Speech articulation and quality tests were made on an all‐pass system capable of advancing or delaying one frequency band relative to the rest of the spectrum. Parameters in the investigation were (a) width of the advanced or delayed band, (b) amount of advance or delay, and (c) position in the frequency spectrum of the advanced or delayed band. Data were taken at signal‐to‐noise ratios of 30 db and 0 db. The results indicate that maximum impairment of speech intelligibility and quality occurs when the delays and advances are of the order of14 second, and when the band that is advanced or delayed is near the center of the speech spectrum and has an articulation index equal to 0.5. These findings are related to data from statistical studies of the timing of speech energy bursts.