> I think it would require a calculation of how fast the wave would disperse in the tube, which OP has not done. True. Two points though: a) The rate of dispe

> I think it would require a calculation of how fast the wave would disperse in the tube, which OP has not done. True. Two points though: a) The rate of dispersion would be enormously dependent on the "stiffness" of the tunnel wall. If the wall is flexible at all a lot of energy would escape in its distortion. How much is a very complex calculation. b) Ideal propagation for a ultra low frequency sound wave is probably very good. Why do I think so? I've heard that "shock waves" (actually their low frequency components) from various blasts often travel several times around the earth and are registered at least on their second lap. This knowledge has been incorporated in my gut and used to guide this "design".