The intellectual and empirical reasoning presented in this treatise, however, gives rise to scepticism with respect to this understanding of the sound of the vowel. The reflections and materials presented provide reason to argue that, up to now, a comprehensible theory of the acoustics of the voice and of voiced speech sounds is lacking, and consequently, no satisfying understanding of vowels as an achievement and particular formal accomplishment of the voice exists. Thus, the question of the acoustics of the vowel – and with it the question of the acoustics of the voice itself – proves to be an unresolved fundamental problem.
9 Ambiguous Correspondence between Vowels and Patterns of Relative Spectral Energy Maxima or Formant Patterns or Complete Spectral Envelopes
All these reflections and observations come down to the conjecture that two sounds of two different vowels, produced at two different fundamental frequencies, can exhibit quasi-identical relative spectral energy maxima and quasi-identical formant patterns within their vowel-specific frequency range. Indeed, many patterns of spectral envelope peaks and formants prove to be ambiguous empirically. As such, they often physically represent two (or even several) different vowels.
In many cases the patterns of relative spectral energy maxima do not prove to be vowel specific, but ambiguous. The same holds true for formant patterns.
This observation becomes particularly evident when comparing vowel sounds for their entire range of fundamental frequencies for which vowels are recognisable and distinguishable and when evaluating the correspondences between relative spectral energy maxima and minima also in a direct comparison of harmonic spectra, aside from determining spectral envelopes and formant frequencies.
In certain cases, this ambiguity also concerns the entire course of the spectral envelope.
Spectral envelopes can be equally ambiguous.
For all German vowels discussed here, there are cases of sounds with ambiguous patterns of relative spectral energy maxima or with ambiguous formant patterns within the respective vowel-specific frequency ranges. ← 64 | 65 →
To what extent this is also true for complete spectral envelopes is left open for discussion.
If vowel sounds are compared for their entire range of fundamental frequencies for which vowels are recognisable and distinguishable and if a possible correspondence of relative spectral energy maxima and minima is evaluated in a direct comparison of harmonic spectra, then, the above ambiguity can be observed not only for sounds of neighbouring vowel pairs but also for other sound pairs and sometimes for sounds of more than two different vowels. This holds particularly true when comparing sounds produced by all of the three age- and gender-related speaker groups.
The ambiguity described is not limited to only a part of the vowels or to neighbouring vowel pairs, and it can affect more than two vowels simultaneously.
The question of whether there are sounds of certain vowels that exhibit strict vowel-specific patterns of relative spectral energy maxima and strict vowel-specific formant patterns, which cannot be found in sounds of any other vowel—for example for sounds of /a /—is left open for further discussion.
The ambiguity discussed above in relation to natural vowel sounds can be evaluated and replicated using resynthesis.
The same also applies to synthesis involving formant patterns or harmonic spectra not derived directly from natural vowel sounds. ← 65 | 66 →