you may have already seen this but it's cool

https://www.youtube.com/watch?v=uENITui5_jU

Purty kewl.

It seemed to me that the hose itself was moving thereby creating this exiting water pattern.

It's very cool, Pat - now share it's implications with us : )

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It's very cool, Pat - now share it's implications with us : )

Don't try this in your bathroom or you will pee the same way...lol

Interesting about sine waves, I'm doning the introcutcion to music course on coursera and the sine wave is a fundemental wave at only one frequency, unlike a musical tone.

Musiclover

Hi Pat.

Thank you for posting this. I'll be setting this experiment up for my physics class. Lot's of great physics to discuss in this one!

Regards,
Noel

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Hi Pat.

Thank you for posting this. I'll be setting this experiment up for my physics class. Lot's of great physics to discuss in this one!

Regards,
Noel

Noel,
If you reconstruct this experiment, please report back and tell us how it went. I have to admit, this seems like such an unexpected phenomenon, I first thought it must be computer animation.

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It seemed to me that the hose itself was moving thereby creating this exiting water pattern.

in the very first example, the hose did not move, yet there was a wave pattern in the water's flow.

In the second and third examples, it DID move, and I think it is the movement of the hose that created the spiral that simulated direction (up and down... like the illusion that a wagon wheel is going backwards at certain speeds)

The hose moves in all examples - but you can't see it moving in the first example - what you see is a result of basically a different form of stroboscope behavior.

The video camera has a 24 fps rate - it is taking 24 still shots per second. The reason you don't see the hose moving in the first example is that the subwoofer is moving it at exactly the same rate as the frame rate of the camera - so at 24 Hz, the position of the hose is precisely the same place for each still shot so it appears that it is not moving, just like using a strobe timing light on a the crank pulley on a car - it looks like the mark isn't moving, but that's only because you see it at the same location on each rotation due to the srobe light.

Now, when you slow things down a bit, then the hose doesn't quite make it to the same location it did for the previous shot and you see what is an aliased image of a slow moving wave towards the hose. Speed it up faster than 24 Hz, and the opposite is the case.

What this demonstrates is a visual analogy to sampling aliasing in audio - so it totally belongs in this forum.

Also similar to when you used to see CRT flashing with slow moving images up or down the screen, with film shots of said CRT - the slight mismatch of the CRT refresh rate with the frame rate of the camera would alias and yield those images. Likewise when wheels on cars are filmed and it appears as if the wheel is slowly rotating compared to what the speed should actually look like. All the same phenomenon.

-Scott.

What is the wave that we are seeing? A 24Hz wavelength would be huge wouldn't it (since sound travels about 1000ft/sec, a 24Hz wave would be wavelength about 40feet) yet the water seems to be captured by the strobe in a wavelength of a few inches. Also, I thought sound waves are in the direction away from the speaker, not tangential to it. ( keep in mind that I know next to nothing about this type of audio, so I'm just asking here )

Fascinating video. I agree, this couldn't be the fundamental at that frequency; it must be a higher harmonic.

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It's very cool, Pat - now share it's implications with us : )

implications? Ummmm...

based on the shape generated by the last 2 experiments, I'm thinking this phenomenon was first discovered by the IRS

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What is the wave that we are seeing? A 24Hz wavelength would be huge wouldn't it (since sound travels about 1000ft/sec, a 24Hz wave would be wavelength about 40feet) yet the water seems to be captured by the strobe in a wavelength of a few inches. Also, I thought sound waves are in the direction away from the speaker, not tangential to it. ( keep in mind that I know next to nothing about this type of audio, so I'm just asking here )

The wavelength of a 24 Hz soundwave is relatively long because the speed of sound is relatively high.
During one cycle, 1/24 sec, sound will travel about 46 ft.

In this case the wavelength depends on the speed of the water stream. The water will not have traveled
very far in 1/24 sec.

Cool. So if it was a string instead of a column of water, it would just be an unmoving straight line (or points equidistant from the speaker), depending on where it was in the cycle it was captured by the strobe.

