The spectra of the stimuli for the vowels /a/ (upper row) and /u/ (lower row), representing how articulation modifies stimulus structure. The stimuli were created using three different types of phonation: the natural periodic glottal pulseform (sounds /a/per and /u/per in the left column), the aperiodic noise sequence (/a/aper and /u/aper, center column), and tonal excitation (/a/tone and /u/tone, right column). The vowels excited by the natural periodic glottal pulseform are characterized by a harmonic comb structure, that is, distribution of sound energy at multiple integers of the fundamental frequency. This regular spectral fine structure is absent from the spectra of the vowels produced by the aperiodic excitation. The spectra of the sounds generated by tonal excitation are further impoverished, comprising only two spectral components. The spectral characteristics of the stimuli of all three excitation types are affected by the formant structure of the underlying vowel. Due to this, the vowel /a/ comprises more high frequencies than the vowel /u/.
Tiitinen et al. BMC Neuroscience 2005 6:62 doi:10.1186/1471-2202-6-62