The wave in which the particles of the medium oscillate parallel to the direction of propagation of the wave is called a longitudinal wave. Sound waves similarly oscillate parallel to the direction of propagation, forming the compressions and rarefactions. Hence sound waves are called longitudinal waves.

Definition of Longitudinal Wave
A longitudinal wave is a type of mechanical wave where the particle displacement is parallel to the direction of wave propagation. This means that as the wave travels, the particles of the medium move back and forth in the same direction as the wave itself.
Characteristics of Longitudinal Waves
- Particle Movement: In longitudinal waves, particles of the medium oscillate along the same axis as the wave. For example, when a sound wave travels through air, air molecules vibrate back and forth in the direction of the sound.
- Compression and Rarefaction: Longitudinal waves consist of regions of compression (where particles are close together) and rarefaction (where particles are spread apart). These alternating zones create the wave's structure.
- Examples: Common examples of longitudinal waves include sound waves in air, seismic P-waves, and pressure waves in fluids.
Propagation of Longitudinal Waves
- Medium Requirement: Longitudinal waves require a medium (solid, liquid, or gas) to travel. They cannot propagate through a vacuum.
- Speed of Sound: The speed of longitudinal waves, such as sound, varies depending on the medium. For instance, sound travels faster in water than in air.
Summary
Longitudinal waves are essential in understanding sound and other mechanical phenomena. Their unique properties, including compression and rarefaction, help explain how energy moves through different media.