A sound wave is essentially fast changing sound pressure. Longitudinal sound waves need a medium to travel through in order to exist e.g mostly air, water, metal...
A sound wave is the pattern of disturbance caused by the movement of sound pressure changes traveling through a medium (such as air, water, or any other liquid or solid matter) as it propagates away from the source of the sound. The source is some object that causes a vibration, such as a ringing telephone, or a person's vocal chords. The vibration disturbs the particles in the surrounding medium; those particles disturb those next to them, and so on. The pattern of the disturbance creates outward movement in a wave pattern, like waves of seawater on the ocean. The wave carries the sound through the medium, usually in all directions and less intensely as it moves farther from the source. Think, what moves your ear drums and what moves the diaphragm of a microphone? It's the changing of acoustic pressure.
The idea that sound moves in waves goes back (at least) to about
240 B.C. The Greek philosopher Chrysippus (c. 240 B.C.), the Roman
architect and engineer Vetruvius (c. 25 B.C.), and the Roman
philosopher Boethius (A.D. 480-524) each theorized that sound
movement might take a wave form.
A sound wave is a longitudinal wave with frequency between 20 to 20,000 Hertz.
It can travel through solids, liquids, and gases but not vacuum.
Longitudinal wave are the ones in which the particles of the medium oscillate along the direction of propagation of the waves.