Evolution of the tube wave amplitude with the distance along the plume for different values of the effective viscosity (thick lines). The dotted, solid, dashed, and dash-dotted lines correspond to the viscosity of 4.0 × 10¹³,2.4 × 10¹⁴,7.0 × 10¹⁴, and 2.0 × 10¹⁵ cm² s^-1, respectively. Here V_A = 130 km s^-1, c₀ = 175 km s^-1, and the wave period is 600 s. The thin lines correspond to plane sound waves.