Travel-time perturbations versus sound-speed perturbation amplitudes A, for p₁ wave packets. The results from two SLiM numerical experiments are shown. In the first procedure, the red diamonds show the travel-time perturbations resulting from sound-speed perturbations to a background model that includes the reference sunspot, ⟨ Δ_QSτ(RS + δc) ⟩  −  ⟨ Δ_QSτ(RS) ⟩. In the second procedure, the black diamonds show the travel-time perturbations resulting from the same sound-speed perturbations to a quiet-Sun background model, ⟨ Δ_QSτ(QS + δc) ⟩. In both experiments, the travel times (diamonds) are measured using sliding quiet-Sun waveforms. For the first experiment, the red asterisks show the results using a different measurement procedure, ⟨ Δ_RSτ(RS + δc) ⟩. The solid lines show linear fits to the three smallest perturbations with A =  −1%, 0, and 1%. The inset shows a zoom around the origin of the plot. The dashed curves are parabolic fits. The p₁ travel-time noise level for 7 days of observation is 3.5 s as indicated by the arrow.