Reverberation is the collection of reflected sound waves that arrive at a listener after the direct sound from the source. In an empty room, sound bounces off walls, ceiling, and floor, creating a complex series of echoes that blend into a 'tail' that decays over time.
The duration of reverberation is measured as RT60 — the time for the sound level to fall 60 dB from its peak. A concert hall might have an RT60 of 2–3 seconds. A professional recording studio has an RT60 of 0.2–0.4 seconds. An untreated bedroom can easily exceed 0.5–1.0 seconds.
Excessive room reverb in a recording makes voices sound distant, reduces intelligibility, and creates an unpolished result. Acoustic treatment reduces reverb by adding absorptive materials that convert sound energy to heat rather than reflecting it back.
Reverb added in post-production (digital reverb plugins or hardware units) is completely controllable and sounds far more professional than room reverb captured during recording. The standard approach is to record as dry as possible and add controlled reverb afterward.