Measuring the attenuation with the temporal code. Top: Coherent wave packet at x₁ = 126 Mm (blue) and x₂ = 144 Mm (red). Bottom: Natural log of the spectrum of the coherent wave packet at different frequencies. We fit its slope between the vertical dotted lines, corresponding to [x₁, x₂]. The vertical dashed lines delimit the location of the perturbation in time (for a wave packet propagating at c₀) and in space. In the figure, the Fourier components have been normalized so that they have the same amplitude before entering the perturbation.