In this video, Stephen Mendes demonstrates a 5U modular synth patch to create a descending Shepard Tone.
Shepard Tones, named after Roger Shepard, are sounds that create the illusion of infinitely ascending or descending pitch. The effect is created by cross-fading between multiple sounds. For example, as a pitch descends, it fades out and is replaced by fading in a higher pitch.
Here’s an example of synthesizing an ascending Shepard Tone.