I think the best you could do would be to stay near the trailing end of the pressure wave. This would be where the air was moving (slightly) in the same directi

I think the best you could do would be to stay near the trailing end of the pressure wave. This would be where the air was moving (slightly) in the same direction as your travel. You would also travel through the lowest density. But I suspect you'd always be better off by simply evacuating the tube as much as possible. Or possibly there is a medium other than air being considered, one in which the speed of sound was much lower at a given density. Perhaps interesting things could happen if the working fluid was a refrigerant and the sound wave was intense enough to induce a phase change cycle.