What actually moves
A shove, a hard push, passes along a line of people. Each person leans and steps back. Mark one person. That person ends where they began.
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Step 1 of 4
- The marked piece sits at its own spot.
- The shake arrives — the piece shifts a little.
- It swings back the other way.
- The piece is home. The shake has moved on.
Rule
A sound wave is a run of compressions and rarefactions. It travels through a medium. The pieces of the medium only shake about their own places. Energy travels, not the medium.
A disturbance is a shake that starts in one place.
- Wave — a disturbance that travels through a medium.
- Longitudinal wave — the pieces shake along the way the wave travels.
Sound is a longitudinal wave. A speaker sends a wave to you. The air near the speaker stays near the speaker. Each piece shakes and returns. So no air travels from the speaker to you. At your ear, the wave makes the eardrum vibrate.
A smell is different. Its particles really travel from an open jar to your nose.