Representation of a typical Γ₁ profile in the ionisation region as a function of the acoustic depth τ (the surface corresponds to τ = 0). The helium abundance in this plot is Y ∼ 0.25. The contributions of the three main ionisation zones have been distinguished, i.e., the hydrogen (H), the first (HeI), and second (HeII) helium ionisation zones. Each of them causes a deviation from the Γ₁ reference value of 5/3.