The intelligibility experiment

5.3.1 Summary

This experiment was based on speech material produced by the same 14 speakers as in the previous experiment. Three utterances from each speaker were included. Each of the seven L1s was represented by 6 utterances. Thus, a total of 42 different sentences were used in the intelligibility experiment. Each N2 utterance was mixed with noise in order to avoid ceiling

10 The measure of rate in this thesis is called articulation rate rather than speaking rate because it did not include pauses.

effects. The listeners’ task was to write down the words they perceived of each utterance, and intelligibility was measured as percentage word-identification per sentence. The stimuli in this experiment were grouped in different stimulus sets, one for each type of stimulus. The effect of intelligibility was measured as the difference in word-identification score across two stimulus sets, for instance across the duration manipulated and the intonation manipulated stimuli.

The original stimuli were found to be unsuitable for this experiment as the original stimuli were more intelligible than any of the manipulated stimuli. This was due to side effects from the manipulations which will be discussed in section 5.5. For that reason, two close-original stimulus sets were generated to replace the original stimuli: The close-original duration stimuli (COD) were generated for comparison with the duration manipulated stimuli and the close-original intonation stimuli (COI) were generated for comparison with the intonation manipulated stimuli.

When intelligibility was measured across the COI and I stimulus sets, the results showed that the intonation manipulation significantly enhanced N2 intelligibility for the English and German L1 groups. The comparison of the COD and D stimulus sets showed that the duration manipulation significantly enhanced the N2 intelligibility for the French, Tamil and Persian L1 groups. Unexpectedly, the I/D, ID/D and ID/I comparisons showed results that were not entirely compatible with the results from the COI/I and COD/D comparisons. It was suggested that this incompatibility was due partly to various side effects from the manipulations. These manipulation problems will be discussed in section 5.5. Because of these problems, it was decided to discard the unreliable results from the I/D, ID/D and ID/I comparisons, and only use the results from the more reliable COD/D and COI/I comparisons (see e.g. 4.4.3, Chapter 4). The magnitude of the intelligibility enhancing effect of the manipulations differed according to the particular L1 group.

Subsequent analyses were carried out to identify those aspects of the duration manipulation that had caused the intelligibility enhancement. Durational adjustment was analysed in a) all segments, b) all consonants, c) all vowels, d) all phonologically long vowels, e) V/C ratio, f) articulation rate and g) pauses. Similar analyses were carried out at the end of Chapter 3 investigating degree of foreign accent for the same durational aspects except pauses. Pauses were added as a factor for the intelligibility analyses because pauses had been shortened in

some of the utterances used in the intelligibility experiment in order to maintain naturalness in the duration manipulated stimuli. Surprisingly, no effects were found for any of these durational aspects. A possible reason why articulation rate was found to significantly affect degree of foreign accent, but not intelligibility, is that a listener may need increased coarticulation effects in order to perceived faster rates, especially when the speech is foreign accented and thus in general less redundant.

Analyses were also carried out to identify those intonational aspects that caused the intelligibility enhancement. F0 slope and F0 direction in three content words per sentence were analysed. The analyses were undertaken a) across the 6 syllables (2 syllables per word), b) in each of the 6 syllables, and c) between the onsets of each stressed syllable. The results of these analyses showed that the intelligibility enhancement had been due to F0 slope changes in the second syllable of the first word and in the second syllable of the third word. These syllables were unstressed. This result may therefore indicate that N2 speakers’ deviant F0 slopes in unstressed syllables can interfere with native listeners’ identifications of N2 words.

The manipulation analyses in the intelligibility experiment had been carried out with data pooled across the two speakers representing the same L1, called a speaker pair, because it was assumed that the two speakers would be similar in the sense that their N2 speech would gain most from the same manipulation. At the end of Chapter 4, investigations were carried out in order to assess the similarity across the speakers in each speaker pair. Similarity was investigated in three ways: as the degree of N1-N2 production deviation, as the magnitude of the manipulation effect, and as the manipulation that most affected the intelligibility. The results from these similarity assessments showed that there was some variability across the two speakers in a speaker pair regarding the N1-N2 production deviation and the magnitude of the manipulation effects. In general, there was more inter-speaker consistency in the speaker pairs regarding N1-N2 production deviation than regarding magnitude of effects.

