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Sorcery! (30 Hertz water wobble filmed at 30 FPS)

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Published on Jul 4, 2012

C playing in the water.
iPhone with freqgen app is generating 30Hz tone and this is driving a subwoofer. Subwoofer has bass port covered with flap. 1/4" hose is duct taped to flap. Filming at 30fps "freezes" the spiral the water stream makes in the air.
I got a computer speaker set that included a subwoofer with a bass port. I connected the line-in to an iPhone. I ran an app called freqgen on the iPhone and set it to generate a pure 30 Hz sinusoidal tone - 30 beats per second with no harmonics. If you hold your hand over the bass port you feel air pulsing against it. I taped two pieces of foam over the bass port such that the outer one "flapped" with this beat. Then I taped a 1/4" plastic water line connected to a hose to the flap. Turning on the water and the speaker means the water stream gets perturbed 30 times per second in such a way that the water stream forms, at any instant, a spiral. So in bright sunshine, with a fast shutter speed, and a tree behind the stream for contrast, a single photo would show a spiral. If you look with your eyes you just see a "fan" of water (in reality a cone). The water spiral, if you could look at it with a hummingbird eyes, is "rotating" 30 times per second. If you took two photos *exactly* 1/30th of a second apart you would see a spiral of water in exactly the same position as before. Now, here is the kicker, the iPhone 4s records movies at 30 frames per second. So it, in bright sunshine, uses a fast shutter speed to take 30 pictures every second, each exactly 1/30th of a second apart and strings them together to make a movie. What you are seeing is an illusion created by the 30Hz perturbation of the water hose being exactly in sync with the frame rate of the camera. What looks like water travelling in a spiral is in fact water heading away from the hose in a roughly cone shape stream. If it helps, if you were to compare two frames from this move, any one drop of water at a particular position in the spiral (say the left most portion of a particular turn) will have moved upwards by one "turn" of the spiral in the subsequent frame. So that drop of water in the left most portion of a particular turn of the spiral will have moved to the left most portion of the turn in the spiral above in the next frame. The water is travelling in "straight" lines (ignoring gravity), the hose mouth is moving in a circle. The spiral looks "compressed" as it travels upwards because gravity is slowing the water. If you include gravity (but not air resistance) each water drop describes a parabola.

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