Formant synthesis

Formant synthesis involves an electrical buzz generator (for voiced sounds) and a hiss generator (for unvoiced sounds) and three or more electrical resonator circuits (to simulate the resonances of the vocal tract). The resonators could be connected in series or in parallel. In a serial synthesiser, the formant amplitudes adjust to the natural slope in the spectrum typical of the human vocal tract, which makes the serial synthesiser suitable for synthesising vowels. Consonants and nasalised vowels, on the other hand, are easier to synthesise with a parallel synthesiser.

Link
Utrecht Lexicon of Linguistics