Tim,
your new capsule is looking very interesting! I've come back to post a couple of further updates on what I've been doing.
Thanks for the tips so far, I've had better luck cutting out the spacer and tuning the membrane. I've used a sine wave on the keyboard now - I'm still not convinced of how accurately I'm getting the tuning though. When you say you've tuned your capsules to 1200Hz - I'm curious to know what margin of error you think there is in that?
Anyway, I've hit another snag.
First a little background on how I've made the capsule as the snag relates to the materials I've used. I started with a one inch brass rod cut and belt sanded it to the correct thickness. For the outer plastic part of the the backplate, I was a bit creative. I don't have a lathe, and I'm not as skilled with the belt sander as you are Tim, so in order to get the outer part of the capsule circular I found a tube that had a 34mm inside diamater (was the end of a piece of PVC I had around). I then placed the brass part of the backplate in the centre of this tube. If you can imagine, there was then space/air where I wanted the plastic part of the capsule to be. I used this as a mould and used epoxy resin to fill the space, leaving me with the plastic outer part of the capsule when it cured. I then took this epoxy ring a made a silicone mould of it so I could repeat the process easily, to create both retaining rings and further rings to fit around the brass part of the backplate if needed. I hope that description makes sense - it may be a bit hard to visualise without photos.
Anyway, the snag I have is that the epoxy retaining rings (which are reasonably thin) are a tiny bit flexible. This means that after I've added a diaphragm to them, when screwing them on to the backplate I tend to get wrinkles in the diaphragm as the screws seem to slightly alter the tension of the diaphragm by slightly warping the retaing ring.
So from this, I've decided it's going to be easier to use a metal retaining ring (I have quite a bit of 1.6mm aluminium aroud that should be fine). This will also solve getting an electrical connection, (though Tim your suggesion of another screw to make contact worked fine on the epoxy ring).
My problem is though I don't have any real way to get the retaining rings circular. I've got a hole saw to cut out a 1 inch hole, so the inside is ok, it's just the outer part. What I was thinking though was this, could the retaining rings actually be a square shape with a circular cutout? (Much easier to make!) The capsule backplate will of course still be circular, so it wont look neat, but it should be ok? I figure as the clamping motion of the retaining ring seals the capsule to the backplate it doesn't matter if the actual retaining ring is larger and of a different shape. The only problem I could think of is that the capsule will no longer be streamlined per say, so that may effect sound reaching the capsule - I don't know?
any thoughts?
Tim
