Echo is a reflection , means change in direction of a wave front at an interface between two different media that arrives at the listener with a delay after the direct sound. The delay in sound wave arrival is proportional to the distance of the reflecting surface from the source and the listener. Echo can be extremely disruptive to a audio / music system. It can so critically impair audio quality very unpleasant and distracting. Enclosed hall / room need acoustic treatment to control Echo from wall, roof and floors.