Dieter Maurer

Fo-Dependence, Number Alteration, and Non-Systematic Behaviour of the Formants in German Vowels

In acoustic theory, formant pattern differences within one vowel identify are related to differences in the speaker groups or in the types of vocalization. Yet, studies of vowel synthesis indicate that formant patterns can also vary strongly in relation either to pitch or to formant number alterations. Within this study, natural vocalizations of nine German vowels were investigated with regard to different formant patterns representing the same vowel identity, apart from differences in speaker groups and vocalization types. The results show i) FO-dependence of the lower formants < 1.5-2 kHz; ii) occurrence of natural one-formant back and two-formant front vowels; iii) a non-systematic relationship between FO and the formants, and a non-systematic relationship between formant patterns of different formant numbers. The implications for psychophysics, physiology and perception of speech are discussed.

Formant Movement and Formant Number Alteration With Rising Fo in Real Vocalizations of the German Vowels [U:], [O:] and [A:]

Production theory explains vowel sounds by formants in terms of a resonance pattern of the vocal tract. In perception theory normalization must also be undertaken because of different formant values for one vowel category, as found in comparisons of men, women and children, and in studies of sung vowels and of synthesized vowel sounds. Synthesis indicates a dependence of the formants on FO. Therefore, the question arises whether the formant pattern is directly related to FO when studied in real vocalizations. This study presents: a. a method to determine formant movement in real vocalizations for all FO, and b. the results of spectral and LPC analysis of the three German vowels [u:], [o:] and [a:] in real vocalizations. The need for formant movement with altering FO, as well as a change in the number of formants with altering FO, is demonstrated. The implications of these results for normalization in the description of the physics of the vowel sound wave are discussed.

Formant Pattern Ambiguity of Vowel Sounds

The formant frequencies of a particular vowel vary according to the speaker group and to Coarticulatoin. Therefore, overlapping formant patterns of different vowels are commonly related to sex and age differences and to coarticulation, and are considered to concern mainly the F1-F2 pattern of adjacent vowels. However, several studies have reported indications of a correlation between the lower formant frequencies and F0. as well as of the appearance of different formant numbers relevant to vowel identity. As a consequence, the overlap between formant patterns of different vowels might be more substantial than has traditionally been assumed. Within the present study, therefore, the extent to which a given formant pattern can represent different vowels was investigated for natural Swiss German vowels produced monotonously and in isolation by men, women and children at F0 of 85–870 Hz. Similar formant patterns were found for vocalizations of different vowels with both small and large phonetic distances, and within the entire frequency ranges of the formants relevant for phoneme identity. For vowel sounds displaying ambiguous formant patterns, the main spectral characteristics related to differences in their perceptual identity were found to concern F0 and relative formant amplitudes. Results are given in exemplary vowel series, and consequences for the psychophysics of the vowel are discussed.

Fourier spectra and formant patterns of German vowels produced at F0 of 70–850 Hz

Previous studies of vowel synthesis and of natural vowels have indicated: (1) a correlation between the lower formants and F0 for F0>175 Hz; (2) intelligibility and spectral differences of vowels with high F0>500 Hz; (3) severe methodological problems of formant frequency estimation for F0>300 Hz; (4) formant number alterations (appearance of different numbers of relevant formants for one vowel); (5) formant pattern ambiguity (appearance of the same formant pattern for different vowels). The purpose of this study was to replicate these findings. A large sample of the German vowels /i,y,e,o/,e,■,o,u/ produced in isolation as well as in CVC context was investigated. There were 18 674 recordings made of 35 men, 44 women, and 20 children. Isolated vowels were produced at different levels of F0, and with different intensities. A listening test was performed. Fourier spectra, LPC spectra and spectral constancy were analyzed. The results strongly support the five indications mentioned. In particular, for isolate...

The spectral difference of different vowels: Toward a new acoustical concept

When isolated vowels were produced beyond F0 of speech (F0>150 Hz for men, F0>300 Hz for women and children), the related formant patterns were found to deviate substantially from the values of formant statistics, and the formant frequency variations proved to be nonsystematic (for details, see poster Klinkert and Maurer). Moreover, formant frequency estimation for vocalizations with F0>400 Hz is highly problematical, although many vocalizations remain unambiguous in the vowel identity up to F0=700 Hz and even above. The nonsystematic formant variations and the methodological problem of formant frequency estimation hardly allow for a normalization of formant patterns. Dynamic spectral properties do often not appear in isolated vowels. Thus, for the acoustic theory of vowel sounds, the question arises as to whether there is a concept to determine the physical correlates of the sounds apart from formants. A first approach toward such a concept is described for voiced vowels: A hypothesis is presented which ...

