In the following image, we revisit our sequence of rising major sevenths. The red-colored
vertical lines indicate the spectral components for the first tone, the green vertical
lines are the spectral components for the second tone, and the blue vertical lines
correspond to the spectral components of the third tone. You will note that, visually,
the sequence of tones suggests downward movement by semitones rather than upward
movement by sevenths.
This coraborates with our auditory experience. Here you will hear 4 major seventh Shepard tones.
Interesting. In fact this sequence sounds much more like descending semitones.