Hmm, interesting. Fin whales and Blue whales call at 20 Hz. I believe that this is a frequency that travels well in the SOFAR (??) sound channel in the sea, so it can be heard a long distance. 20 Hz is also about the middle of the frequencies used in seismic reflection data to image the sedimentary layers down to 5 km depth or so (oversimplified....). You might want to check if I am correct about SOFAR-I'm remembering this from a decade ago and my memory is not as good as it once was-Don, my beer is Rolling Rock-think I'll have one-I have to program for the first time in 2 years and second time in 10 years.......Chris