Relative amplitude in discrete frequency bands

The two places of articulation, labial and alveolar, were clearly distinguishable by their relative amplitude in discrete frequency bands within the respective obstruent groups, word-initially and -medially. Very good results were found in the higher formant frequency regions to distinguish place of articulation in stops, fricatives and affricates, as expected due to former investigations (cf. Jongman et al., 2000b).

The full-factorial ANOVA (Obstruent x Vowel x Subject x Task) revealed a main effect for the variable Obstruent for all of the frequency bands in wordinitial and -medial positions in their respective groups (cf. Table IX and X). The results of the post hoc tests for the word-initial obstruents are presented in Table IX:

Table IX: ANOVA results testing the variable obstruent in word-initial position per frequency band. The three columns on the right show the results of place distinction by the post hoc tests. “n.s.” is used as abbreviation for “not significant”.

Frequency ANOVA Post hoc tests

Variable Obstruent [pf] vs. [ts] [f] vs. [z] [p] vs. [t]

Post hoc tests indicated that place of articulation can be distinguished within affricates and stops in the higher frequency bands between 4-5 kHz up to 7-8 kHz. Fricatives show different results in word-initial position; the best results are achieved in the lower frequency regions between 0-1 kHz up to 3-4 kHz. The difference between fricatives on the one hand, and affricates and stops on the other, is due to the voicing feature of [z], contrasting with the voiceless [f]. Nevertheless, place distinction for fricatives is also possible in the frequency bands between 5-6 kHz and 6-7 kHz. In the lower frequency regions between 0-1 kHz up to 3-4 kHz, the relative amplitude values are positive, indicating that the amount of energy is higher at the vowel center, whereas in the center of the frication only little energy is measurable. In the higher frequency regions, alveolar phonemes have greater relative amplitudes compared to the labial ones. This applies to nearly all frequency bands. The actual results are illustrated in Figure 11.

Relative Am plitude - w ord-initially - all frequency bands

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Figure 11: Mean relative amplitude as a function of place of articulation across all frequency bands (0-1 kHz up to 7-8 kHz), word-initially.

Word-medially, the ANOVA calculated for the relative amplitude of discrete frequency bands revealed a main effect concerning the variable Obstruent. Post hoc

tests exposed a clear pattern. As in word-initial position, the higher frequency regions (4-5 kHz up to 7-8 kHz) showed the ability to distinguish place of articulation in all obstruent groups. Again, the alveolar obstruents have greater relative amplitude values compared to labial obstruents (cf. Figure 12 and Table X).

Table X: ANOVA results testing the variable obstruent in word-medial position per frequency band. The three columns at the right of the table show the results of place distinction by the post hoc tests.

“n.s.” is used as abbreviation for “not significant”.

Frequency ANOVA Post hoc tests

Variable [pf] vs. [ts] [f] vs. [s] [p] vs. [t]

Re lative Am plitude - w ord-m e dially - all frequency bands

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Figure 12: Mean relative amplitude as a function of place of articulation across all frequency bands (0-1 kHz up to 7-8 kHz), word-medially.

The results of the relative amplitude calculation for the obstruents wordinitially and -medially revealed the same tendencies. In general, obstruents are preferably distinguished by their relative amplitude in the frequency regions between 4-5 kHz up

to 7-8 kHz. For example, in the frequency band between 5-6 kHz, the relative amplitude is -10.5 dB for [pf] and -23.7 dB for [ts]. Place of articulation in stops is also significantly different in the frequency bands between 2-3 kHz and 3-4 kHz, in both word-initial and -medial positions. Affricates do not show this pattern, they revealed significant place differences in the frequency bands between 1-2 kHz and 2-3 kHz in word-initial and -medial positions. The results of fricatives correspond to those of affricates in word-medial position. A significant place difference is observed in the frequency bands between 1-2 kHz and 2-3 kHz. In word-initial position, all lower frequency regions reveal the possibility of significant place distinction for fricatives.

Every significant place contrast showed the same pattern, alveolar obstruents had greater relative amplitudes compared to labial obstruents (apart from the frequency band between 0-1 kHz).

4.5.3.1 Summary

Relative amplitude serves as the most stable and reliable function to distinguish place of articulation within affricates, fricatives and stops. Highly significant results were achieved word-initially and word-medially, demonstrating the ability of the relative amplitude metric to distinguish place of articulation in the higher frequency bands in the three respective obstruent groups. All frequency bands between 4-5 kHz up to 7-8 kHz were appropriate. It was found that the relative amplitude is smaller for labials compared to alveolars, regardless of the frequency band.

The results of the relative amplitude measurements support the findings of Jongman et al. (2000b) with respect to its ability of being an indicator for place distinction in fricatives. The validity of these results was even extended to stops and affricates.

The relative amplitude calculation is a dynamic approach in contrast to the static approach of the logarithmic distance measure. Two events that are spread across two phonemes (the obstruent and the vowel) are related to each other. Unlike the formant frequency analysis, no influence of the sequence of these phonemes was found. The results were the same no matter whether a CV initially) or VC sequence (word-medially) was analyzed. Therefore, the results gained by relative amplitude

calculation in discrete frequency bands were even more stable in distinguishing place of articulation than those of the logarithmic distance measure.