Phase machine: Steve Reich and the music of process

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Steve Reich and process music

June 1, 2026

Written by Jordan Elias, MT-BC

In 1965, Steve Reich placed two identical tape loops on two separate machines and played them at the same time. Because the machines ran at slightly different speeds, the loops began to drift. What unintentionally emerged was phasing. This is a browser version of those tape machines to experiment with yourself. Load any audio, set a drift rate, and listen.

It's gonna rain

The piece that started it was called It's Gonna Rain. The source material was a field recording of Brother Walter, a preacher Reich had encountered in Union Square, San Francisco in 1964. He had recorded Walter's sermon on a portable machine, and later extracted a single phrase, "it's gonna rain", which he looped on two separate tape machines.

The machines were meant to run in unison but were slightly off, with one running slightly faster than the other. Over the next several minutes, the loops drifted apart and gradually back together, creating a process of rhythmic effects: canons, syncopations, unison moments, dense polyrhythm as the loops slowly converged again.

This discovery became the foundation of his compositional practice. What had been a slight inconsistency of motor speed between two tape machines became a deliberate compositional method. In later works like Piano Phase (1967) and Violin Phase (1967), Reich transferred the phasing process to live performers: two pianists or two violinists playing the same repeating figure, with one player instructed to gradually speed up. The score specifies the starting material and allows the process to play out.

The sync function

In the phase machine above, tape A runs at normal speed and tape B runs slightly faster, accumulating offset over time until the loops have drifted a full cycle and re-align. The K key is a sync hold: while pressed, both tapes run at the same speed, the offset freezes, and for a moment you hear the loops in unison. Releasing the key allows drift to resume from wherever the offset had reached.

This feature was present in Reich's live performance practice. A performer could hold a phase position rather than continuously drifting but it is almost universally omitted from digital phase machine implementations. Without sync hold, a digital phase machine can only simulate the original tape accident: continuous, uncontrollable drift.

Reich's tape pieces were, in his own words, an encounter with a process he could not control. The machine did what the machine did. The performer's job was to listen to what emerged. The fact that phasing began as an artefact of imprecise mechanical equipment is part of what makes it interesting. Reich noticed that drifting was interesting, and built a practice around attending to it carefully.

What the ear does with drift

When two identical loops are slightly out of phase, something surprising happens in the auditory system: you begin to hear melodies, rhythms, and patterns that are not present in either loop individually. The brain does not hear two separate streams. It actively groups and re-groups the incoming material, finding structure in the relationship between the two signals, structure that exists only in the perceptual interaction between them.

This is a vivid demonstration of what Albert Bregman called auditory scene analysis: the brain's continuous work of organising acoustic input into coherent perceptual objects. In the context of phasing, that work produces emergent melodic patterns, apparent accelerations and decelerations in the rhythm, moments where the two streams seem to merge into one, and moments where they become clearly separate voices. None of these are composed.

The perceptual effects are most pronounced at the mid-phase position when the offset is roughly half the loop length. This is where the material from the two loops is most interleaved, and where the auditory system has the most work to do in constructing a coherent percept.

Process music and psychotherapy

Reich's 1968 essay Music as a Gradual Process articulates something that sticks with me: the idea that the listener's engagement with process music is not passive reception but active participation. The slow perceptual tracking of a change that is too gradual to pinpoint but too large to miss. You cannot hear the moment the loops drift but you can hear that they have drifted. The experience is one of continuous presence with something that is always already different from how it was.

Change in the therapeutic process is rarely visible at the moment it occurs. The session where something shifts usually does not feel dramatically different from the sessions before and after it. What becomes apparent is a retrospective sense that something is now different, that a pattern has moved, that a position once held with great force is now held more lightly. There was a gradual process.

Check out the earlier indeterminacy post exploring our relationship with uncertainty.

The standalone version of this tool, with additional options including loop length control, is on my GitHub.

Experiments

The first encounter

Use the default loop. Press play and do nothing for two full minutes. Don't adjust any controls. Simply listen as the loops drift apart. Notice when you begin to hear emergent patterns. Notice when the loops seem to merge. Notice when the original material becomes unrecognisable. Notice when it returns. Try to locate the moment the transformation happened. You will find it is difficult to identify.

Using sync hold

Let the loops drift to a point that feels rhythmically or texturally interesting. Hold K. You can now listen to that phase relationship as a sustained rhythmic pattern without it changing. Release K and drift resumes. Notice what it feels like to have agency over a process that was previously automatic.

Your own material

Record a short phrase, a word, or a sustained note into your microphone. Start the phase machine and listen as your own voice multiplies and drifts. At different phase positions, the phrase becomes a different phrase. The meaning of the words, if there are words, shifts. Notice which phase positions you prefer and which feel uncomfortable. The material is unchanged. Your relationship to it is not.

If this resonates

This is part of a series on sound, perception, and experimental music as therapeutic frameworks. Related posts: timbre and how sound waves become meaning, binaural beats and what EEG actually shows, generative systems, and the indeterminacy post exploring the role of uncertainty in our lives.

If you are curious about music therapy in practice, you can learn more about music therapy here. You can also book a free 30-minute consultation to discuss how music therapy could benefit you, with no commitments.

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