Avalon Acoustics Sentinel - Manual (Page 44)

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10.2 Flutter Echo
These parallel, reflective surfaces can also produce a different audible
problem. If there is little absorption at higher frequencies, a musical transient
containing high frequencies, such as a hand clap or the strike of a percussion
instrument, can be heard bouncing repeatedly between the surfaces. Called
flutter echo (or slap echo), these multiple reflections can obscure musical
detail. The situation is analogous to standing between two parallel mirrors,
when the outline of your reflection becomes more difficult to discern, due to
the additional reflected images present.
Again, it is only necessary to reduce the reflectiveness of one of the surfaces
in each pair of surfaces to eliminate flutter echo. However, we are also
constantly concerned with retaining left/right symmetry in order to maximize
imaging quality. Since we are concerned with the high frequencies, any soft
material is appropriate. Drapery or fabric wall hangings are quite effective on
the walls. Bookshelves also work well by breaking up the flat surfaces.
Carpeting should eliminate most potential problems between the floor and
ceiling.