Why is there a node in the middle of an open-open tube?

  • #1
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Hello. If you blow across an open-open tube, according to the khan academy video, the molecules at both ends contract towards the middle, and then go back outward towards the edges, and the middle molecules don't move. Why? If you are blowing over let's say the top of the left open side, why don't all the molecules move to the right? What is causing the right side molecules to come inward?
 

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  • #2
lekh2003
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Could you refer us to the specific video? Maybe a link?
 
  • #3
CWatters
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You aren't blowing through the tube but across the end. If you blow across the end do you think the air flow will be smooth or turbulent?
 
  • #4
sophiecentaur
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Hello. If you blow across an open-open tube, according to the khan academy video, the molecules at both ends contract towards the middle, and then go back outward towards the edges, and the middle molecules don't move. Why? If you are blowing over let's say the top of the left open side, why don't all the molecules move to the right? What is causing the right side molecules to come inward?
The motion in a tube is not right and left - it is end to end. The diagrams that you are shown are graphs showing the pressure (or displacement along the tube and not the direction of air movement. This is not a string but it is often not made clear.
It's easier if you first think in terms of exciting the resonance with an outside source. There will be a node inside if the tube is long enough to have a resonance at the 'test' frequency. An antinode each and and a node in the middle will work for a wavelength of twice the tube length (approx) so that will be the wavelength of the sound you hear. When you blow across the tube, you are supplying Energy and the Impedance at the resonant frequency will allow energy of that frequency to build up. If you excite a tube with white noise (all frequencies) then you will get a 'ringing' at the natural frequency of the tube, when will persist when the noise is turned off.
 
  • #6
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You aren't blowing through the tube but across the end. If you blow across the end do you think the air flow will be smooth or turbulent?
Turbulent, but I still don't get the result
 
  • #7
sophiecentaur
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https://www.khanacademy.org/science...-sound/modal/v/standing-waves-in-tubes-part-1

What I was referring to was not from the graphs. at 2:50 he shows the displacement of the air molecules going inward, and then both sides going outward at the same time.
Of course. It's just what you would expect. For the mid spot not to be oscillating, the movements have to cancel out. His graph is just as I described (without needing to see it). I think you must be having a problem interpreting what he is actually saying because there is no contradiction.
I re-read my post and I really should have put this:
The motion in a tube is not right and left
differently.
I should have said "Lateral." But the video does a perfect job, in any case.
 
  • #8
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But he doesn’t even show the graph at first. Ok let’s forget the middle air molecules. Why does the air oscillate All inward and then outwards in both directions simultaneously? Rather then first right, then left?
 
  • #9
sophiecentaur
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But he doesn’t even show the graph at first. Ok let’s forget the middle air molecules. Why does the air oscillate All inward and then outwards in both directions simultaneously? Rather then first right, then left?
I see your problem now.
This is what would happen with a very short tube but a standing / stationary wave only occurs where waves are travelling in different directions due to reflections at 'the other' end. When this happens, the waves cancel at the nodes. There is a rather poor animation in this link which shows what I mean. The blue and green waves are bouncing against the ends and the black is the resultant. In this case (just to make things more difficult for you) an antinode is formed in the middle because the wave in this case is between two closed ends. When the waves are reflected at open ends, a node is formed. If that still gives you a pain then google standing waves and look for something better than my link. I am sure you will find something.
 
  • #10
CWatters
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+1 Initially turbulence causes pulses of air to set off down the tube, these get reflected off the end of the tube. It takes time for a pulse/wave to travel down the tube and back and that changes the phase of the returning signal in relation to the original. If the time and phase are right you get constructive and destructive interference at the right places to cause nodes and antinodes.

If only i could remember the maths i could show that more power is delivered from the source/mouth to the load/resonating air column when it's at resonance, that's why if you change the driving frequency it suddenly gets louder at one frequency.
 
  • #11
sophiecentaur
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If only i could remember the maths i could show that more power is delivered from the source/mouth to the load/resonating air column when it's at resonance, that's why if you change the driving frequency it suddenly gets louder at one frequency.
Iirc, it is a matching thing. More energy can go into the tube at a frequency that presents the right impedance, I think I can report having actually felt this when playing a bottle. When you get it to resonate, you can actually feel different in your lungs and there is an 'awareness' of something different in the resistance. With brass instruments (slightly different mechanism) it a definite change in the feeling, particularly down at the fundamental. It almost sucks air out of your lungs. (Or perhaps I am odd in this respect ????- anyone else??)
For an open ended resonance, the pressure variation at the central region is at a max while the displacement is at a min.
 

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