Probability density distributions showing the results of our automated noise estimation for the sample of synthetic spectra containing: only white noise (upper left panel); white noise and signal (upper right panel); white noise, signal, and negative noise spikes (lower left panel); white noise, weak signal, and negative noise spikes (lower right panel). The abscissa shows the determined root-mean-square noise value σ_rms normalised by the true root-mean-square noise value σ_{rms,  true} that was used to generate the white noise. Hatched areas and vertical dotted lines show the respective interquartile ranges and median value of the respective distributions. The black solid line shows the distribution obtained by using all spectral channels from sample A for the noise calculation. See Appendix B.2 for more details.