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 theshort‐timeamplitude spectra on phase changes.