The gap among different measures of clear speech perception

speech enhancements were manifested differently depending on the type of perception tasks that listeners engaged in. Particularly, despite the clear speech intelligibility benefit shown for native English talkers’ and higher-proficiency talkers’ speech (Experiment 2A),

native English listeners did not perceive these talkers’ clear-style sentences to be easier to understand than their plain-style sentences (Experiment 2B). Listeners even perceived higher-proficiency talkers' plain-style sentences to be easier to understand than clear-style sentences in the quiet listening condition. However, listeners were sensitive to the increased effort in the clear-style sentences as compared to the plain-style sentences for all the talker groups’ speech. Though it may be puzzling to observe such discrepancies among different measures of clear speech perception, it is possible that acoustic features of clear speech enhancements influenced listeners’ perception differently for different tasks. For example, some acoustic features, such as decreased speaking rate, may have contributed to

improving intelligibility of the speech, but not perceived degree of comprehensibility. Though a decrease in speaking rate has been one of the most prominent features of clear speech enhancements in previous studies (e.g., Bradlow et al., 2003; Picheny et al., 1986) as well as in the current study (see Chapter 2), previous results have also demonstrated that slower speaking rates are associated with poorer comprehensibility ratings for perception of non-native speech (Munro & Derwing, 1998). Specifically, Munro and Derwing (1998, 2001) claimed that very slow speech may place extra processing demand for listeners by requiring them to retain information in working memory for a longer period of time. Further, this slower speaking rate may also allow listeners to notice more phonological errors. Smiljanić and Bradlow (2005) also suggested that there may be a limit to the perceptual benefit influenced by clear speech strategies; modifying acoustic-phonetic features beyond a certain threshold may result in speech sounding unnatural and even less intelligible. The current results contribute to these lines of research by demonstrating that clear speech enhancements that are effective at improving one measure of listeners’

evaluation may not improve other perceptual measures. That is, while larger degrees of acoustic modifications (e.g., slowing down the speech, increasing the pitch range) may contribute to the greater talker effort perceived by listeners, they may not impact other types of perception in the same way. Particularly, for improving intelligibility and perceived degree of comprehensibility, there may be an optimal amount for acoustic- phonetic modification, and modifications exceeding the optimal amount may not result in improvement in these perceptual measures.

The gap between the two subjective measures of clear speech perception observed in the current study (i.e., perceived degree of talker effort vs. comprehensibility) may originate from the particular clear speech elicitation method used here. That is, reading the same sentences once in a plain-speaking style and once in a clear-speaking style, without actual listeners present in the room, may have induced some acoustic-phonetic

modifications that sound unnatural to the listeners in the perception experiment. This may have contributed to the clear speech perceived to be not easier to understand than plain speech, but perceived to have been produced with increased effort compared to plain speech. Previous studies have demonstrated that acoustic-phonetic characteristics of clear speech modifications vary depending on how the speech is elicited. For example, clear speech elicited with read speech, using the instruction to speak clearly as if talking to someone who is hearing impaired, involved more extreme changes in some acoustic- phonetic characteristics (e.g., pitch range and speaking rate) than the spontaneous speech produced in a challenging listening condition (Hazan & Baker, 2011). Read clear speech elicited in noise (i.e., the talkers read sentences while simultaneously listening to noise) involved even more extreme changes than the read clear speech produced in quiet (in

speaking rate, pitch mean, energy in the 1-3 kHz range: Gilbert et al., 2014). As suggested by Hazan and Baker (2011), read clear speech likely involves a relatively constant degree of clarification as compared to the spontaneous clear speech produced in a challenging listening condition, which involves a larger variance in degrees of clarification. The

varying degrees of clarification may better model the tension between a talker and a listener (i.e., to minimize articulatory effort and to clarify speech to ensure successful

communication: Lindblom, 1990), which fluctuates over the course of the interaction, as compared to the constant degree of clarification found in read clear speech. Thus, it is possible that the acoustic-phonetic modifications made in the read clear speech in the current study were perceived to be somewhat unnatural, making the speech perceived to be not easier to understand than plain speech. However, it is an open question to what extent different clear speech elicitation methods influence the way clear speech impacts different aspects of perception (e.g., how intelligibility improvement relates to subjective evaluations of the same speech).

