Comparison between glitch-based solutions for the acoustic radius of the base of the convection zone in Felix (solid vertical red lines and shaded pink area for the error bars), and the dc/ dτ profile (solid black curve). The true solution, the He II ionisation zone, and the peak in the Γ₁ profile, located between the He I and He II ionisation zones, are indicated by the vertical dashed blue lines at 3830 s, 5816 s, and 6041 s, respectively. The dotted and the dot-dashed green curves show dc/ dτ as a function of acoustic depth. Given the uncertainties on the determination of the total acoustic radius, the dotted curve uses τ_Tot. whereas the dot-dashed curve uses 1/2Δν (see Table 6). The upper x-axis also uses 1/2Δν as the total acoustic radius.