i made a speaker from a cup, a coil of wire and a magnet. I hooked up both of the ends of the wire to a headphone jack and it works. Albeit very low sound, but it works. Can you point me in the direction of some equations that will show me the most influential factors on how to make it louder? I've been also looking at impedance and their effect at high and low frequencies. I know my differential equations so lay it on me if you can. I would like to know more about Φ(flux), too. How can I measure the strength of a magnet? I'm playing around with creating small ac current by spinning a motor with a magnet on it inside a coil.... pretty simple.