This reminds me of something I observed with water. If you get a thin, smooth, and clear (no air bubbles) stream of water coming out of a faucet, you can put your finger into the stream and it will cause a disruption of the smooth flow of water up to an inch above your finger. It's subtle and you might have to look at it in the right lighting to notice. I guess it's not too surprising since the speed of sound through water is faster than the water is falling, so the waves are able to propagate up the falling stream.
