Why there are fewer echoes in the school hall when it is full of children than when it is empty?

When a school hall is full of children, the sound waves produced by their voices and movements are absorbed and scattered by the bodies and clothing of the students, reducing the amount of sound that reflects off the hard surfaces. In contrast, when the hall is empty, sound waves bounce off the walls, floor, and ceiling without obstruction, leading to more echoes. This absorption and scattering effect diminishes reverberation, resulting in a quieter and less echo-prone environment when the hall is occupied.