Linearstability curve (Eq. (16)) corresponding to the Pr76-parameters (solid curve). The dashed lines indicate the different values of the viscosity parameter β (Eq. (7)) that are used for the large-scale integrations of resonantly forced density waves discussed in Sect. 7.4. Viscous overstability is expected to develop for all values of β larger than the minimal value which occursfor λ ~ 260 m. For β ≳ 1.03 only a narrow band of wavelengths is unstable. For β ≳ 1.22 all wavelengths larger than a critical one are unstable, implying instability of the resonantly forced density wave.