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Old 09-13-2008, 02:01 PM   #124
the all new rob
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Originally Posted by J Kennedy View Post
My idea here is that a couple thin iron bars flat against the side of the coils would keep some of the flux within the coil boundary. Shouldn’t alter the density thru the coil enough to lose windings and might even be a plus. This could make major headway in reducing single coil hum. I don’t know if they make iron foil, but this would probably do the job better.
Disclaimer: it's been over a decade since I did the relevant coursework, but I believe you are partly correct. The mechanism is that the iron provides a path with a lower reluctance (the magnetic dual of electrical impedance) than the air path, and more magnetic lines of flux will take the lower reluctance path.

The thing that's hard to figure is whether you are correct about lowering the field density in the coils. I'm guessing anything that cheap and easy would have been taken up already by the manufacturers, so it may actually lower the field density in places you don't want it to.

Anything ferromagnetic ought to work for doing your tests.
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