SOURCE-FILTER SEPARATION VIA THE CEPSTRUM
An example of source-filter separation using voiced speech:

(a) Windowed Signal (b) Log Spectrum (c) Filtered Cepstrum (n < N) (d) Smoothed Log Spectrum (e) Excitation Signal (f) Log Spectrum (high freq.)

An example of source-filter separation using unvoiced speech:

(a) Windowed Signal (b) Log Spectrum (c) Filtered Cepstrum (n < N) (d) Smoothed Log Spectrum

The reason this works is simple: the fundamental frequency for the speaker produces a peak in the cepstrum sequence that is far removed (n > N) from the influence of the vocal tract (n < N). You can also demonstrate this using an autocorrelation function. What happens for an extremely high-pitched female or child?