What type of mechanical wave requires a medium to travel?

Longitudinal waves are characterized by the displacement of the medium being parallel to the direction of the wave's travel. This type of wave requires a medium, such as air, water, or a solid substance, to propagate. In a longitudinal wave, regions of compression and rarefaction move through the medium as energy is transferred.

In contrast, electromagnetic waves, such as light, do not require a medium; they can travel through a vacuum. Transverse waves, where the displacement of the medium is perpendicular to the direction of wave travel, can also exist on a surface, but their energy transmission still relies on having a medium to oscillate. Surface waves are specific to interfaces (like the surface of water) and also require a medium, but longitudinal waves are a more general category that encompasses a broader range of phenomena, particularly in gases and liquids. Thus, longitudinal waves specifically highlight the necessity of a medium for wave propagation.