Here’s a cool sound and water experiment that gives off the illusion of water drops stopping in mid-air. Video cameras capture motion by taking 24 individual still pictures per second and playing them back at the same speed. What these guys have done, is stream water in front of a large sub woofer speaker playing a sine wave (speaker goes fully in and fully out) at a rate of 24 times per second (24 hz) which exactly matches the speed of the camera’s picture taking rate. This causes the water drops to appear as if they are frozen in mid-air. When the sound is played back at 23 hz, a bit slower than the photography the drops appear to slowly descend. The magic happens though at 25 hz, with the sounds being slightly faster than the photography, and the drops appear to float upward, defying gravity. This is a very neat experiment with a very cool visual effect.
source: water experiment on youtube