Numerical errors in the FDTD algorithm as a function of frequency for a model as in Fig. 3, computed at a propagation angle of 0°. (a) Variation of the normalized sound speed ratio . The solid line are the values computed for a model with no attenuation. The dots indicate values computed for η=ζ= 1.8 × 10⁻⁵ kg m⁻¹ s⁻¹ at a density ρ_o= 1 × 10⁻⁷ kg m⁻³. (b) Variation in attenuation ratios for a model with attenuation as in (a).