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But filmed with a 24 fps camera. I think it wouldn't appear to wave in the camera as it would be filmed in the same position each time.




I guess that would depend on synchronizing the camera to start just before the deviation starts. And even then I'm not sure about it. The reason is the mass of the string, it's kinetic energy, and inertia. But in an ideal universe: Yes.

Guido


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To add to an already academic conversation; that was cool!
Wonder how much effect the scientifically exact application duct tape mattered?
The duct tape holding the hose to the speaker was a limiting factor on the hose movement at times. I was amazed at how quickly the hose calmed down (if this video is accurate) at the very end, once the sound was shut down. Water weight must dampen it (?)

Yet another example of duct tape and WD-40.
If it won't stay put; duct tape it.
If it stays put and you don't want it to, WD-40 it.


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megafiddle has it. The only thing the subwoofer is providing here is a means to shake the hose at a controlled shake frequency - that is all.

The hose moving back and forth is flinging the water coming out of the hose. Gravity and flow rate are providing the movement of the water away from the hose.

To answer PG's question, if it were a string, the effect would be less easy to see because there is going to be lots of damping in the string and the fact that the string isn't liquid. When the hose shakes the water in one direction, the water 'globules' separate from the stream. With a string, the molecules are all attached to each other and prevent horizontal movement much more than you get with the water and it's significantly higher mass and resulting inertia.

One can be fooled into thinking that the acoustic energy of the subwoofer is affecting the stream - that is not what is happening in this demonstration at all.

It is simply the subwoofer moving the hose end in translation coupled with the fact that video is not smooth, but sampled in a way, just like audio. The fact that you can't see the hose moving when it's exactly at 24 Hz is because the hose position is in exactly the same spot for each still shot that the camera is taking 24 times a second. The reason the hose looks like it moves at the other excitation frequencies is that the hose is NOT in exactly the same position for each still shot, but slightly off position from the previous shot. The water droplets are not suspended in space, as they appear, but are in the next closest place to the ground on each subsequent frame - but our brains play tricks on us and connect the frames together to give the appearance of movement. This is the whole premise of how video and film and refresh rates work with TVs, CRTs, LCDs, etc. What appears smooth is actually a quick presentation of still images or refreshed screen images. The same applies in the capture of the images as well.

Make sense?

Last edited by rockstar_not; 03/16/13 07:34 PM.
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))). It is simply the subwoofer moving the hose end in translation

Oh, I had missed that, in this experiment, the hose is duct taped to the speaker, and so it is just being physically shaken by the speaker.

So the same thing would happen in a vacuum, and sound waves have nothing to do with it. Seems incorrect that it is called an amazing sound experiment.


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I missed that they called it a sound experiment. It isn't that at all. It's merely a demonstration of aliasing in videography - very cool demonstration - but the implication that it is acoustic in nature is pretty misleading. I didn't pay attention to the title of the video.

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well, it's still worth noting that one of the key elements in the demonstration is the ability to create a controlled sound frequency in order to get a specific movement. It's not just the fact that the hose moves... You could make the hose move any number of ways without getting these results.

An experiment, by definition, is the setting up of a test to see if a repeatable phenomenon can verify the existence of a postulated law. You already know enough about the laws that govern frequency, so you aren't inclined to assume the position of someone who needs to see evidence that proves the nature of waves and measurable frequencies.

Most of the fundamental laws of science were identified by experiments much like this

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well, it's still worth noting that one of the key elements in the demonstration is the ability to create a controlled sound frequency in order to get a specific movement. It's not just the fact that the hose moves... You could make the hose move any number of ways without getting these results.

An experiment, by definition, is the setting up of a test to see if a repeatable phenomenon can verify the existence of a postulated law. You already know enough about the laws that govern frequency, so you aren't inclined to assume the position of someone who needs to see evidence that proves the nature of waves and measurable frequencies.

Most of the fundamental laws of science were identified by experiments much like this




Agreed - but I also agree with Peter that calling it a sound experiment is misleading and has folks thinking the wrong physics aspects. At first glance, one would easily assume that the sound waves are 'bending' the water stream - and that isn't what is happening at all.

Here's what I really think is cool about the demo: Note that in order to get what looks like suspended droplets, or even the droplets slowly moving up/down, those are not the same droplets, but similarly sized/placed droplets from each successive 'wave' passage. The flow control to get similarly sized droplets might be hard to pull off.

-Scott

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The 24Hz is merely the vibrating frequency of a mechanical function here.

You could go outside, pick up the end of your common garden hose and SHAKE it evenly while the water is running out in a downhill direction - and create much the same pattern.

The youtube poster is using a subwoofer, amplifier and signal generator set to the very low audio region, but in effect it is NOT being used to create an audio function here. It is being used to create a MECHANICAL function, in other words the excursions of the speaker cone outward and inward at the chosen frequency rate.

This particular phenomenon, along with certain other paramaters, such as modulating the pressure of the stream, is exploited in the various popular "Dancing Water" fountain displays:

http://www.bing.com/images/search?q=danc...n&FORM=IGRE

Just taking advantage of old Newton's laws, for every action there is an equal and opposite reaction, & etc...


--Mac

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