In small rooms, the spatial variation of the sound pressure at low frequencies is very large. In order to reduce the time‐average spatial variations in a designated test section within the reverberant room, in the region below the frequency of modal overlap, a rotating vane diffuser is employed. Effects of the rotating vane, such as changing the character of the acoustical space from a time‐invariant into a time‐variant system, which in turn introduces new frequencies, is considered. Some experimental findings are presented. [This research was supported by the Naval Applied Science Laboratory, Brooklyn, N. Y.]