Flutter echo

A flutter echo is a periodic sequence of an echo that arises from the fact that a sound signal propagates along a path that leads back to the starting point via two or more highly reflective surfaces. The audibility of a flutter echo presupposes that the propagation in other spatial directions is more attenuated, more precisely, that the flutter echo reverberation time is greater than the reverberation time of the entire room.

If the distance between the surfaces between which the sound is thrown back and forth is small, the flutter echo has a tonal character. In the case of long distances between successive repetitions, on the other hand, the latter are perceived as separate signals: A shot or clap of hands then sounds like machine gun fire becoming weaker. Such flutter echoes can arise in very high rooms or (even with a relatively low ceiling height) between a flat floor and a curved ceiling or a bridge arch above. Depending on the ratio of the radius of curvature of the curved surface and the height of the ceiling, in these cases the sound only returns to the starting point after four, six or eight reflections . The audible repetitions are therefore rare.

Flutter echoes are perceived as annoying in all rooms for sound presentations and even more so in recording studios and should be avoided. This can be achieved through the geometric room design and / or by partially covering reflective surfaces with absorbent material ( sound absorption ), whereby the flutter echo reverberation time is reduced below the reverberation time of the entire room.