Shepard Tones

Tones consist of a number of sinusoidal components, an octave apart, with a fixed spectral envelope that goes to zero at low and high frequencies.

Figure 9: Shepard tone spectrum.

When frequencies of the sinusoidal components are raised, we get the sense of an increase in pitch.

Shepard tone:

• sinusoidal components are raised repeatedly (or continuously) such that the pitch seems to rise;
• new low-amplitude components appear at the lowest frequency according to the sound's fixed envelope;
• once 12 semitones is reached the tone is back to the beginning (yet having left the impression it is continually rising) showing the impact of common fate.

The pitch appears to increase endlessly:(play auditory demo: ``circularity in pitch judgement'').