Cross-correlation function F₄(x,y,τ) (solid line) between the cross-covariance C₄(x,y,t) at (x,y) = (60,0) Mm and the corresponding quiet-Sun cross-covariance . The envelope (dashed line) is determined by demodulation. The travel-time shift, Δτ, and the wave packet amplitude, A, are indicated by arrows. In practice, we fit the peak nearest to τ = 0 with a parabola to measure the time shift. In this particular example, the time shift and reduced amplitude are Δτ =  −57 s and A = 0.44 with respect to the quiet Sun.