Consider the following activity to better understand longitudinal waves. Make a forward and backward motion with a coil or spring. What are your observations? You'll notice that the turns in some parts of the coil are closer together, while others are further apart. Sound travels through a medium in the same way. We will look into it right now.
From the above activity, you can see that in some parts of the coil, the turns are closer together. These are regions of compressions. In between these regions of compressions, we have regions where the coil turns are far apart called rarefactions.
As the coil oscillates, the compressions and rarefactions move along the coil. The waves that propagate with compressions and rarefactions are called longitudinal waves.
The particles of the medium move back and forth along the wave's propagation direction in longitudinal waves.
Sound is a longitudinal wave as well. Only when there are particles that can be compressed and rarefied can sound travel. The regions where particles are crammed together are known as compressions. Rarefactions are low-pressure areas where particles are widely separated. A sound wave is an example of longitudinal mechanical wave.