Longitudinal Waves
A longitudinal wave is a wave in which the particles of the medium are displaced in a direction parallel to the direction of energy transport. A longitudinal wave can be created in a slinky if the slinky is stretched out
horizontally and the end coil is vibrated back-and-forth in a horizontal direction. If a snapshot of the resulting wave could be taken so as to freeze the shape of the slinky in time, then it would look like the following diagram.
Because the coils of the slinky are vibrating longitudinally, there are regions where the coils become pressed together (compression) and other regions where the coils spread apart (rarefaction). A compression is a point in the medium of maximum density, while a rarefaction is a point of minimum density.
Periodic waves have a regular spatially repeating pattern. In transverse waves, crests and troughs alternate. In longitudinal waves, compressions and rarefactions alternate. In either case, the distance from one location on the wave to the next corresponding location is called the wavelength - the length of one complete cycle of a wave. In the case of a longitudinal wave, a wavelength is the distance from one compression to the next, or from one rarefaction to the next.
Amplitude is easy to see on a transverse wave diagram. It is the vertical distance from the resting position to the crest (or from the resting position to the trough). Amplitude is harder to visualize with a longitudinal wave diagram.
In a high amplitude longitudinal wave, the particles in a compression are very tightly packed and the molecules in a rarefaction are very spread out. In a low amplitude longitudinal wave, there is very little difference in particle spacing between compressions and rarefactions.