Final answer:
Air particles vibrate parallel to the wave's direction in a longitudinal wave when sound travels through air, which creates areas of compression and rarefaction.
Step-by-step explanation:
When sound travels through air, the air particles vibrate parallel to the direction of the wave's propagation. This type of wave is known as a longitudinal wave, where the particles of the medium move in a direction that is parallel to the energy transport. Unlike transverse waves where the motion is perpendicular to the direction of propagation, in a longitudinal wave, areas of compression and rarefaction are created as the sound waves travel through the air.
It's important to note that sound cannot travel in a vacuum because there are no particles to transmit the vibrations. The speed of sound in a medium also depends on its ability to compress. The more easily compressible the medium, the faster the sound will travel through it.