Gravitational-wave energy spectrum, dE/dν, as a function of frequency. Top: energy spectrum for different non-spinning equal mass BBHs according to the legend. The more massive the BBH systems, the more energetic the GW signals and the smaller the merger frequencies. More precisely, the GW energy spectrum scales as dE/dν ∝ ν^−1/3ℳ^5/3 for ν < ν_merger and dE/dν ∝ ν^2/3ℳ^5/3 for ν < ν_ringdown (cf. Eq. (B.1)), while ν_merger ∝ M⁻¹ where M = m₁ + m₂ is the total mass (cf. Eq. (B.4)). Bottom: energy spectrum for a BBH system composed of two BHs of mass 10 M_⊙ and different effective spins according to the legend. Larger χ values lead to more energetic GW signals as dE/dν ∝ ν^−1/3(1 + 𝒪χ)) for ν < ν_merger and dE/dν ∝ ν^2/3(1 + 𝒪χ)) for ν < ν_ringdown (cf. Eq. (B.1)).