Past exam of the physics A course of the University of Cambridge / 2012 / Paper 2 / Section A / 3

Reading the time axis, the rapid oscillation has period about $2\mathrm{s}$, while successive envelope maxima are about $10\mathrm{s}$ apart. The envelope stays nonzero and has a slowly sinusoidal modulation about a positive mean. The corresponding amplitude-modulation model is $y=[A+B\cos (2\pi 0.1t)]\cos (2\pi 0.5t)$, with $A>|B|$. Product-to-sum givesThus the intended positive-frequency components are $0.4$, $0.5$ and $0.6\mathrm{Hz}$. The envelope frequency $0.1\mathrm{Hz}$ is not itself a separate additive displacement component. These values use the illustrated sinusoidal-modulation interpretation; a finite qualitative trace alone does not uniquely specify every possible spectral component.