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Cool. So if it was a string instead of a column of water, it would just be an unmoving straight line (or points equidistant from the speaker), depending on where it was in the cycle it was captured by the strobe.

Glad it helped.

Oops, sorry, you posted while I was posting this below.
I'll add some more shortly -

We often think of waves as they usually drawn, as an x-y plot. This also
is the same thing you see on an oscilloscope and chart recorder.
The only waves that actually looks like that (that I can think offhand),
are water waves like on the ocean, and a rope being shaken from one end.

In a soundwave, the air does not move from side to side or up and down;
in other words it does not move at right angles to the direction of travel.
The sound can spread out to the side, but the air motion is in the direction
of travel. The air moves towards and and away from the source. The soundwave
is an alternating series of compressed and rarefied regions of air.
If you could freeze a soundwave and walk along it in the direction of travel
with a pressure gauge, you would see the air pressure rising and falling as
you moved along it. If you now plot these pressure readings with pressure on
the vertical axis, you get the familiar sine wave shape, if the sound is of
a pure frequency.

The wave seen in the water stream is more like a plot of a wave, rather than
an actual wave. It is in effect a plot of the motion of the hose. The water
stream is much like the paper strip in a chart recorder (like a polygraph or
electrocardiograph, and the hose tip is like the chart recorder pen.

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Cool. So if it was a string instead of a column of water, it would just be an unmoving straight line (or points equidistant from the speaker), depending on where it was in the cycle it was captured by the strobe.

I may have misunderstood, I think you already know what I was trying to explain?

I beleive the water stream is not in the same direction as the sound wave would be,
but at right angles to it. If the stream (or a string) were in the same direction as
the soundwave from the speaker, both would remain straight.

I think that is what you meant?

))) I may have misunderstood, I think you already know what I was trying to explain?

Yes. I saw this nice animation of it, which shows the back and forth motion of the air molecules, as the compression wave moves along, and the pressure at any point rises and falls.

http://dev.physicslab.org/Document.aspx?doctype=3&filename=WavesSound_IntroSound.xml

This is another video, with cool patterns from sound, based on resonant frequencies of rectangular plate glass, with the sound source coming from the center of the glass.. Note: a different phenomenon than the other video. Standing waves are produced by specific frequencies, as they reflect back from the edges, and cancel at certain points, resulting in no vibration. It doesn't require a special camera to see them. The salt accumulates in areas where the plate is not vibrating.

http://m.youtube.com/watch?v=Zkox6niJ1Wc

Ok, thanks for confirming.

A string, as you described, would remain straight if it were in line with
the speaker axis, the direction of the soundwave. The speaker in the video
is projecting horizontally. But the water stream is vertical. If the string
were also vertical, it would also wave, much like a rope being shaken from
side to side at one end.

But there is probably a more important difference between the water stream
wave and a soundwave. A soundwave propogates; energy is continuously being
transformed back and forth between two different forms. Air motion causes
compression and rarefaction, compression and rarefaction causes air motion,
etc, etc.

The wave in the water stream is purely ballistic. Bits of water are being
sent out along varying paths in a repeating pattern. But there is no wave
propogation involved. You can see similar "waves" in traced machine gun fire.
Or if you just shake a garden hose rapidly.

The "wave shape" is there, even if directly observed. The strobe effect of the
video camera affects the wavelength and motion of the wave.

(edited - that should be "...affects the motion and direction of the wave".)
.

))) If the string were also vertical, it would also wave, much like a rope being shaken from
side to side at one end.

Yes, I was referring to a vertical string. 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. This is when the frequency is 24 Hz or any freq that is a multiple of 24, like 240 Hz. If the frequency was slightly off 24, the string should appear to move slowly, like a slow motion view of the flapping that goes on with a guitar string.

That would be true if the string moved as one solid piece, like a rod.
I think it would kind of wave or wiggle, though.

A better example might be a very high speed stream of water, one moving
at the speed of sound. Over the distances in the video, the water stream
would appear very nearly straight.

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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

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.

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?

))). 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.

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.

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

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