You're right that you can use a speaker as a microphone, because they're pretty much dynamic microphones run in reverse. However, speakers are exceptionally low

You're right that you can use a speaker as a microphone, because they're pretty much dynamic microphones run in reverse. However, speakers are exceptionally low sensitivity when used as microphones. You could amplify them and get a usable signal level out of them, but the noise would probably be much worse than a purpose built microphone. Microphone size doesn't really matter that much for low frequency sound. The Earthworks M50 suggested above? I wouldn't recommend it for this application because if you're going to spend a lot of money, it might as well be on something well-suited to the application. However, the approach it takes works and can be done for much less money. Cheap omnidirectional electrets go down pretty far[1], and you don't need the ruler flat response of the M50. You just need amplification circuitry that doesn't filter out the low frequency stuff you want. DSP is commonly used to correct the frequency response of microphones. However, measuring frequency response is difficult and expensive. [1] http://volcanomodels.sr.unh.edu/jbj/MICROPHONES/microphone_list.html http://volcanomodels.sr.unh.edu/jbj/MICROPHONES/microphone_l...