Sound waves in air are longitudinal waves. This means the particles of the medium (air) vibrate back and forth parallel to the direction the wave is traveling. This creates regions of compression (high pressure) and rarefaction (low pressure).

This is different from transverse waves, like light, where particles vibrate perpendicular to the wave's direction. The wave equation for a sound wave propagating in the x-direction is given by the pressure variation:

$P=P_0\sin (kx-\omega t)$

Therefore, the correct answer is longitudinal.