Frequency spectrum of waves likely to be excited by the tidal perturbation and of the corresponding tidal regimes. The characteristic frequencies of the system are, from left to right, the drag frequency σ_R, the inertia frequency 2Ω, the surface-gravity waves cut-off frequency , the Brunt–Väisälä frequency N, and the acoustic cutoff frequency σ_s = c_s∕r. Their hierarchy can vary as a function of the physical and dynamical properties of the planet. In the case of the Earth’s ocean, σ_R ≈ 10⁻⁵ s⁻¹ (Webb 1980), 2Ω = 1.46 × 10⁻⁴ s⁻¹, σ_g = 3.1 × 10⁻⁵ s⁻¹, N ≈ 2.2 × 10⁻³ s⁻¹ (Gerkema & Zimmerman 2008), and σ_s = 6.3 × 10⁻³ s⁻¹.