What is the key characteristic of a longitudinal wave?

A longitudinal wave is characterized by the movement of its particles in a direction that is parallel to the direction the wave travels. In this type of wave, as the wave moves through a medium, the particles of that medium vibrate back and forth along the same line that the wave propagates. This creates regions of compression, where particles are closer together, and rarefaction, where particles are spread apart. A common example of a longitudinal wave is a sound wave, where air molecules vibrate in the same direction as the sound travels. This distinct particle motion is what fundamentally distinguishes longitudinal waves from other types, such as transverse waves, where particle movement is perpendicular to the wave direction.

Moreover, longitudinal waves do not require a vacuum to travel; they can propagate through various media, including solids, liquids, and gases. While the concept of frequency may be applicable, it is not a defining characteristic of the wave type itself. Thus, the key aspect that defines a longitudinal wave is indeed the parallel movement of particles to the wave direction.