In Shepard tones, one hears a scale that sounds like it increases in pitch continually. (The illusion can also be designed so that one hears a scale that sounds like it is decreasing in pitch.) Each sound in the scale seems a bit higher than the one preceding it, but the listener eventually realizes that the tone one is hearing is back at the pitch at which the scale started, although all one hears is increasing pitches. That is, the pitches sound like they are getting continually higher, but in fact, the sound frequencies return back to lower frequencies without our noticing it in any change from one note to the next. The illusion can also be run in reverse, with the perception being that the notes get lower and lower, when actually they do not.
The Shepard tones have been used even in movie soundtracks. Below is a video talking abou the use of the Shepard tones in Dunkirk to increase the tension in the movei.
In this illustration, you can play the Shepard tones and compare them to just the fundamental of the tone. Notice how the Shepard tones are created.
To see the illustration in full screen, which is recommended, press the Full Screen button, which appears at the top of the page.
Below is a list of the ways that you can alter the illustration. The settings include the following:
Play/Stop: start or stop the tones. They will play until you press stop.
Keyboard: click or touch a key to play the tone associated with that key.
Plot Frequency: You can have the frequency graph plotted Linearly where each Hertz step is the same size or Logarithmically where the ratio between given frequencies stays the same size.
Tone Type: play Shepard Tones or just the Fundamental of the Shepard Tone.
Scale to do: When the tones are playing chose one of these scales to play.
Pressing this button restores the settings to their default values.