboard is stiffer and harder than MDF - resonance should be easier to damp - but they are both ugly. If I were interested in open back, I would start with the bamboo ply that I used for the front of my speakers. It is strong, stiff, hard but not ugly. It also is not very resonant. I would mount the driver and use an accellerometer to determine where the braces need to go. It can be stained any color - or left natural. I am surprised that more people have not used bamboo. It is so easy to cut and mill.

That's some good experience there. It prompts a few questions, but I suppose the one I would ask is do you think sand is best regardless of wood type, or just with chipboard?

For a smallish open baffle from say 200Hz upwards what would you say is the best combination? Perhaps 6mm mdf or 5.5mm ply with a 10mm? sand filled layer between? Interested in your thoughts on this.

[steve s]

Really nice speakers there, classy

[simon]

Thanks Rowuk, some food for thought.

I've never seen bamboo ply but googling it, it's the same price as birch ply, £220+ a 8x4 sheet - the price of sustainability I suppose.

I've sometimes wondered about expanding foam rather than sand. It should fill all the voids if done carefully and wouldn't settle over time. It's light so wouldn't do the same job as sand but should help to create a thicker, more rigid sandwich that's light weight. But I've never seen anyone do it so what do I know?

[rowuk]

Thanks Rowuk, some food for thought.

I've never seen bamboo ply but googling it, it's the same price as birch ply, £220+ a 8x4 sheet - the price of sustainability I suppose.

I've sometimes wondered about expanding foam rather than sand. It should fill all the voids if done carefully and wouldn't settle over time. It's light so wouldn't do the same job as sand but should help to create a thicker, more rigid sandwich that's light weight. But I've never seen anyone do it so what do I know?

Now I know what I will try next - just not on my speakers. The whole idea of the constrained layer is to turn acoustic energy into heat between the layers. The constrained layer determines at what frequency. With expanding foam, we must insure that the outside and inside walls are stationary during the foaming other wise we have pain putting the box together. I wonder how consistent the foam density is?

I did some testing this weekend (bass heavy music "too damn loud") and am very happy with the performance of the cork. There is only one position close to the bottom woofer that will get an additional brace.