There did not seem to be any systematic relationship between N1-N2 production deviation and magnitude of manipulation effects. The similarity analyses also showed that not all speaker pairs consisted of speakers who had gained most from the same manipulation. Table 5.2 shows the manipulation that most affected intelligibility for each L1 group, and the two speaker pairs in which the individual speakers gained most from different manipulations (marked with *).

Table 5.2: The manipulation that most affected the intelligibility for each L1 group as measured with data pooled across the two speakers from each speaker pair. The L1 groups in which the speakers’ N2 speech gained most from different manipulations are marked with asterisks.

L1 Most important

manipulation

French Duration German Intonation English Intonation Tamil Duration

Chinese Equally important

Persian * Duration

Russian * Equally important

The table shows that when the data was pooled across the two Russian speakers, the manipulations affected intelligibility equally for the Russian L1 group. An asterisk shows that the individual speakers in the Russian L1 group had gained most from different manipulations. This means that the equal effect of the manipulations for the Russian L1 group was caused by opposite effects between the two individual speakers. The table further shows that when the data was pooled across the two Persian speakers, duration was found to affect intelligibility more than intonation. An asterisk shows that the two Persian speakers actually gained from different manipulations. This means that the superior effect of duration for the Persian L1 group was due to a superior D effect for only one speaker. The other Persian speaker instead had a superior I effect. The fact that there were opposite effects between the two speakers in the Russian speaker pair and between the two speakers in the Persian speaker pair, makes the results for the Russian and Persian L1 groups inconclusive.

5.3.2 Discussion

In this section, the results from the intelligibility experiment will be related to the literature presented in the introductory chapter (section 1.2.3, Chapter 1) that is judged as relevant.

Unfortunately, only two of those investigations actually compared the effects of intonational and durational aspects on intelligibility. These two investigations were Almberg & Husby (2000), who investigated Russian-accented Norwegian, and Bannert (1995), who investigated Punjabi- and Persian-accented Swedish. Both of these investigations used listener ratings of perceived comprehensibility. It is difficult to directly compare these studies with the present investigation because the literature has shown that such ratings can yield different results than intelligibility as measured through transcriptions (e.g. Matsuura, Chiba & Fujieda, 1999;

Derwing & Munro, 1997). Almberg & Husby (2000) investigated Russian-accented N2, and

thus shares the target language and one L1 with the present investigation. Their result showed that durational aspects affected the perceived comprehensibility more than intonational aspects. They used only one speaker, as opposed to the two speakers in the present investigation. In this investigation, it was found that when the data were pooled across the two Russian speakers the two manipulations affected intelligibility equally, but further analyses showed that the two Russian speakers had in fact gained most from different manipulations.

For one of the Russian speakers, duration was thus more important than intonation in line with the result from Almberg & Husby (2000). In his study, Bannert (1995) found that the Punjabi- and Persian-accented Swedish was more affected by intonational aspects than by durational aspects. In the present investigation, one of the Persian speakers had in fact gained most from the intonation manipulation while the other Persian speaker had gained most from the duration manipulation. Because of the opposite effects for the individual speakers in the Persian speaker pair and in the Russian speaker pair, it is difficult to make meaningful comparisons with the literature regarding these particular L1 groups. The remaining investigations that were presented in the literature review of the introductory chapter did not compare durational and intonation aspects. Instead, these studies mainly investigated durational aspects, and showed significant effects of such aspects on intelligibility. Across these studies, the perceptual effect of speaking rate was frequently studied (Anderson-Hsieh

& Koehler, 1988; Derwing & Munro, 1997; Munro & Derwing, 1998; Munro & Derwing, 2001). One of these investigations, Anderson-Hsieh & Koehler (1988), showed that faster speaking rates had adverse effects on comprehensibility as measured through questions on text content. They used naturally varied speaking rate as opposed to the digitally manipulated speaking rate in the present study. Because it seems likely that faster rates can increase the amount of error and pronunciation inaccuracy in L2 speech, it is unclear whether that investigation actually measured the effects of speaking rate. The other investigations of speaking rate showed that a moderately accelerated speaking rate was beneficial in terms of perceived comprehensibility (Derwing & Munro, 1997; Derwing & Munro, 1998: Derwing &

Munro, 2001) but that it had no effect on intelligibility as measured through word-identification scores (Derwing & Munro, 1997). The present investigation also found that there was no effect of speaking rate on intelligibility as measured through word-identification scores, and this result is therefore in accordance with the results from Derwing & Munro (1997).