Here's why:
* Compression waves: In these waves, the particles of the medium are squeezed together (compressed) and then spread apart (rarefied) in a rhythmic pattern. The energy is transferred through these compressions and rarefactions. Since the particles are moving closer together and farther apart in the same direction as the wave travels, the particle displacement and energy transport are aligned.
* Examples: Sound waves are a classic example of longitudinal waves. When you speak, your vocal cords vibrate, compressing and rarefying the air molecules, creating sound waves that travel through the air. The air molecules themselves are vibrating back and forth in the same direction the sound wave is traveling.
Contrast with transverse waves: In transverse waves, the particles of the medium move perpendicular to the direction of energy transport. Think of a wave on a string – the string moves up and down (perpendicular) as the wave travels horizontally.
