Leahy-normalized power density spectra (PDS) annotating the identified low-frequency features for a) XTE J1550-564 (ν_sub ~ 2.8,ν_f ~ 5.3 and ν₁ ~ 10.9 Hz); b) XTE J1859+226 (ν_f ~ 1.2 and ν₁ ~ 2.4 Hz); c) GRS 1915+105 (ν_f ~ 67,ν₁ ~ 136,ν₂ ~ 205,ν₃ ~ 267 and ν_u ~ 781 mHz) and (d) Cygnus X-2 (ν_f ~ 14.9 and ν₁ ~ 29.1 Hz) with the spectral resolution and (fundamental) QPO cycles f ≡ ν_fT indicated where T ⋍ 1 ks (see the text for notations).