This particular geologist is typically sceptical, but so are most....I do agree in some ways with the difficult part...and that is in what I always called filtering....'specially when it comes to ear tones....

If you want an experience in ear tones....this is the place to be....We live on top of a large crystal node which hums at a low frequency. It takes a while for folks to 'learn' how to hear it but almost everyone whom I have taught knows how...amazing stuff...not piezo either because of its VLF......just the crystals way of talking to Gaia I suppose....BTW I think your geologist friend will eventually come around....if it is who I think it is...he has an open mind for a sceptic.....I can hear his wheels turning:-)
Bob