The effects of dialect variation on speech intelligibility in a multitalker background

Speech intelligibility in a multitalker background can be affected by the language of both the talker and the interfering speech. This study investigated whether this interaction is modulated by dialect variations of the same language. American English listeners were presented with target sentences in either their own General American English (GAE) or a different accent (Southern American English [SAE]) masked by either GAE or SAE two-talker babble at three sound to noise ratios (SNRs): +3, 0, and –3 dB. All speech materials were produced by male talkers. Across all conditions, SAE target was more intelligible than GAE. Intelligibility of either target decreased as the level of the interfering babble noise increased. Target accent interacted with masking accent: at +3 dB SNR, GAE (and not SAE) was the more effective masker. The target-masker interaction was different as listening conditions deteriorated: at 0 and –3 dB SNR, masking accent did not affect GAE target, but when the target was SAE, the SAE masker (and not GAE) was more effective. Thus, at increased noise levels, listeners benefited from the mismatch between the target and masking accents only when the target was in a nonnative accent. These results demonstrate that dialect variation can influence listeners’ performance in a multitalker environment. The apparent asymmetry in intelligibility of accents may be in part related to dialect-specific prosodic and phonetic features. Listening to speech in noisy environments, such as in the presence of multiple talkers, is challenging as these conditions limit its intelligibility. The growing literature on the subject repeatedly points to two factors which contribute most to the perceptual degradation of the target speech as determined in laboratory testing: (a) the increased sound level of the multitalker babble relative to the target and (b) the increased number of interfering talkers in the babble (e.g., Bronkhorst, 2000; Bronkhorst & Plomp, 1992; Brungart, Chang, Simpson, & Wang, 2009; Brungart, Simpson, Ericson, & Scott, 2001; Cooke, Garcia Lecumberri, & Barker, 2008; Miller, 1947; Simpson & Cooke, 2005; Van Engen & Bradlow, 2007). Performance also decreases when the target and interfering talkers share similar vocal characteristics (Brungart, 2001; Brungart et al., 2001). In © Cambridge University Press 2014 0142-7164/14 $15.00 Applied Psycholinguistics 2 Jacewicz & Fox: Dialect effects in speech-on-speech masking general, multitalker environments produce two types of masking effects: energetic and informational. Energetic masking is assumed to occur at the periphery of the auditory system where the target and interfering voices “compete,” that is, overlap in time and frequency. Informational masking is thought to be more central, when the segregation of the target and the interfering voices becomes difficult due to stimulus uncertainty although both types of signals are audible. The intelligibility of the target in the presence of the masking speech is also affected by the linguistic system of each. Recent work has shown that the language of the masker can differentially affect the intelligibility of the target. In particular, a mismatch between the languages of the target and the masker produces intelligibility benefit compared to when both the target and the masker share the same language (Freyman, Balakrishnan, & Helfer, 2001; Garcia Lecumberri & Cooke, 2006; Rhebergen, Versfeld, & Dreschler, 2005; Van Engen & Bradlow, 2007). Exploring these effects, a number of studies have compared a range of listener–target–masker combinations that also included foreign or “accented” language either in the target sentence (Gordon-Salant, Yeni-Komshian, & Fitzgibbons, 2010; Pinet, Iverson, & Huckvale, 2011; Rogers, Dalby, & Nishi, 2004; Van Engen, 2010) or in the masking babble (Calandruccio, Dhar, & Bradlow, 2010; Freyman et al., 2001; Van Engen, 2010) and with native or nonnative listeners (e.g., Cooke et al., 2008; Garcia Lecumberri & Cooke, 2006; Mayo, Florentine, & Buus, 1997; Pinet et al., 2011). In general, native listeners performed better compared to nonnative listeners in a variety of listener–target–masker combinations, and the intelligibility of accented speech decreased in the presence of the masker (see Garcia Lecumberri, Cooke, & Cutler, 2010, for a comprehensive review). These findings prompted the question that motivated the present study: Can dialect variations of the same language differentially affect intelligibility of the target speech in the presence of the multitalker masker? The goal is to establish whether varying dialect in the target speech and in the masker produces effects comparable with those reported for native and nonnative (or accented) languages. In this pursuit, the study aims to determine whether speech recognition is also affected by fine-grained acoustic phonetic details that are present in dialect-specific pronunciation patterns in different regional varieties of the same language. The existing experimental evidence shows that listeners are sensitive to pronunciation differences across dialect regions in the United States (Clopper & Pisoni, 2004, 2007) and that familiarity with a dialect improves its processing and comprehensibility. That is, comprehension difficulties and processing costs decrease as listener’s familiarity with features of that dialect increases (Adank, Evans, Stuart-Smith, & Scott, 2009; Floccia, Goslin, Girard, & Konopczynski, 2006; Scott & Cutler, 1984; Sumner & Samuel, 2009). Two recent studies are of direct relevance to our present investigation because both examined intelligibility of regional dialects in masking noise, thus increasing cognitive effort of listeners due to difficult listening conditions, which tend to reduce redundant speech cues that are otherwise abundant in stimuli presented in quiet (e.g., Meador, Flege, & Mackay, 2000; Pisoni, Nusbaum, & Greene, 1985; van Wijngaarden, Steeneken, & Houtgast, 2002). In the first study, Clopper and Bradlow (2008) Applied Psycholinguistics 3 Jacewicz & Fox: Dialect effects in speech-on-speech masking presented sentence stimuli produced by talkers representing four broad American English dialect regions (Mid-Atlantic, North, South, and General American) to three groups of listeners who lived, prior to age 18, in either Northern or General American English (GAE) dialect regions or were mobile, living in more than one region. In Experiment 1, the stimuli were presented in speech-shaped noise at –2 and –6 dB sound to noise ratios (SNRs) and in Experiment 2 at –2 and +2 dB. The results showed that, in the most difficult listening condition (–6 dB), the intelligibility scores were significantly