Product ⟨ τ^oiτ^ac ⟩ as a function of days of data averaged over, for f-mode-filtered HMI observations at − 40° and 40° latitude as well as τ^ac computed for the noise model. The filled areas denote the 3σ estimates. All data have been averaged over space (about 180 × 180 Mm²), four angles β and three 8 h datasets per day.