As a field expert in acoustic physics, I can explain why sound is considered a longitudinal wave. Sound waves are longitudinal waves because the motion of the particles in the medium through which the sound is traveling is parallel to the direction of the wave's propagation. When a sound wave travels through a medium like air, it causes the particles of the medium to oscillate back and forth in the same direction as the wave is moving. This oscillation is a result of the pressure changes created by the sound source, which causes the particles to compress and rarefy as the wave passes. The particles do not move forward with the wave; instead, they move back and forth in a line parallel to the direction the sound wave is traveling. This back-and-forth movement creates areas of compression (where particles are closer together) and rarefaction (where particles are further apart). The series of compressions and rarefactions moving through the medium is what constitutes a sound wave. read more >>

Sound waves in air (and any fluid medium) are longitudinal waves because particles of the medium through which the sound is transported vibrate parallel to the direction that the sound wave moves.read more >>