Answer:
In longitudinal waves, such as sound, the vibration is parallel to the propagation direction of the wave itself. These disturbances are due to the successive compressions of the medium, where the particles move back and forth in the same direction as the wave.
If we want to measure the amplitude of this type of wave we need to know the distance between particles of the medium that is being compresed by the perturbation. So, the closer together the particles are, the greater the amplitude of the wave.
