Given an FFT chain this calculates the Spectral Flatness measure, defined as a power spectrum's geometric mean divided by its arithmetic mean. This gives a measure which ranges from approx 0 for a pure sinusoid, to approx 1 for white noise.
The measure is calculated linearly. For some applications you may wish to convert the value to a decibel scale - an example of such conversion is shown below.
an FFT chain.