A few friends have asked me for an explanation of what’s going on in the video above as it seems pretty weird. It’s actually pretty simple…
The camera records at 24 frames every second. The speaker is at 24hz (cycles per second) and ‘knocks out’ the drops of water in a constant 24 times per second. Think whacking a ketchup bottle once and then taking a picture just after the sauce flies out, you catch the ketchup blob in mid air. Now whack it 24 times a second and take a snap each time… you kinda take the same pic each time but clearly the blobs of ketchup will be a slightly different shape each time… but in pretty much the same position. That’s all that’s going on here.
But what about it going backwards? Well, at 23hz (not 24hz as before), the camera is relatively faster by 1 frame so the water *seems* to go slower. Seeing as it was stationary before, *slower* than stationary is backwards. In this case, the same water drops are snapped slightly earlier each time giving the effect of going backwards.
And, you guessed it, the opposite happens at 25hz. Water goes ‘faster’, relatively speaking. It’s a completely linear thing so you can change the frequency slightly and see the result instantly.
Main thing is to generate the drops at a nice constant rate, which in this case is what the speaker vibration is doing. Other methods involve precise pumps or special ‘gates’ that snip the water stream into droplets very accurately.
Here’s a quick flash example I made to show the ‘shutter’ effect on regular falling drops. Just move your mouse left and right then see if you can get the shutter and blob frequency to match. Note how there two streams of drops are created at slightly different speeds so that one can be made to look like it’s going up while the other is going down.
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