Bandwidth of time-varying harmonics of speech For speech signals, do the harmonics have a constant bandwidth above some threshold (arbitrarily, -6 dB  dropoffs)? Or does this bandwidth decrease with the upper harmonics? If this is not known, how can it be measured? (I could imagine one way would be find -6 dB dB points around peaks of a given frame of samples).
If the process that creates a spectrogram includes the use of a windowing function, then the artifacts of that windowing will have a nonzero bandwidth associated with that particular window function (including its length).

If a signal is modulated, then it will have a non-zero bandwidth.  Any tiny variations in the mouth cavity shape or in the amplitude of the glottal closures or in the time period between glottal closures of voiced speech could be a cause of modulation of the harmonics.  These factors seem unlikely to be a fixed constant for all humans at all times.  Which might lead one to doubt a hypothesis of fixed bandwidth.