# How does a two open ended pipe resonate?

1. Jul 19, 2013

### iScience

For a pipe with both ends closed, with sound being produced from one end, sound is reflected off of the other end and is able to produce a standing wave, however, for pipes with both ends open or one end open, how can the air inside this pipe resonate? or is it not really there air inside that is resonating? is it the pipe itself that is resonating?

2. Jul 19, 2013

### voko

Look up open-end air columns.

3. Jul 19, 2013

### iScience

it only tells me that there must be antinodes at the open ends it doesn't say why. i understand why there cannot be antinodes at the end of a closed end, but why does it have to be an antinode at an open end?

4. Jul 19, 2013

### voko

That is the whole point of resonance.

It does not have to be.

But it is physically possible.

And if there is something that comes close, it will be there.

5. Jul 19, 2013

### iScience

....Whaaaaaaat?

can you put context behind everything you just said i did not follow..

6. Jul 19, 2013

### Staff: Mentor

Let's consider the the left-hand end of an open-ended tube oriented horizontally. Suppose that end were not an antinode. Then there are two possibilities:
1) The pressure a bit to the left, outside the tube, is even higher than the pressure at the end.
2) The pressure a bit to the right, farther into the tube, is higher than the pressure at the end.

#1 doesn't make a lot of sense; that would have the pressure outside of the tube, where it is free to expand up and down as well as sideways, higher than the pressure inside the tube where it can only expand sidewise.

#2 looks more plausible, but by the argument above the pressure just outside the tube cannot be higher than the pressure just inside the tube, so a higher-pressure region inside the tube will want to move to the left.

So #1 says that you can't have an antinode outside the tube; #2 says that the antinode nearest to the end of the tube but still inside the tube will want to move towards the end of the tube (antinodes deeper in the tube are being pressed from both sides so tend to stay put). Put them both together and you end up with the antinodes right at the end of the tube.

7. Jul 22, 2013

### iScience

so basically the pressure is highest at the ends right?

You cannot have an antinode outside the tube because the pressure outside the tube radially degrades and so you cannot make a standing wave out of this. which means obviously that whatever standing waves that exist have to exist inside the tube. is this right? it seems rather obvious now but just wanted to check if this is the right line of reasoning

8. Jul 22, 2013

### Staff: Mentor

Yep - that's the right line of reasoning.

9. Jul 23, 2013

### sophiecentaur

At each end of the pipe there is a change in acoustic impedance (in the pipe and in the open air). Wherever there is a change in impedance when a wave propagates, there will be some reflection. Sometimes there is a phase inversion and sometimes not. If the distance between the discontinuities in impedance (the ends) is the right value for constructive interference (waves travel in both directions and be 'in step'), there will be a build up of energy stored in the column. That is what resonance is.
The reflection of energy at a closed end is more efficient than at an open end, as it happens, and the resonance is not so good with open ended pipes as with closed pipes.

10. Jul 23, 2013

### technician

Where?

11. Jul 23, 2013

### sophiecentaur

Did you consider an internet search engine?

12. Jul 23, 2013

### technician

What search engine does Physics forums suggest for physics explanations and advice?
PS....dont want to stray to far away from the original post !

13. Jul 23, 2013

### Staff: Mentor

Google works just fine for this, although as with any internet search you still have to separate out the good sites from the plausible-sounding crap.

Some wikipedia pages are good, some are awful. If you get into the habit of looking at the discussion page as well, you'll generally get a pretty decent sense of how trustworthy/mainstream a given page is. The references and external links on a wikipedia page are often more valuable than the page itself.

(I'm wondering if we shouldn't fork a new thread for this discussion)

14. Jul 23, 2013

### technician

You can imagine a short 'plug' of air in each end of the tube. Because the ends are open there is little restraint on these 'plugs' moving in and out. Their movement will send pressure variations along the tube from each end and here is where you have the essentials of resonance.
2 waves travelling towards each other (from each end of the tube)
The one condition that must be satisfied is that the (open) ends of the tubes are antinodes.
Look at Google images for open tube resonance for some great illustrations.
Hope this helps

15. Jul 23, 2013

### sophiecentaur

The Hyperphysics pages are a good source of info on most topics. Pretty reliable stuff, too but not always a lot of chatty explanations. Wiki is usually ok on the basics. Check back with PF to confirm what you have understood is pukkah.