The Power Spectral Density (PSD) and the Autocorrelation.
A periodogram is a sample estimator for the PSD. See here too..
Power spectral density power law PSD more on power laws and 1/f noise Fourier Transform--Exponential Function Wiener–Khinchin theorem http://personal.egr.uri.edu/chelidz/courses/mce567/handouts/psdtheory.pdf