The Longitudinal Wave, or "Compression Wave" is shown below (If you are viewing the HTML (web based) or CHM version of this document you will see the animation)




There are areas where the particles are "bunched" or "compressed" together, and areas where they are further apart.



Consider the top diagram. The red vertical line is intended to represent a plunger, say for example in a bicycle pump. If the plunger is pushed sharply to the right and then drawn back to its starting position, the particles of gas immediately in front of the plunger will be momentarily bunched together.


Consider the second diagram. The "undisturbed" particles are represented in blue, and those that are momentarily bunched together are represented, more closely packed in red. The particles in the red section represent a "compression", and blue particles (when they are more spread out) represents what is known as a "rarefaction".


The "compression" will travel through the particles, as energy is transferred. The compressed particles oscillate, the energy carried by the red particles will be transferred to the neighbouring blue particles which will form another compression and this process will be repeated as long as the compression wave is travelling, however you must remember that the particles themselves are only oscillating, not moving. Imagine, if you wish, that this is a long line of people playing pass the parcel, you can understand from this analogy that the people (particles) don't move, only the parcel (energy).