We simulated several arrangements to drive a membrane for a speaker. Coil-coil intereractions are nice because they don't require permanent magnets, but the force produced in our geometry is very small (~1000x smaller) compared to that of coil-magnet interactions.
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First prototype:
Given this, we decided to use arrays of neodymium disc magnets, which are cheap and easily available in a variety of dimensions. Planar magnetic speakers often use flat bar magnets, but these are more expensive and difficult to arrange on the substrate. The design challenge is to route copper coils to perpendicularly cut the line segments between all adjacent magnets, while minimizing the additional run (wasted copper) and maintain symmetry of the membrane (so the deformation modes aren't deformed). Below is a design with three-fold symmetry that can be extended to an abritrary radius by extending the spiraling pattern.
