The vox humana is a short-resonator reed stop on the pipe organ, so named because of its supposed resemblance to the human voice. As a rule, the stop is used with a tremulant, which undulates the wind supply, causing a vibrato effect. The vox humana is intended to evoke the impression of a singing choir or soloist, though the success of this intent depends as much upon the acoustics of the room in which the organ speaks as it does the voicing of the pipes. It is almost invariably at 8 ft pitch, though on theater organs it is not uncommon to encounter a chorus of vox humana stops unified at 16 ft, 8 ft and occasionally 4 ft pitch on the Great manual and 8 ft and 4 ft pitch on the Accompaniment manual.