Longitudinal Traveling Wave:

1. The arrows in the above diagram are displacements, not velocities.
2. All particles inside a compression are moving in the same direction of the propagation of the wave, while those inside a rarefaction are moving in the opposite direction.
3. At the central part of a compression or a rarefaction, the displacement is zero and the speed is the greatest.
4. But in a longitudinal stationary wave, the centers of compressions and rarefactions are NODES. Their displacements are zero and always at rest.