Affective state and voice: the specific properties of overtone distributions

Motivated by psychiatric interests and as part of our investigations into the basic properties of human speech, we carried out a normative study with 192 healthy subjects--stratified according to sex, age and education--in order to derive reference values of the general population and to learn to distinguish between normal fluctuations and significant changes over time. In the present investigation, our interest focused on the individual sound characteristics of speakers ("timbre") rather than on speech behavior. Accordingly, we determined the optimum parameter setting for a problem-specific, reliable estimation of time-dependent spectra. An interval of one second length was found to be optimum for reproducibly assessing formants and corresponding bandwidths for more than 95% of the cases. Based on these findings, we adapted the concept of "spectral patterns" to speech analysis. It turned out that spectral voice patterns are stable over time and measure the fine graduations of mutual differences between human voices. A highly reliable computerized recognition of persons was possible by means of these quantities, on the basis of 16-32 s time series: 93% of persons could be uniquely recognized after a 14-day interval. Hence, we succeeded in developing specific means for modelling intra-individual changes of voice timbres over time. This is of particular interest for investigations of the speech characteristics of affectively disturbed patients, since the tonal expressiveness of human voices, or the lack thereof, essentially depends on the actual distribution of overtones and the corresponding variabilities.