Bode diagram (modulus and phase) of the vocal tract H_VT. The variation in the value of the ϕ_i parameters is studied in order to observe the changes in the transfer function. The errors $E_{{\varphi }_i}$ and sensitivities $S_{{\varphi }_i}$ are also presented for variations of ϕ_i between −10% and +10%. The variations in Bl, S_d and β_m (left-hand column) considerably impact the amplitudes and positions of the formants. In contrast, variations in ω_m and τ_m (right-hand column) have negligible effect on H_VT.