Design of a specific electromagnetic field

I am seeking the design parameters for constructing a 250 gauss electromagnetic field to be used in conjunction with commercial audio equipment. I am seeking the coil specifics to be used in an audio circuit with an 8 ohm resistance. I do not know how to get from microHenries to gauss. I would like to purchase a coil that I can use in place of a speaker to produce the electromagnetic field of 250 gauss. Otherwise I need to be able to make my own coil. I would appreciate the assistance. :D