A Phase Shifter creates a complex frequency response containing many regularly-spaced “notches” in an incoming signal by combining it with a copy of itself out of phase, and shifting the phase relationship cyclically. The phasing effect creates a “whooshing” sound that is reminiscent of the sound of a flying jet. This effect dominates the sound in the song Star Guitar by Chemical Brothers. The song was not played with any guitars but you can hear the phasing effect. The instrument being phased was actually a synthesizer. Some electronic “rotating speaker simulators” are actually phase shifters. Phase shifters were popular in the 1970s, particularly used with electric piano and funk bass guitar. The number of stages in a phase shifter is the number of moving dips in the frequency response curve. From a sonic perspective, this effect is equalization-oriented. However, it may be derived through moderate time-based processing.
Some phaser pedals include:
MXR Phase 90, Phase 45, and Phase 100
Electro-Harmonix Small Stone
Moog MF-103 12 Stage Phaser