If you watch this Amazing Water & Sound Experiment #2 YouTube video, you can see an amazing effect with water, a sinusoidal signal and a woofer.
Is this a true effect due to vibration of the pipe generated by the sine wave? Is it just a hoax?
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Yes, what you see is real. However, as @Oddthinking points out, this happens because of the sync between the frequency of the camera and the frequency of the sound wave. If you make this experiment at your home without a camera that is shooting at the same frames per second (fps) rate as the frequency of the sound wave you'll not see the same effect with your naked eye.
The drops don't flow in a sinus form. There is a drop of water at the same place every 1/24 second due to the frequency of the wave sound. Each drop goes in a straight line, but the stream of water returns to the same position every 1/24. When looking at this at a frequency of 24 frames per sec, you see as if there is a stream that is "suspended" in air in a shape corresponding to the wave. If the water source is allowed to move then you get a "sinus wave", if you hold the water hose and change the flow strength at 24 Hz, you will see droplets "suspended" in air, as shown in some of the videos below.
The same phenomena can be seen on the following videos as well:
The phenomena was explained by @RedGrittyBrick on Physics.SE:
It's a visual illusion akin to the Wagon Wheel effect
The stream of water is being waggled back and forth by a 25 Hz audio signal and being filmed at 25 frames per second.