Why does a hall keep a sound after its source stops?

A note stops at its source, yet a hall can seem to hold its fading sound. The room is not continually producing new notes. Sound already traveling through the air follows many paths, reflecting from boundaries before reaching a listener. Numerous delayed arrivals overlap into reverberation, rather than necessarily forming separate recognizable echoes.

Each encounter with a boundary can reflect some energy and remove some through absorption or transmission. Openings allow energy to escape, and air also absorbs sound. Once the source stops supplying energy, these losses make the remaining field decay. A large volume with little effective absorption can retain sound longer than a more strongly absorbing space.

Reverberation time, often written T60, measures a 60-decibel decay in the remaining sound level. That corresponds to an energy ratio of one millionth, not absolute silence. In a simplified diffuse field, sound energy is approximately well mixed across directions and positions. Its energy decays exponentially, while its decibel level falls approximately linearly with time.

Sabine’s approximation gives T60 in seconds as 0.161 times room volume in cubic meters divided by equivalent absorption area in square meters. Equivalent area combines each surface area with its absorption fraction. The example gives 1.61 seconds for 600 cubic meters and 60 square meters of equivalent absorption. This is a model, not a guarantee for every hall: absorption varies with frequency, and uneven absorption, strong individual echoes or non-diffuse low-frequency behavior can defeat its assumptions.