A possible variation in research contexts could also include examining perceptual consequences of clear speech enhancements with different listener populations. That is, listeners with different backgrounds may perceive clear speech produced by native and non-native talkers of English differently than native English listeners. For example, because the talkers in the current study, as in previous studies (e.g., Smiljanić & Bradlow, 2005), were instructed to speak as if talking to a hearing-impaired listener, their clear speech enhancements may be effective at improving perceived degree of comprehensibility for hearing-impaired listeners, in addition to improving intelligibility for these types of listeners as shown in previous studies (e.g., Picheny et al., 1985; Schum, 1996). Further,

given the previous results demonstrating that non-native talkers’ speech is better

understood by non-native listeners than by native listeners (Hayes-Harb, Smith, Bent, & Bradlow, 2008; Imai et al., 2005; Munro, Derwing, & Morton, 2006), it is possible that non-native listeners, who share an L1 background with the talkers, would benefit from the non-native clear speech enhancements more than native listeners do, not only in terms of intelligibility improvement but also in subjective measures of perception. Particularly, the native English listeners in the current study perceived higher-proficiency non-native talkers' plain speech to be easier to understand than their clear speech in the quiet listening

condition, suggesting that higher-proficiency talkers' clear speech enhancements had a detrimental effect on perceived comprehensibility of their speech. However, these talkers’ acoustic modifications may result in an improvement in perceived comprehensibility for native Mandarin listeners, who share the L1 background with the talkers. A future study may investigate how non-native talkers’ proficiency level impacts different types of clear speech perception (e.g., intelligibility, subjective evaluations) for native and non-native listeners.

Finally, the current results demonstrated that native listeners’ subjective evaluations differed for the speech produced by talkers of different L1 backgrounds and L2 proficiency levels. Specifically, the size of task-based differences in perception (i.e., the difference in the proportion of times the listeners chose the clear-style sentence for the effort vs.

comprehensibility task) was much smaller for lower-proficiency talkers’ speech compared to higher-proficiency talkers’ and native talkers’ speech. The results further indicated that detecting the increased effort in the clear-style sentences (as compared to the plain-style sentences) was more difficult for lower-proficiency talkers’ speech than for native talkers’

and higher-proficiency talkers’ speech. Here, it is important to point out that the current perception task was designed to examine the difference in listeners’ perception between plain- and clear-style productions, not their perception of the talkers’ speech more generally. That is, the small plain-to-clear difference in perceived talker effort for lower- proficiency talkers’ speech does not suggest that listeners did not perceive lower-

proficiency talkers’ effort producing the clear speech. It is possible that listeners perceived that lower-proficiency talkers’ plain speech was produced with a relatively high level of effort (perhaps with more effort than higher-proficiency talkers' and native talkers’ speech). This may have resulted in the small difference in perceived effort between plain and clear speech for lower-proficiency talkers’ speech. Examining listeners’ subjective evaluation using a Likert scale (e.g., 1: speech is produced with the least effort; 9: speech is produced with the maximum effort) may be able to test the questions of how perceived degree of talker effort increases from plain to clear speech, in relation with whether foreign-accented speech is generally perceived to be produced with increased effort compared to native speech.

3.4.4. Conclusion

The current study examined multiple aspects of clear speech perception:

intelligibility (Experiment 2A) and subjective evaluation of the speech (Experiment 2B). Specifically, Experiment 2A examined whether clear speech enhancements produced by native and non-native talkers of different proficiency levels result in a similar intelligibility improvement for native English listeners. Further, Experiment 2B examined whether native and non-native clear speech enhancements improved native listeners’ subjective evaluation,

in terms of perceived degree of comprehensibility (how easy the speech is to understand) and talker effort (how hard the talker is trying to speak clearly). The results of Experiment 2A showed the effect of non-native talkers’ target language proficiency level on their speech intelligibility. That is, higher-proficiency talkers’ speech was generally more intelligible than lower-proficiency talkers’ speech, and higher-proficiency talkers’ clear speech resulted in a larger plain-to-clear speech intelligibility improvement than lower- proficiency talkers’ clear speech did. The results of Experiment 2B demonstrated that talkers’ clear speech enhancements improved listeners’ subjective evaluation of talker effort more than comprehensibility across different talker groups’ speech. However, lower- proficiency talkers’ increased effort was more difficult to detect than that of higher-

proficiency talkers and native talkers. Together, these results suggest that non-native talkers’ ability to increase intelligibility of their speech improves as their target language proficiency develops. However, for both native and non-native talkers’ speech, an improvement in intelligibility does not necessarily correspond to an improvement in subjective evaluations of the speech. A future investigation may examine which acoustic features of clear speech enhancements are responsible for different types of listeners’ perception.

CHAPTER IV: PRODUCTION OF CONTEXTUALLY-RELEVANT SPEECH