← all cardsTopic 11 · Wave Motion · Longitudinal & sound
Neighbour analysis
● VERBATIM — examined word-for-wordAt a zero crossing, check the neighbours (with + = right): left neighbour displaced towards the point AND right neighbour displaced towards it → converging → compression. Both displaced away → rarefaction. This is how you tell WHICH zero crossing is which.
What this actually means
Zero crossings alternate C, R, C, R along the wave. To identify which is which, interrogate the neighbours on either side.
With rightward taken as positive: left neighbour displaced right (towards the point) and right neighbour displaced left (also towards it) means both converge, so that crossing is a compression. Both fleeing means rarefaction.
This is two lines of working, and drawing the two little arrows on the graph is what convinces the marker you did it rather than guessed.
Prove it — watch it be true
- Pick a zero crossing on the s–x graph
- Read the displacement of the particle on each side: towards or away?
- Both towards: watch a compression halo sit exactly there