Various shapes along with their associated parameters that are used to describe a structural perturbation. (a) Dirac function used in Monteiro et al. (1994) to model the variations in the acoustic potential (in the case of an overshoot) passing from a convective to a radiative region. For this modelling, it is only necessary to specify the acoustic depth τ_d and amplitude A_δ of the perturbation. (b) Triangular shape of the Γ₁ perturbation in the second helium ionisation region as used in Monteiro & Thompson (2005). An additional parameter controls the width (β) of the perturbation. (c) Gaussian shape of the Γ₁ perturbation in the second helium ionisation region as used in Houdek & Gough (2007). and Γ_II describe the variance and the area of the distribution, respectively.