(Color online) Synthetic vocalizations produced by numerical integration of Eqs. (1) and (2), considering source-filter independence (left panels) and source-tract interaction (right panels). Sound (top panels) is produced when the system is driven by a simple pressure gesture (bottom panels). The sonograms (middle panels) show that the spectral content of the vocalizations are qualitatively different. Parameters used for numerical integration were (γ, β, c, r, α) = (7000π, 0.01, 1 × 10−6, 0.71, 0.0014) in dimensionless units, τ = 1.43 × 10−4 s.