Here is a link to inductors that might help. Don’t fret over the formulas, read the first paragraph and see the pictures. http://en.wikipedia.org/wiki/Inductor

An inductor will store energy in a magnetic field and later release it as a current. It actually wants to keep the current going through it continuously. Therefore once the field is built up if the current is interrupted the voltage will rise very high trying to restore the current. This will cause a spark of much higher voltage than the applied voltage. If you put a 1.5 volt battery across an inductor and then release the battery while holding the wires you will get quite a shock. Anyone using a traditional ohm meter has likely experienced this when measuring the resistance of an inductor. This same principle is what allows the 12 volt battery in your car to make a good hot spark in your engine. It is interesting to note that the spark-plug fires when the points disconnect the battery, not when it is connected as the collapsing field is much faster (stronger) than the building field.

Using Iron or some other magnetic material will enhance the effect many times. That factor of enhancement is called permeability. The permeability of air is one. The permeability of transformer iron is many thousand, there for it increases the inductance of a coil of wire by that amount. See the explanation and table of materials in this article.

http://en.wikipedia.org/wiki/Permeability_(electromagnetism)

Of the examples I gave last night the most applicable to audio is the speaker crossover where there is an inductor in series with the woofer. In this application the inductor’s reactance (think resistance) increases with frequency thus reducing the current going to the woofer as we go up in frequency. Not all speakers have this inductor and let the woofer roll off through cone breakup, though this is not the best way to make a speaker. Better speakers roll the woofer off before cone breakup and hand the signal off to the tweeter.

Good speaker inductors, also called chokes, use a lot of copper and are thus expensive. The best ones are air core which uses even more copper. One can calculate the inductance needed for a woofer by knowing the impedance of the woofer and the cut-off frequency desired. This is usually 8 ohms and 1 KHz. In that case the inductor can be found L=8 ohms/6.3 x 1000 hz. (6.3 is 2 pi) or one can use a online calculator. http://www.sengpielaudio.com/calculator-XLC.htm

Using those numbers an inductor of 1.27 millihenries will roll off the woofer at 6 dB/octave starting at 1000 Hz. If the desired crossover point is 2000 Hz the inductor would be half as large, 0.64 mHy. Here is a 1 mHy air core inductor typical for a speaker. It also has 0.48 ohms resistance which will hurt the damping of the speaker somewhat. Thicker copper wire is needed to reduce that. http://www.parts-express.com/jantzen-10mh-18-awg-air-core-inductor-crossover-coil–255-250. Here is the same inductor with heavier wire but still 0.3 ohms. http://www.parts-express.com/jantzen-10mh-15-awg-air-core-inductor-crossover-coil–255-422

Please ask if you have questions or suggestions.