Vowel recognition at fundamental frequencies up to 1 kHz reveals point vowels as acoustic landmarks

The phonological function of vowels can be maintained at fundamental frequencies (fo) up to 880 Hz [Friedrichs, Maurer, and Dellwo (2015). J. Acoust. Soc. Am. 138, EL36-EL42]. Here, the influence of talker variability and multiple response options on vowel recognition at high fos is assessed. The stimuli (n = 264) consisted of eight isolated vowels (/i y e ø ε a o u/) produced by three female native German talkers at 11 fos within a range of 220-1046 Hz. In a closed-set identification task, 21 listeners were presented excised 700-ms vowel nuclei with quasi-flat fo contours and resonance trajectories. The results show that listeners can identify the point vowels /i a u/ at fos up to almost 1 kHz, with a significant decrease for the vowels /y ε/ and a drop to chance level for the vowels /e ø o/ toward the upper fos. Auditory excitation patterns reveal highly differentiable representations for /i a u/ that can be used as landmarks for vowel category perception at high fos. These results suggest that theories of vowel perception based on overall spectral shape will provide a fuller account of vowel perception than those based solely on formant frequency patterns.

Formant pattern ambiguity of vowel sounds revisited in synthesis: Changing perceptual vowel quality by only changing fundamental frequency

The influence of varying fundamental frequency on the perception of vowel quality in synthesized vowels was tested in two experiments. In experiment 1, based on investigations of natural Standard German vowel sounds, various model formant patterns F1’ to F3’ were created and, for each single pattern, sounds were synthesised on two or three fundamental frequencies (range 200-600 Hz). In experiment 2, corresponding to open-tube resonance characteristics for men, women and children respectively, sounds were synthesised with formant patterns F1’ to F5’, formant frequencies being odd multiples of 500, 600, or 700 Hz and fundamental frequencies being 1/3, 1/2 or 1/1 of the first formant frequency. Five phonetic expert listeners identified all synthesised sounds in a multiple-choice identification tasks. The results of both experiments revealed that the perceived vowel quality can be changed systematically by varying fundamental frequency only and that the changes can exceed the perceptual boundaries of two neig...

“Flat” vowel spectra revisited in vowel synthesis

Some studies of natural and of synthesized vowel sounds indicate “flat” vowel-related spectral envelopes or envelope parts in terms of vowel-related frequency ranges with harmonics equal in amplitude. The present investigation addresses this question in a vowel synthesis experiment in which sounds related to series of harmonics, multiples of 200 Hz in frequency and equal in amplitude, were created. Thereby, for various frequency ranges, the number of harmonics was increased stepwise from a single lower harmonic to an increasingly broader harmonic series, and, inversely, it was also decreased from a broad series of harmonics to a single higher harmonic. The entire frequency range of investigation was 0.2-4 kHz. Vowel recognition was investigated by means of a listening test in which five phonetic expert listeners were asked to assign the synthesized sounds to Standard German vowel qualities. The results of the experiment reveal that synthesized sounds with frequency bands of series of two or more equal-amp...

Vowel synthesis related to equal-amplitude harmonic series in frequency ranges > 1 kHz combined with single harmonics < 1 kHz, and including variation of fundamental frequency

Front vowels can be synthesized on the basis of series of harmonics equal in amplitude, with frequencies only above 1 kHz. In these cases, spectral energy usually attributed to the first formant frequency is lacking. The present paper reports results of an experiment in which sound synthesis was performed on the basis of harmonic series covering higher frequency ranges above 1 kHz, combined with a single lower harmonic < 1 kHz, all harmonics equal in amplitude. Thereby, two or three sounds were synthesized for which the higher frequency range and the frequency of the lower harmonic is identical, but the frequency distance of the higher harmonics differs resulting in different perceived pitches of the sounds. Vowel recognition of all sounds was investigated by means of a listening test in which five phonetic expert listeners were asked to assign the synthesized sounds to Standard German vowel qualities. The results of the experiment reveal that the perceived vowel quality of such types of sound pairs or so...

How listeners recognise vowel sounds under highpass or lowpass filtering of vowel-specific frequency ranges

The present paper reports findings of two experiments on filtered sounds of the Standard German vowels /i-y-e-o-ɛ-a-o-u/ produced by a female speaker at two fundamental frequencies fo  = 220 Hz and 659 Hz and a male speaker at fo  = 131 Hz and 523 Hz. High-pitched sounds were included in order to account for a possible impact of the fo level on the perception of filtered vowel sounds. In the first experiment, the frequency region of the first formant of the sounds was highpass filtered, and in the second experiment, the frequency region of the second formant of the sounds was lowpass filtered. Vowel recognition of all sounds was investigated in a listening test. Results revealed shifts in the perceived vowel quality which varied across (i) vowel categories, (ii) fo , and (iii) filter cutoff frequencies. Details of the filter parameters and of the perceived vowel quality shifts are given in the paper and implications for the relationship between acoustic cues and vowel recognition are discussed.