@SPUD GUN: As
@Audioguru said in his post #2 above, you have failed to mention ANY of the important characteristics of "a low frequency emitter capable of 20hz." Perhaps you could get a used guitar and remove all except the heaviest string, which produces the lowest note. Then tune that string to produce a decaying 20Hz note when plucked. Or purchase a steel tuning fork whose "natural" frequency is somewhat lower than 20Hz and carefully file the two tines until the tuning fork vibrates at 20Hz when struck sharply. If you want an electronic solution, construct a simple Wien-bridge oscillator. If you want some really good advice, please tell us wtf you want to DO and leave the how-to-do-it to the forum members who respond. Personally, I like the idea of using an open pipe, cut to be resonant at 20Hz, and excited with low-pressure compressed air... like a pipe organ.