Difference between pressure antinodes and displacement node?

[eprparadox]

Homework Statement

An organ pipe with one end open and the other closed is operating at one of its resonant frequencies. The open and closed ends are respectively:
A. pressure node, pressure node
B. pressure node, displacement node
C. displacement antinode, pressure node
D. displacement node, displacement node
E. pressure antinode, pressure node

Homework Equations

The Attempt at a Solution

So the answer is B.
[/B]
The pressure node makes sense because the pressure is just whatever atmospheric pressure is. IE, there is no over-pressure at the open end.

But I get confused about why a displacement node is the same as an antipressure node.

I've looked at http://hyperphysics.phy-astr.gsu.edu/hbase/Waves/standw.html and it says

"A node for displacement is always an antinode for pressure and vice versa, as illustrated below. When the air is constrained to a node, the air motion will be alternately squeezing toward that point and expanding away from it, causing the pressure variation to be at a maximum. "

Intuitively, where there are a lot of particles, there will be high pressure. But that's not what this is saying. And what exactly does "displacement node" actually mean?

Thanks so much.

 

[kuruman]

Intuitively, where there are a lot of particles, there will be high pressure. But that's not what this is saying. And what exactly does "displacement node" actually mean?

Displacement node means that a very thin slice of the medium at the node does not move (zero displacement). For example, right at the wall of a closed pipe the air can't move. However the pressure at the wall rises and falls while the particles vibrate with a small amplitude. You seem to think that particles actually move and congregate near the wall thus raising the pressure. This is not the case. The pressure at the wall increases because slice 1 of air at a node pushes slice 2 next to it and the combined pushes of slices 1 and 2 push slice 3 and so on until the wall, which cannot move, experiences the combined pushes of all the slices from the node to it thus forming a pressure antinode. If you understand this, then you will understand why at any displacement node, whether there is a wall to constrain the medium from moving or not, there will be pressure antinode.

BTW, there is no such thing as an antipressure node.