Slow Bass Reverses Time
Using some physics trickery, it’s possible to create the illusion of water flowing backwards by running it in front of a subwoofer speaker, as shown in this no-frills video demonstration…
Sounds Like Time
This is a fairly well-known physics illusion that depends upon the synchronisation (or almost synchronisation) of an audio pulse with the framerate of the video camera. In most cases, video cameras capture 24 frames per second, which is a capture frequency of 24Hz. If you run a stream of water in front of a powerful speaker outputting a sine wave which is also at a frequency of 24Hz, it will modulate the droplets in sync with the capture rate of the video camera. The upshot of this is that when you look at the video footage, it appears as if the water is levitating, suspended in mid-air…
As a further bonus, if you slightly increase the pulse speed of the sine wave, the water actually appears to move slowly backwards up towards the tap.
This is a phenomenon that is more commonly demonstrated using a strobe rather than a subwoofer – here’s a video from Cornell showing the effect, with some extra information on how this behaviour can be used to measure the Earth’s gravitational pull…