http://www.instructables.com/id/SPKR-MiK–How-to-make-a-microphone-from-a-speaker/

How to make an inexpensive microphone capable of picking up low frequencies that doubles as a speaker and direct box.

The large diaphragm of this microphone will pick up more of the low frequencies when recording a kick drum or bass guitar.

Sound recording engineers have been using this trick for years, and Yamaha has also made a commercial speaker microphone called the SubKick, that usually retails for around USD$300.

I was able to build this mic for under $20 by «scrounging» various parts out of old junk. Even if you need to buy all the components, you should be able to build this mic for a fraction of the price of the retail version.

This design goes slightly beyond the SubKick, as far as electronics are concerned, with a dual coil design, and internal direct injection (DI) box.

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