If a semiconductor contains two generation–recombination centres, A and B, the noise spectrum either is the simple addition of the two characteristic A and B spectra, or a Lorentzian spectrum with a relaxation time t given by t-1=tA-1+tB-1. The latter situation is called mixing. We can derive the mathematical criterion that decides whether addition or mixing occurs: Addition if na*,nb*. Mixing if na*,nb*. The number n is the number of free electrons in the conduction band. The number a* either is a, the number of occupied A states, or the number of empty states, (A–a), depending on which is smaller. A qualitative physical interpretation is given of the precise mathematical condition.