The essence of sound is pressure wave, that is, pressure that changes with time and space position. Such interference frequency falls within the acceptable range of human ears, which is sound.
The spread of sound
The propagation of sound needs matter, which is called medium in physics. This medium can be air, water or solid. Of course, in a vacuum, sound cannot spread.
Sound travels at different speeds in different media. The propagation speed of sound is related to the equilibrium resistance of the medium. Equilibrium resistance means that when a molecule of a substance deviates from its equilibrium position, the surrounding molecules will push it back to its equilibrium position. The greater the equilibrium resistance, the faster the sound travels. The resistance balance of water is greater than that of air, and that of iron is greater than that of water.