Sound waves bouncing off hard surfaces cause reverberations in concert halls. When the sound wave reflects in 0.1 seconds or fewer, the human ear hears both the previous and current sound at the same time, leading to the reverberation.
To reduce reverberations, architects use acoustic materials, such as fiberglass, to absorb the sound rather than reflect it the way harder materials, such as concrete, do. Architects use combinations of absorbent and reflective materials to create the most pleasing sound for the auditorium based on its size, shape and capacity. The squares that hang from the ceiling in many concert halls are called "acoustic tiles" because they absorb and redirect sound waves.