When sound travels through air or any other medium, the particles of the medium vibrate back and forth, but not in a fixed direction. The motion of the particles is parallel to the direction of wave propagation, which is known as a longitudinal wave.
Let's consider a simple example of sound propagating through air. When a sound is produced, such as a clap or a musical note, it creates a disturbance in the air particles. The disturbance causes the air particles to compress and rarefy, forming a pattern of alternating compressions and rarefactions.
As the sound wave propagates, each air particle oscillates around its equilibrium position. The particle undergoes small displacements back and forth along the direction of the wave's travel. However, the particles themselves do not move significantly in one direction.
For instance, if you clap your hands in front of you, the sound waves generated travel outward in all directions. As the sound waves reach different regions of the air, the particles in those regions vibrate back and forth in a manner that is parallel to the direction in which the sound waves are moving.
To summarize, when sound travels through a medium like air, the particles of the medium vibrate back and forth in a direction parallel to the wave propagation. This vibration allows the energy of the sound wave to be transmitted from one particle to another, facilitating the propagation of sound.