Antinode
An antinode is a point on a standing wave where the oscillation reaches maximum amplitude, because the two counter-propagating waves always reinforce there. Antinodes sit halfway between adjacent nodes.
AP Physics 2 EK 14.6.B.1.i: an antinode is a point on the standing wave where the amplitude is always at maximum. It is the mirror image of a node, and the two alternate along the pattern a quarter of a wavelength apart.
The boundary rules that decide where they sit. EK 14.6.B.1.iii names four kinds of region. The CED does not tabulate which end gets which, so here is the rule:
| Boundary | What sits there |
|---|---|
| String, fixed end | Node |
| String, loose end | Antinode |
| Pipe, closed end | Node |
| Pipe, open end | Antinode |
Sketch the two ends first, draw the simplest pattern that satisfies both, and the wavelength follows from the length of the region.
Sound is where this turns counterintuitive. The antinode at the open end of a pipe is a displacement antinode: air there is free to move furthest. In terms of pressure it is the opposite, because the open end is held at atmospheric pressure. AP Physics 2 works in displacement, so draw displacement.
A region with a node at one end and an antinode at the other supports only odd harmonics (EK 14.6.B.2), which is why a pipe closed at one end sounds an octave below an open pipe of the same length.