The law of conservation of energy tells us that energy can neither be created nor subtracted. But energy can be converted from one form to another. Sound-absorbing materials convert the vibration energy of the particles into heat energy, thereby reducing sound.

How does the propagation of sound change through the air? 

First, sound waves propagate in the air, particles vibrate, and sound energy is converted into a small amount of heat energy lost. Then, when the sound hits the wall, part of it is reflected back by the surface of the acoustic material, called echo. In addition, some sound waves enter the acoustic material and continue to propagate. The direction of sound propagation shifts downwards because the density of acoustic materials is greater than the density of air. Due to the frictional resistance of the acoustic material, some of the sound energy is transferred into a little heat energy. As the sound continues to travel, its energy becomes weaker and weaker. Every reflection and refraction loses sound energy.