PHASER became a classic effect in 70s rock, used by everyone from the Eagles to Van Halen to add a swirling psychedelic edge to electric guitar sounds. To understand how phasers work, let’s remind ourselves of a little basic physics.

In their simplest form, soundwaves have regular peaks and troughs, like waves rolling towards a beach. Now imagine two identical soundwaves aligned so that when the one is peaking, the other is in a trough. These two soundwaves are said to be ‘out of phase’. In the most extreme out-of-phase alignments, they cancel each other out completely, resulting in silence.

Phaser pedals split your guitar’s signal into a dry half and a wet half that passes through multiple filters, knocking the two signals out of alignment. When the filtered (wet) signal is mixed with the dry signal again, phase interference occurs that creates cancelled-out gaps in your overall signal’s frequency range.

These gaps are then moved across the spectrum by a low-frequency oscillator (LFO, controlled by the rate knob). This constant movement morphs your guitar’s tone between bassy and warm, to toppy and brittle in regular cycles, creating the ‘swooshing’ sweep of a phaser effect.