Reverberation (sometimes called reverb or " echo ") occurs when a sound persists after its source stops emitting it. Sometimes reverberation is intentional - as in sound "effects" which can be added to an audio source during a recording or a live presentation. At other times reverb can be unintentional or even a problem. Acoustical engineers spend significant effort trying to reduce reverb in large venues, such as cathedrals or opera houses. A far cry from the Greek myth of Echo, modern engineers can reduce reverb with new acoustically absorbent material, and/or add reverb to sound tracks with the touch of a button and sophisticated digital software.