Sine Waves And FPS

Sound waves have an incredible ability to make other objects match their frequency. If you’ve ever listened to music with a heavy bass beat in your car, you’ve probably noticed the mirrors rippling when the sound waves hit them. What’s happening in the video above is essentially that, though the end result is much more dramatic.

A 24 Hz sine wave travels through a speaker under a water hose. The hose starts vibrating 24 times per second. When the water comes out, it forms waves that match the 24 Hz frequency. Here’s the trick though: Seen in real life, it would only appear to wave back and forth on its way to the ground.

The real hero here is the camera—the phenomenon of shifted perspective. By filming the falling water at 24 frames per second, the camera makes the water stream appear to freeze in midair. Each wave of water hits the exact same space, 24 times every second. On film, it seems like the same wave sits in the air indefinitely, when in reality a different wave has taken its place each frame. If you switched the sine frequency to 23 Hz, it would actually look like the water was falling upwards into the hose because of the tiny offset between the camera’s frame rate and the sine waves.