I'm trying to learn more about breathiness. I understand that the cepstral peak is less prominent in the cepstrum of breathy phonation than plain ol' modal phonation -- but I'm not sure why this may be.
It goes back to the properties of the glottal wave. With modal phonation, there is a sharp discontinuity to what is fundamentally a sine wave coming from the glottal airflow function, when the vocal folds slam shut and are blown open. That gives the modal phonatory source a move triangular shape. With breathy voice, there may not be much (or any) of a closure period, so the source wave is closer to a sine wave. A sine wave has just one component, whereas a triangular wave has many components, the most prominent being the fundamental but higher harmonics are also quite prominent. So it really goes back to the spectrum (or cepstrum) of different glottal airflow functions.