Why is a sound wave called a longitudinal wave ?

A sound wave is called a longitudinal wave because the particles of the medium through which the sound is traveling move parallel to the direction of the wave propagation. In other words, the displacement of the medium's particles occurs in the same direction as the wave is moving.

Here's a more detailed explanation:

Longitudinal Waves: In longitudinal waves, the particles of the medium vibrate back and forth in the same direction as the wave travels. This creates regions of compression (where particles are close together) and rarefaction (where particles are spread apart).

Sound Waves: When a sound wave travels through a medium (such as air, water, or a solid), it causes the particles of the medium to oscillate parallel to the direction of the wave. This back-and-forth motion of particles creates alternating high-pressure (compression) and low-pressure (rarefaction) regions.

This is different from transverse waves, where particles of the medium move perpendicular to the direction of wave propagation (like waves on a string or electromagnetic waves).