Evolution of perturbations in radial velocity δv_r (top three rows), magnitude of the outgoing Elsässer variable δz⁺ (middle three rows), and magnitude of the incoming (towards the Sun) Elsässer variable δz⁻ (bottom three rows). For each quantity, the results for the flux tube expansions, f_max = 3, 5, and 10, are shown in the left, middle, and right columns, respectively. The frequency of the injected pump wave is indicated in the inset of each panel.