different among all four dialects. As the overall performance improved in the easiest +2 dB condition, some of the significant differences disappeared, showing that only Mid-Atlantic talkers were less intelligible than the talkers from the other three dialects. In both experiments, however, the highest intelligibility scores were obtained from productions by the GAE talkers, indicating that GAE was the most intelligible dialect for all listeners in all testing conditions. Overall, this study shows that dialect differences among talkers have a differential effect on intelligibility of degraded speech and tend to be attenuated in more favorable listening conditions that provide listeners with greater redundancy of cues. In the second study, Adank et al. (2009) examined processing cost (measured in terms of response times) associated with comprehension of sentences produced by Southern Standard British English talkers and Scottish English (Glasgow) talkers by native speakers of these two regional varieties. The Standard English listeners were unfamiliar with the Scottish variety, whereas the Scottish listeners were equally familiar with both Standard and Scottish English. The stimuli were presented in quiet and in speech-shaped noise at three SNRs: +3, 0, and –3 dB. The results of Experiment 1 showed that the Standard English listeners were significantly slower when responding to sentences in an unfamiliar dialect (Scottish) at moderate SNR levels (+3 and 0 dB), but the difference between the two accents became much smaller at –3 dB, that is, as the listening conditions deteriorated and the response times were the slowest. Thus, Standard English listeners showed less efficient speech processing for an unfamiliar regional accent in moderate listening conditions, most likely due to their lack of experience with the Scottish English. The Scottish English listeners were equally fast when responding to both varieties, apparently due to their familiarity with both accents. It needs to be underscored that the difference between the familiar and unfamiliar accents was not significant in quiet, suggesting that both groups of listeners benefited from the redundancy of the speech signal in favorable listening conditions. In the present study, we investigate whether listeners’ familiarity or unfamiliarity with the dialect of American English (General or Southern) affects intelligibility scores when the target speech is presented in two-talker babble. While varying the dialect in the masking babble, conditions were created for additional interactions involving the dialect of the masker and dialect of the target sentence. Based on the results of the two studies reviewed above, we can tentatively predict that listeners familiar with GAE will perform better when target sentences are produced in that variety and will perform worse when responding to productions in a local Southern American English (SAE) dialect, which is relatively unfamiliar to them. This scenario may not hold true when the familiarity with the dialect in the target interacts with familiarity with the dialect in the masking babble. Applied Psycholinguistics 4 Jacewicz & Fox: Dialect effects in speech-on-speech masking To our knowledge, no study has as yet explored the effects of dialect of the babble on intelligibility of the target. Therefore, to spell out predictions about possible outcomes of the interaction between the two, we turned to available literature that examined such effects for native and nonnative language configurations. Van Engen and Bradlow (2007) found that for American English listeners responding to an American English target, English two-talker babble was more detrimental than Mandarin two-talker babble, but this effect disappeared in six-talker babble. Calandruccio et al. (2010) provided American English listeners responding to American English targets with five distinct two-talker babble maskers, which used American English, three levels of Mandarin-accented English (mildly, moderately, and heavily accented) and Mandarin. There was a significant difference in performance among all five maskers in a more difficult listening condition (–5 dB). Listener performance was poorest in English babble, followed by that in Mandarin babble. However, their performance was better in the presence of the three Mandarin-accented English maskers. Altogether, American English listeners performed best in the presence of nonnative, heavily Mandarin-accented English masker and worst in the presence of their native American English babble. In another study, Van Engen (2010) used native American English listeners and native Mandarin listeners who spoke English as a second language (L2). The listeners were presented with American English target sentences masked by a two-talker babble in either English or Mandarin. In this design, L2 listeners responded in the presence of either a native language (Mandarin) or L2 (English) masker and native English listeners responded in the presence of their native language (English) and foreign language (Mandarin) masker. The results showed that English listeners performed poorly in the presence of their native language (English) masker but were significantly better in Mandarin babble, thus replicating an earlier finding by Van Engen and Bradlow (2007). However, performance of L2 listeners was also worse in the presence of the English masker (a masker in their L2) and better in the Mandarin masker, which was in their native language. Thus, the English masker was more detrimental for both groups of listeners, regardless of their native language background (while processing English sentences). In the present study, we varied two regional dialects, GAE and SAE, in target sentences and in the masker (two-talker babble) and presented the speech material to GAE listeners at three SNRs (+3, 0, and –3 dB). Taking into consideration the main findings of the research reviewed above, we predict the following pattern of responses. First, we expect that intelligibility of the target will decrease as the SNR decreases. Second, listeners are expected to perform better when responding to GAE target (Adank et al., 2009; Clopper & Bradlow, 2008) masked by SAE babble rather than by GAE babble (based on Van Engen, 2010). This prediction, however, rests on the premise that the native-dialect status and target– masker similarity effects correspond to the effects reported for the native and L2 languages (Garcia Lecumberri & Cooke, 2006). Finally, the target–masker dialect interactions are predicted to have a greater effect on listener performance at the more difficult SNR (–3 dB) than in moderate listening conditions (+3 and 0 dB). Applied Psycholinguistics 5 Jacewicz & Fox: Dialect effects in speech-on-speech masking