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Old 17th December 2009, 05:17 AM   #11
diyAudio Member
Join Date: Aug 2003
Location: Calgary on the Bow
What kind of a coil driver did you have in mind? If you are considering a conventional coil and motor (as in a standard dynamic driver) you will have to make some adjustments. You will require a shorted turn (copper or silver sleeve on the pole piece) on the pole and you will require shorted turns at the top of the motor top plate and at the bottom of the pole. You will need to vent the pole and probably the cavity inside the motor. Build a single layer 4 ohm coil as short as possible with a top plate and magnet as large as possible. You will not be able to utilize any form of standard spider, the best option is a liquid bearing instead, use a medium to medium thick ferro fluid in the gap to centre the coil rather than a spider (will generate talk and distort the sound). You will want to keep the coil driver dead centre to balance tension from the diaphragm. You will also need to use a much heavier diaphragm so save the micron stuff for stats. The diaphragm in this design is your primary suuspension so keep the coil as light as possible. You will want to use a biaxially oriented film to keep things balanced and I would recommend 150 to 200 gage Mylar. If you like I can send you some DuPont HS150 (works very well but heavier gage would be nice if you want to make any real bass). Damping can be applied close to the rear side of the diaphragm and must vary across the diaphragm being heaviest at the outter edge of the film. This will allow you to build a cardioid polar patten which is very usefull. This speaker will radiate high frequiencies in all directions (left, right, up and down). In most cases left to right radiation is not an issue but floor and ceiling reflections are a concern). You may think that the film thickness I have recommended is much too thick to generate any high frequiencies but this design can easily run out to 10 K at the outter edges of the diaphragm. For the last octave you only require a light pulped paper dust cap on the front side of the voice coil. Mount the cap inverted into the pole (machine the pole out to accomodate this) and extend the outter edge of this dust cap outward past the diaphragm surface plane by about a milimetre or so (not flush with the diaphragm) to increase the structural integrity of the cone. All that and a lot of hard work should get you there. Hope that this has been of some use. None of this is made up or just my opinion, this is all based upon real R&D and a sucessful product which made it to market. Regards Moray James.
moray james
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