OK, now you've got me curious. I had to dust off Excel for this one, but I think I've got it sorted out now. If we are 57 octaves below 261.6 Hz, that puts us at (261.1/(2^57)) -- or somewhere around 1.81*10^-15 Hz. That comes out to one cycle every 17,456.9 millennia - once every 17.5 million years. I don't think the Radio Shack SPL meter's going to know what to do with that test tone.

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