I posed the same question to an ex-military comms intercept chappess. Her first comment was that:
"you're all a bunch of deaf old f__rts who can only ever hear low tones" (they're a rude lot, ex military types) then commented that there were considered to be two 'magic frequencies', but wouldn't say initially what they were. Today she came back with:
"432Hz is favoured by many in the healing game, occasionally there are them that think 528Hz works better".
Which has got nothing to do with comms intercept work, and seems more about meditation and cult stuff, but I tried listening to those tone frequencies, including buried in noise. And she's right. They do sound 'nice' and quite detectable. They also happen to be in the ratio 11/9. So just for the fun of it, there are now eight user-selectable tones in my gizmo, in the ratio 1.2222222 starting 432, 528, 645 etc up to 1760Hz. Details will be published in due course (writing it up now, hopefully in time for Scatterpoint)