Flat-spectrum speech
Manfred R. Schroeder, Hans Werner Strube · The Journal of the Acoustical Society of America · 1986
Flat-spectrum stimuli, consisting of many equal-amplitude harmonics, produce timbre sensations that can depend strongly on the phase angles of the individual harmonics. For fundamental frequencies in the human pitch range, many realizable timbres have vowel-like perceptual qualities. This observation suggests the possibility of constructing intelligible voiced speech signals that have flat-amplitude spectra. This paper describes a successful experiment of creating several different diphthongs by judicious choice of the phase angles of a flat-spectrum waveform. A possible explanation of the observed vowel timbres lies in the dependence of the short-time amplitude spectra on phase changes.