flutter echo: They are created when sound bounces back and forth between two parallel walls. Their perception is favored if the other spatial direction - for example due to strong damping - only throws back little sound. In large rooms you can then hear a clear echo, if the room is smaller, there is more of a kind of reverberation of the sound, the flutter echo takes on the character of a tone. To avoid a flutter echo, its reverberation time below that of the entire room.
