Characterisation of tremor in normophonic voices

Vocal tremor is a low frequency instability of the voice that causes modulation of its amplitude and fundamental frequency. Among these two, frequency modulation is more relevant for perception and it has been shown to be present both in normophonic and dysphonic voices and to happen in similar frequency bands for both voice types. This paper presents a characterisation of the frequency modulating signal estimated for normophonic voices in terms of both its spectral characteristics and its statistical distribution. By using the discrete Fourier transform for data non-uniformly spaced in time domain, it is shown that the modulating signal may be either low-pass or band-pass (i.e. oscillating), though the low-pass case dominates in the analysed data. As for the values of the modulating signal, their distribution is shown to fairly fit a Gaussian distribution with a standard deviation that significantly depends on the average fundamental frequency.