Examples of longitudinal waves are sound waves in air and primary waves, known as P waves, in earthquakes. Longitudinal waves are waves in which the displacement of particles occurs in parallel to the direction the wave travels. These waves are also called l-waves.
When a sound wave occurs, it causes a sinusoidal pressure variation in the air, which, in turn, causes the air's motion to move back and forth with the sound. The P waves in earthquakes have the fastest velocity and are the first waves to arrive. The S waves, or secondary waves, of earthquakes are slower and arrive after the P waves. These are transverse waves, which means the displacement of particles occurs perpendicular to the direction of the wave.