Thx, looks like I have to go for trial and error, I have here a basic open cells foam that's usable till 8000 Hz (it's effects can even be measured !) but becomes a first order low pass after 8000...
I tried the carving electric knife the other day - it must need to be sharp, my parents knife was useless.
Thanks for the tip on the angle grinder. There are many ways to do it, but that's not one I would have thought of. Now if you could rotate the foam and the grinder that would be good! Trouble is, all the methods require skill unless you go to the effort to make up some kind of machine. Not sure it's worth it unless like Geddes you are setting up a little production line. Hence my quick n dirty method is good enough. What you see on the front can be made neat enough. You have to use a fan with my method so you don't breathe in the ugly fumes.
I have some info about the foam on the blog:
http://redspade-audio.blogspot.com/2011/03/miniwaves-foam-mod.html
Where to find it in Australia:
http://redspade-audio.blogspot.com/2011/02/where-can-i-find-reticulated-foam-in.html
Another example:
http://redspade-audio.blogspot.com/2011/03/waveguide-foam-mod.html
Thanks for the tip on the angle grinder. There are many ways to do it, but that's not one I would have thought of. Now if you could rotate the foam and the grinder that would be good! Trouble is, all the methods require skill unless you go to the effort to make up some kind of machine. Not sure it's worth it unless like Geddes you are setting up a little production line. Hence my quick n dirty method is good enough. What you see on the front can be made neat enough. You have to use a fan with my method so you don't breathe in the ugly fumes.
I have some info about the foam on the blog:
http://redspade-audio.blogspot.com/2011/03/miniwaves-foam-mod.html
Where to find it in Australia:
http://redspade-audio.blogspot.com/2011/02/where-can-i-find-reticulated-foam-in.html
Another example:
http://redspade-audio.blogspot.com/2011/03/waveguide-foam-mod.html
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Hey Paul
Seriously - you found a source that would sell you bricks like that? Wow, that would have saved me a ton of money here in the US. All I ever got was the "You have to buy a bun and thats $1500.00." Thats what I still do. A bun is what the size of the foam block that is made at the factory.
The fun part is that when I get a bun it lasts a while. This stuff is my sons favorite toy. I get the bun sliced (best that they will do) and he stacks it and builds with it like big Lego blocks. And then jumps on it and just generally has a ball. Then it disappears and he has to wait until I buy another bun.
I use an ellectric knife to rough cut the bun slices, but the cuts are not good enough to use for a production piece. Then I use a hot wire. The fumes form a hot wire are toxic to poisonous so you have to take some care at this stage. And it takes some experimenting with wire size, current and speed.
Seriously - you found a source that would sell you bricks like that? Wow, that would have saved me a ton of money here in the US. All I ever got was the "You have to buy a bun and thats $1500.00." Thats what I still do. A bun is what the size of the foam block that is made at the factory.
The fun part is that when I get a bun it lasts a while. This stuff is my sons favorite toy. I get the bun sliced (best that they will do) and he stacks it and builds with it like big Lego blocks. And then jumps on it and just generally has a ball. Then it disappears and he has to wait until I buy another bun.
I use an ellectric knife to rough cut the bun slices, but the cuts are not good enough to use for a production piece. Then I use a hot wire. The fumes form a hot wire are toxic to poisonous so you have to take some care at this stage. And it takes some experimenting with wire size, current and speed.
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Hi Earl - yes I was a bit surprised. There is a local foam shop called Clarke Rubber, not sure if it exists anywhere else. They said I'd have to either get thin 12mm stuff from them at $70/m or pay them about $300 - 400 so they could buy a big chunk of it, even though I only wanted a pair of 6" block to try out on my "Miniwaves" which is a bit like a step down in size from Harper with a 2k xo. Surprisingly it worked pretty well not that it will ever see long term use.
So at this local place, they were surprisingly helpful and accomodating. I thought I might be a pest with such a small quantity. Cost me $15 for the foam cut into 6" blocks. I was having a waveguide comparison event, the other guys with an 18 sound waveguide and someone with a 12" oblate spheroid. They got their foam cut to blocks as well. I'd guess you would get the big chunks cheaper. I think it was about $100 for a pair of blocks for the 12" waveguide. I could probably rig something up to cut them better, as you do, but it doesn't seem worth it for the small amount I need.
I must admit, I'm a bit curious how you get a hot wire to work without having the wire get floppy due to expansion, but perhaps you don't want to give away too many trade secrets. You can buy wire for such things, but the stuff I've seen is so thin it would not hold shape unless stretched, and I can't see you could get enough tension to avoid the floppy wire issue.
So at this local place, they were surprisingly helpful and accomodating. I thought I might be a pest with such a small quantity. Cost me $15 for the foam cut into 6" blocks. I was having a waveguide comparison event, the other guys with an 18 sound waveguide and someone with a 12" oblate spheroid. They got their foam cut to blocks as well. I'd guess you would get the big chunks cheaper. I think it was about $100 for a pair of blocks for the 12" waveguide. I could probably rig something up to cut them better, as you do, but it doesn't seem worth it for the small amount I need.
