Duration

This section attempts to relate the effect of the duration manipulation for each utterance to specific details in the manipulation of the utterances. The effect of the duration manipulation is called the manipulation effect and is here defined as the rated accent difference between the stimuli in the O_D stimulus pair (mean effect across all listeners). The specific details of the duration manipulation that were investigated are simply referred to as factors. These factors were in the form of segment type (vowels and consonants), V/C ratios, phonologically long vowels and articulation rate, and will be further explained in section 3.6.1.1.

The extent to which a factor (e.g. vowel durations) was adjusted as a result of the duration manipulation is called the manipulation size. Manipulation size was measured as the percent adjustment made to the particular factor. Adjustments to the articulation rate (also called manipulation size) were measured as the difference in number of phonemes per second between the O and D stimuli. Manipulation size was investigated for a correlation with the manipulation effect for each utterance. For instance, consider an utterance with a large duration manipulation effect (i.e. perceived as having a considerably reduced accent in the D stimulus as compared to the O stimulus). The manipulation size for the articulation rate is large (i.e. a large rate difference between the D and O stimulus). Because it is reasonable to assume that large effects should be due to large changes in the signal, it is plausible that the duration manipulation effect was affected by the large adjustment in articulation rate for this utterance.

For each utterance, and for each factor, the manipulation size and manipulation effect were investigated for correlations. In section 3.6.1.1 the various factors are defined before the results from the correlation analyses are presented in section 3.6.1.2.

3.6.1.1 Factors

The first factor that is defined is the overall durational adjustment of the utterances. This factor was measured as the mean percentage durational adjustment across all the segments in an utterance.

Research on the effect of L2 segment production accuracy (native listeners judged how accurate the segments were produced) on speech intelligibility (Bent, Bradlow & Smith, 2007) has found that vowel errors were more important than consonant errors. Perhaps the duration of vowels is similarly highly important for L2 degree of accent. Durational adjustment of the vowels and consonants were therefore defined as two factors.

In Chapter 1, section 1.4.3, it was explained that Norwegian has a two-way vowel quantity distinction such that there is phonological opposition between long and short vowels.

Moreover, all stimuli in the present experiment were based on the same read sentence (see section 3.3), which contained three stressed, phonologically long vowels. The duration of the three phonologically long vowels was defined as a factor in the present investigation.

Speech rhythm was also defined as a factor. In section 1.4, Chapter 1, it was explained that there are as yet no universally standardized method of measuring speech rhythm, but that phonetic approaches typically measure ratios or intervals between successive units in the time domain. It was therefore decided to investigate speech rhythm through the measurement of V/C ratios. If this measurement reveals significant effects, then more elaborate methods of speech rhythm measurement can be applied. In the present investigation, V/C ratios were measured for the three stressed (and phonologically long) vowels and their following consonants.

The duration manipulation of the N2 utterances (Chapter 2, section 2.2.1) affected not only the internal durational organization of the utterances, but also the total utterance durations.

This is because the sum of each phoneme’s duration equals the total utterance duration.

Articulation rate⁴ was measured as the number of phonemes in an utterance divided by the utterance’s total duration⁵.

The factors are summarized below:

• All segments

4 Rate will be referred to as articulation rate as opposed to speaking rate because the manipulations have not involved the remediation of pauses or similar disfluencies.

5 Note that articulation rate is a different phenomenon from duration across all segments, the factor defined in the first paragraph of the present section. The former measures adjustments in terms of number of phonemes per time unit whereas the latter measures the extent to which the phonemes have been adjusted regarding duration.

• All consonants

• All vowels

• All phonologically long vowels

• V/C ratio

• Articulation rate

To sum up, for each utterance and for each factor the manipulation size was correlated with the manipulation effect. The analyses were performed in order to reveal which details (here called factors) in the duration manipulations that had caused the perceptual effects described in sections 3.5.6 through 3.5.12. The correlations were performed as regression analyses. Note that these regression analyses were multiple (with more than one predictor variable) only when the categories in the factors did not overlap. Only two factors did not overlap, i.e. the factor vowels and the factor consonants. For this reason, vowels and consonants were investigated in one multiple regression analysis whereas the remaining factors were investigated in separate regression analyses.

