The Shepard-tone contains anywhere between 8 and 12 partials. Each
partial is spanned by an octave. The amplitudes of the partials has
the following structure:
The structure of a tone's partials (its spectrum) is shown here
on a log2-frequency axis, with amplitude shown along the y-axis (using
some arbitrary, though relative scale) and frequency along the x-axis..
Each solid vertical line represents an octave component. In the following,
two Shepard tones, a semitone apart, are depicted.
The dotted vertical line represents a shepard tone that is slightly higher
than that represented by the solid line. The upward-pointing arrow
indicates the "fundamental" of the second tone. As can be observed,
the lower partials have a greater amplitude than their corresponding partials
in the first tone, while the higher partials have a lesser amplitude than
their corresponding partials in the first tone. Meanwhile, those
partials in the center of the frequency range have pretty much the same
amplitude in both sets of tones.