I must admit, I'm a bit curious how you get a hot wire to work without having the wire get floppy due to expansion, but perhaps you don't want to give away too many trade secrets. You can buy wire for such things, but the stuff I've seen is so thin it would not hold shape unless stretched, and I can't see you could get enough tension to avoid the floppy wire issue.
Yep
Balancing tension with breakage and wire guage is what its all about, but it can be done. When heated the wire stretches and it softens. Too much tension and it breaks, too little and it will not "pull" through the material. As you change the wire guage to get this all to happen, you need to change the voltage to get the proper current for the right temperature. Somehow, after much experimentaion, it all comes together.
All in all people look at what I am doing and say "Oh yea, thats easy" - NOT! And thats just making a foam plug.
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A Yeti ??? We don't have this creature around, just Orang Utang next island in Borneo.
Ok, It's just an over exposure with a slightly lens distorsion. I liked the effect...
Back on the topic, does that cutting the foam by heating ways is not creating a peripheric hardened zone where the cells are burned ?
If it's the case, then, maybe this makes a thin layer with other reflecting properties, could them be positive or negative.
With the grinder, no texture modification . The resulting shape has not facets as with any cutting tool. Somebody has to try, it's really easy.
For an industrial production, as in The Geddes Factories, it's possible to conceive a machine from a sharpening knives thingie. The wheel has to be done with a lathe (can be in wood) with the required profile. On it, have to glue tiles of abrasive big grained sheets.
It can work only if the linear speed in periphery is high enough, if not so the wheel will grab the foam from it's support.
The support is the problem...I guess it has to be done by hand, if not so, the foam has to come already shaped in cylinders.
Sorry, was long...
Ok, It's just an over exposure with a slightly lens distorsion. I liked the effect...
Back on the topic, does that cutting the foam by heating ways is not creating a peripheric hardened zone where the cells are burned ?
If it's the case, then, maybe this makes a thin layer with other reflecting properties, could them be positive or negative.
With the grinder, no texture modification . The resulting shape has not facets as with any cutting tool. Somebody has to try, it's really easy.
For an industrial production, as in The Geddes Factories, it's possible to conceive a machine from a sharpening knives thingie. The wheel has to be done with a lathe (can be in wood) with the required profile. On it, have to glue tiles of abrasive big grained sheets.
It can work only if the linear speed in periphery is high enough, if not so the wheel will grab the foam from it's support.
The support is the problem...I guess it has to be done by hand, if not so, the foam has to come already shaped in cylinders.
Sorry, was long...
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I tried shaping some foam with 36 grit on a pneumatic angle grinder...that didn't work too well for me. So I did the best I could with a razor blade. Thank goodness what I was needing was rectangular and small. Anything other than what I was doing, I would have email Dr. Geddes.
I'm not one of those! It's not too hard if you aren't too fussed about getting the inside just right. To do it nice and neat is a lot more work. Probably not worth it for me.
Depends on the speed of the cut. If you get it just right it slices through nicely without a layer of melted foam. If you do it as I've done, you'll get some of that, but you can cut it off with scissors. Not the most elegant solution, but the ugly part is hidden. I don't see any real issue with such a layer, assuming you get the profile right it sits right up against the waveguide wall. You'll probably use glue anyway, which is a layer of sorts that will absorb perhaps a little. No big deal. If this layer was in the middle of the foam then it might be more of a problem.
I've seen your plugs in your blog, it's a good job.
This makes me think that my foam is harder (too hard) then easier to work with the grinder.
As you said. Even making "just" that is already a source of debate.
This makes me think that my foam is harder (too hard) then easier to work with the grinder.
As you said. Even making "just" that is already a source of debate.
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Yes a hard foam is much easier to work with. The foam that I use is very soft. I have not found a skin effect on using a hot wire. The cuts burn clean and leave no residue. The hot wire cuts are very smooth. I think that you must be using a different foam.
Yeah I think you have a different kind of foam. The foam like I have (which is what I think Dr. Geddes uses) could not be shaped by sanding. Lord knows I tried everything I had to make the cuts look pretty, and nothing I had would do anything to the foam. I did not have a hot wire to cut it with though.
When I did mine in layers, I sculpted it right into the throat and just into the driver. This talk of hot wires (which I assume will cut you a cone) makes me think no one else takes it back that far.
I don't and you are correct that it is a limitation of hot wire on a solid block. My thoughts are that the attenuation is much greater on the primary wave relative to the HOMs in the throat than the portions out from the throat. This might be why Meyer did not continue to use foam in his devices while I found it quite effective. His was way down into the throat only and mine is much furtrher out towards the mouth. That distinction is made in the patent and was required for allowance.