A longitudinal wave is a type of wave in which the particles of the medium through which the wave travels vibrate in the same direction as the wave's propagation. In other words, the displacement of the medium's particles is parallel to the direction of the wave's energy transfer. This is in contrast to transverse waves, where the displacement of the medium's particles is perpendicular to the direction of the wave's travel. A common example of a longitudinal wave is a sound wave.
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A longitudinal wave has oscillating displacement parallel to its propagation direction. Push and pull one end of a long spring along its axis: compressed and expanded regions travel along the spring while individual turns move back and forth. In a small-amplitude acoustic wave in a uniform fluid, the corresponding pressure and mass density variations drive neighbouring fluid elements through pressure gradients.
For a plane acoustic wave, can describe the particle velocity along . The separation between a particle's small excursion and the distance travelled by the disturbance is essential: the speed of sound is the propagation rate, not the particle velocity. Compressional waves in an solid with elasticity provide another example; their speed depends on both the bulk modulus and resistance to shear strain. A longitudinal polarization leaves no independent choice of transverse direction of the kind available to a transverse wave.