Shepard tones, first described by Roger Shepard, are an auditory illusion heard as a continuous increase or decrease in pitch (see this Wikipedia article for more information).
The below tool generates up to four ascending and four descending Shepard tones. Note that the effect may be panic-inducing or otherwise disconcerting to some people. Also make sure that the volume on your device is turned down when experimenting with combinations to avoid overly loud output.
Use the “Ascending” and “Descending” sliders to set the volume for each tone and click “Start” to generate the Shepard tones. The visualization below the controls (shown once playing) depicts volume on the vertical axis and pitch on the horizontal axis.
The audio is created using the excellent Tone.js library; the interface was created with React. The algorithms for calculating frequencies and volumes were adapted from this codepen by Riley Shaw. The full source code is available on GitHub.