Looking around the hall carefully, it is found that there are wall bricks made of special materials with small holes, which eliminate the sound waves. Our classroom is designed in this way. Maybe those advanced places should have better materials, and the principle is similar. The material on the wall has certain elasticity, and those small holes can absorb the transmitted sound waves and will not bounce back to form echoes.

In layman's terms, it is

There are many "nests" on the wall, and the sound will be reflected repeatedly in the "nests", resulting in energy loss. Our physics teacher said.