Principle of sound: sound is a kind of pressure wave. When playing musical instruments, knocking on the door or knocking on the desktop, their vibration will cause the medium-air molecules to vibrate rhythmically, thus changing the density of the surrounding air, forming longitudinal waves with alternating densities and generating sound waves. This phenomenon will continue until the vibration disappears.

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 air, and its resistance balance is greater than water.

In addition, the propagation of sound is also related to resistance. When the wind is strong, the speed of sound propagation will be much slower, and the sound will be reflected by external substances. For example, people can hear their own echoes when facing the call of mountains.

An example of refraction is that sound travels farther at night than during the day, because the sound encounters rising hot air during the day, thus quickly refracting the sound into the air. When the cold air drops at night, the sound will spread slowly and calmly, and it is not easy to refract.