How do compressions and rarefactions form a sound wave?

Sound propagates through a medium, such as air, as a longitudinal wave, which consists of alternating regions of high and low pressure known as compressions and rarefactions.12

These regions form through the mechanical vibration of an acoustic source, which disturbs nearby particles.132

• Compressions: When a source—such as a tuning fork prong—moves forward, it squeezes adjacent air molecules together, creating a region of high pressure.13
• Rarefactions: As the source moves in the opposite direction, it leaves behind an area of reduced air pressure, creating a region of low pressure.13

As these particles are disturbed, they move forward and backward, passing the impulse to neighboring particles. This successive "pushing" and "pulling" creates a periodic variation in pressure that travels through the medium as a sound wave. Because this process requires the movement of particles, sound cannot travel through a vacuum.21