Just a few observations on general construction techniques.
Capsules should be regarded as examples of precision engineering,wether you decide to build K47,K67,resonant chamber or a mixture,the same principals apply. With regard to the back plate attention should be paid to drilling the holes with a high degree of roundness,making sure not to leave overhang on the diameter after lapping.This can be removed by gentle scrubbing with a fiber pot scourer.Dont forget that these cylindrical chambers constitute small air springs helping among other things to maintain equal compliance across the diaphragm. I gave details of precision lapping in a previous post.Make sure that the interference groove is acuratly turned to the specified dimensions.
The anular ring should be turned to the design thickness making sure that the dimension will not cause cavity resonance.The underside clamping face should be lapped after drilling to give a smooth inclusion free surface.The inside quarter of this face should be bevelled and polished to avoid any minute interference of the dynamic portion of the membrain.A light should be held at an angle to the tensioning jig to observe striations occuring at the periphery of the membrain.The inside diameter of the spacer should be cut very cleanly and lined up with the outside diameter of the interferance groove.The holes in the spacer should also be cleanly cut ,the best way is to make a small punch from a suitably sized drill shank.Use the smallest screw thread possible to avoid interference with the PET.If you use slotted screw heads,use a shrouded screw driver,I use 14 BA screws.Tighten the screws equal and opposite as you would a car cylinder head.
Look out for any striations occuring as you proceed,easing off screw tension.or re applying in a different orderwill usually rectify the problem.This is dependant upon you obtaining uniform tension in the frame in the first place!
If you are going to cement the PET to the anular ring apply a minute film of adesive to the outsid diameter only.A small bevel should turned in the ring for this purpose.I'm not going to tell you how to do this.mainly because it's too complicated.Have a lookat the Neuman Pics that Zebra posted you will get the idea.
BTW I dont think Neuman produce production capsules like this.Looks like a lab set up to me. Its pretty much what I used to use in the garage.
I did give a reasonably comprehensive description of how to heat treat and pre tension PET film in a previous post at the old place.I dont know if it got into the meta if not I could re post it if anyone is interested.
I think part of the answer is to strive to get a stable membrain seated with equal tension and a high degree of flatness accross the back plate.You may then have a chance of making a useable microphone.
I am not going in to design criterior or the math. Tim C and others are handling thatvery adaquately.
One observation on design.I think Gus got it right.use the math to get in to the ball park,then its hours and hours of trial and error.
Hope I'm not teaching my grandmother to suck eggs and I hope some of it may be of some use.no way definative,just what has worked for me over the years (mostly).
Andy.
Capsules should be regarded as examples of precision engineering,wether you decide to build K47,K67,resonant chamber or a mixture,the same principals apply. With regard to the back plate attention should be paid to drilling the holes with a high degree of roundness,making sure not to leave overhang on the diameter after lapping.This can be removed by gentle scrubbing with a fiber pot scourer.Dont forget that these cylindrical chambers constitute small air springs helping among other things to maintain equal compliance across the diaphragm. I gave details of precision lapping in a previous post.Make sure that the interference groove is acuratly turned to the specified dimensions.
The anular ring should be turned to the design thickness making sure that the dimension will not cause cavity resonance.The underside clamping face should be lapped after drilling to give a smooth inclusion free surface.The inside quarter of this face should be bevelled and polished to avoid any minute interference of the dynamic portion of the membrain.A light should be held at an angle to the tensioning jig to observe striations occuring at the periphery of the membrain.The inside diameter of the spacer should be cut very cleanly and lined up with the outside diameter of the interferance groove.The holes in the spacer should also be cleanly cut ,the best way is to make a small punch from a suitably sized drill shank.Use the smallest screw thread possible to avoid interference with the PET.If you use slotted screw heads,use a shrouded screw driver,I use 14 BA screws.Tighten the screws equal and opposite as you would a car cylinder head.
Look out for any striations occuring as you proceed,easing off screw tension.or re applying in a different orderwill usually rectify the problem.This is dependant upon you obtaining uniform tension in the frame in the first place!
If you are going to cement the PET to the anular ring apply a minute film of adesive to the outsid diameter only.A small bevel should turned in the ring for this purpose.I'm not going to tell you how to do this.mainly because it's too complicated.Have a lookat the Neuman Pics that Zebra posted you will get the idea.
BTW I dont think Neuman produce production capsules like this.Looks like a lab set up to me. Its pretty much what I used to use in the garage.
I did give a reasonably comprehensive description of how to heat treat and pre tension PET film in a previous post at the old place.I dont know if it got into the meta if not I could re post it if anyone is interested.
I think part of the answer is to strive to get a stable membrain seated with equal tension and a high degree of flatness accross the back plate.You may then have a chance of making a useable microphone.
I am not going in to design criterior or the math. Tim C and others are handling thatvery adaquately.
One observation on design.I think Gus got it right.use the math to get in to the ball park,then its hours and hours of trial and error.
Hope I'm not teaching my grandmother to suck eggs and I hope some of it may be of some use.no way definative,just what has worked for me over the years (mostly).
Andy.
