This is an intriguing proposal, but one that I think it ultimately flawed. Once a wave is strong enough for there be to appreciable density differences (NOT the

This is an intriguing proposal, but one that I think it ultimately flawed. Once a wave is strong enough for there be to appreciable density differences (NOT the usual case for sound waves), the usual wave equation for sound does not apply [1]. In the case of such "Very Strong Sound Waves", you have a non-linear wave equation and thus the shape of your wave is no longer fixed as it moves. Eventually, you end up with shock wave, where the front of your wave is a discontinuity. Why is this problematic? Well, once you have a shock wave (e.g., a wave crashing on the beach), the physics gets a lot more complicated. You are basically riding an explosion in a tube. At this point, it no longer safe to me to neglect factors like dissipation. Your shock wave is going to heat up the air and eventually fizzle out. If you want to keep it going, you'll need to be continually supplying tremendous amounts of energy at exactly the right time along your tube. You'll also have to be continually cooling or cycling the air to relieve the waste energy. To me, it seems a lot simpler and more reliable (not to mention safer) to simply evacuate your tube, which, I believe, is the original hyperloop idea. [1] http://mysite.du.edu/~jcalvert/waves/soundwav.htm#H http://mysite.du.edu/~jcalvert/waves/soundwav.htm#H