Sound is a mechanical wave generated by the vibration of an object and requires a medium to propagate. Whereas hearing is the ability to perceive sound waves by detecting the vibrations in the surrounding medium. Sound waves cause the particles of the medium to vibrate and transport energy through it. Since the wave propagates parallel to the direction of the vibrations, sound waves are referred to as longitudinal waves. During vibration, the particles move back and forth, resulting in high-pressure and low-pressure regions in the medium known as compressions and rarefactions, respectively. In space, sound waves cannot travel since there are no particles to vibrate and propagate the sound wave. Since sound is a pressure wave, it can be modeled similarly to periodic wave equations. When propagating in the air, sound waves can also be modeled in terms of the displacement of air molecules.