Longitudinal wave

A wave in which the medium’s oscillations are parallel to the direction the wave travels. It propagates through alternating regions of compression and rarefaction.

Compression: A region in a medium where particles are closer together and pressure or density is higher than average. Longitudinal waves propagate as moving regions of compression.
Sound wave: A mechanical disturbance that travels through a material medium as variations in pressure and particle motion. In air and fluids, sound commonly travels as a longitudinal wave.
Transverse wave: A wave in which the medium’s oscillations are perpendicular to the direction the wave travels. Its particle motion is perpendicular, rather than parallel, to propagation.
Simple harmonic motion: Periodic motion in which a restoring force is proportional to displacement and directed toward equilibrium. Small-amplitude particles in many longitudinal waves approximately follow this motion.
Sound in air: A pressure disturbance that propagates through air as alternating compressions and rarefactions. It is the familiar gas-borne example of a longitudinal wave.
Rarefaction: A region in a medium where particles are farther apart and pressure or density is lower than average. Rarefactions alternate with compressions in a propagating longitudinal wave.
P-wave: A seismic body wave that compresses and expands material parallel to its direction of travel. P-waves are longitudinal waves that travel through Earth’s interior.
Surface wave: A wave that travels along the interface between two media, often combining transverse and longitudinal particle motion. Surface waves combine motions that a simple longitudinal wave keeps parallel.
Elasticity: The ability of a material to deform under stress and recover its original shape when the stress is removed. Elastic restoring forces let a disturbed medium transmit longitudinal waves.
Sound in water: A pressure disturbance that propagates through water through coupled compression and expansion of the liquid. Water carries longitudinal sound despite having no shear rigidity.

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