In the figure (left):
a) *f*_{modulation}
= 1*Hz* is
represented by the number of twists around the **x**-axis,

b) the degree of the modulation is represented by the radius of the twist, and

c) *f*_{beat}
= 2*f*_{modulation} = 2 is
represented by the number of amplitude fluctuations formed when the 3-dimensional twisted
signal is projected on two dimensions (projection angle represents phase relationship of
the original sines).

This collapse from 3 to 2 dimensions represents the fact that, while

f_{mod}
represents the oscillation of *A*_{mod}
,

f_{beat} represents the oscillation of (*A*_{mod})^{2}.