3.6.1.2 Results

The analyses was carried out in the form of multiple regression analyses to test whether there were correlations between the manipulation size for each of the defined factors (the extent to which the factors have been adjusted) and the manipulation effect (the accent reducing effect of the duration manipulation). Figure 3.14 shows the manipulation size across all segments (vertical bars) related to the manipulation effect (graph) for each utterance. The data were sorted in ascending order according to manipulation size. A trend line was drawn for the manipulation effect.

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Ge2 En3 Fr3 Pe3 En2 Pe2 Chi7 Ru4 Ge3 Ru1 Chi6 Fr2 Ta2 Ta1

Utterances

Figure 3.14: Manipulation size and manipulation effect across all segments for each utterance (n= 14). Data in ascending order according to manipulation size. A trend line was drawn for the manipulation effect.

In the display of the individual utterances’ manipulation size related to the resulting manipulation effect, no clear relationship can be discerned. For instance, the German speaker Ge3’s utterance was duration manipulated to a large extent, but the accent reducing effect of the manipulation was very small. Conversely, the English speaker En3’s utterance was moderately duration manipulated, but the effect on the degree of accent was large. However, the trend line for the manipulation effect indicates a possible correlation. A regression analysis with duration across all segments as the predictor variable (Table 1, Appendix C) shows that there was no correlation with manipulation effect in terms of accent reduction.

This could be interpreted to mean that the degree of foreign accent did not diminish linearly as a function of overall adjustment to the segment durations.

A multiple regression analysis was carried out with the consonants and the vowels as the two predictor variables in order to investigate the impacts of durational adjustments within each of these two segment groups. A significant correlation was found between manipulation size for the consonants and the manipulation effect in the form of accent reduction (Beta= 0.655; p<

0.05), but no effect was found for vowels. In other words, only the consonant durations and not the vowel durations affected the degree of foreign accent. Moreover, the correlation was such that when an N2 utterance’s consonant durations were extensively adjusted, the effect on the degree of foreign accent was larger than when the utterance’s consonant durations were less adjusted. The reason why consonant durations affected the degree of foreign accent significantly whereas vowel duration did not may be because consonants in general are less

compressible than vowels. Listeners may therefore be more sensitive to deviations in consonant durations than in vowel durations.

Three multiple regression analyses were performed with phonologically long vowels, V/C ratio and articulation rate factors as predictor variables (Table 1, Appendix C), but none of these factors were found to correlate with the manipulation effect.

We turn now to the impact of adjustments to the articulation rate. As explained above, the duration manipulation automatically changes the utterance duration. If, for instance, most of the phoneme durations in an utterance are shortened, then the utterance duration is automatically shortened. The same number of phonemes is then uttered in a shorter period of time, which in turn means that the articulation rate becomes faster. Figure 3.15 shows the relation between changes in articulation rate (called “manipulation” in the figure although the adjustment of the articulation rate was merely an automatic side effect from the duration manipulation) shown in the vertical bars and the manipulation effect shown in the graph. A trend line was drawn for the manipulation effect.

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Ge2 En2 Ru1 Fr2 Ge3 Fr3 Ru4 Pe2 En3 Pe3 Chi6 Ta2 Chi7 Ta1 Utterances

Figure 3.15: Manipulation size of articulation rate and manipulation effect for each utterance (n= 14). Data in ascending order according to manipulation size. A trend line was drawn for the manipulation effect.

The visual impression of Figure 3.15 strongly indicates a relation between the articulation rate and the manipulation effect in terms of accent reduction. The articulation rate did in fact significantly correlate with the manipulation effect (Beta= 0.842; p < 0.001). Almost all of the original N2 utterances were produced at a slower rate than the corresponding N1 utterances.

The effect was therefore such that when the N2 articulation rate accelerated, the degree of foreign accent diminished.