Amplitude modulation
The amplitude of an oscillation cos(ω_b t) can have a variable value.
This variable value A cos(ω_a* t) "modulate" the first oscillation:
y = A cos(ω_a* t) [ cos(ω_b t) ]
An oscillation can be considered as the real part of a complex number that "rotate" in Gauss plane ( cos(ω_a t) + ί sin(ω_a t) )
So the modulated oscillation can be considered as a product between the real parts of two rotating complex numbers.
You can animate the point T and change the values of the maximum amplitute and of the angular velocity of the oscillations.