2.1. The correlates of stress in productionTOP

The acoustic correlates of stress have been shown to include duration, F0, overall intensity, frequency-sensitive intensity (spectral balance) and formant frequencies (F1/F2). Gordon and Roettger (2017) surveyed 110 studies on 75 languages and found that although duration was the most frequently observed cue to stress, all of these cues played a role of some kind in most of the languages surveyed. The relative strength of different cues appears to vary across languages, however.

It is widely assumed that F0 is the most prominent and consistent cue to stress in English, based on the influential early study by Fry (1955), which did not, however, examine the correlates of stress in the absence of accent. Studies which avoid the stress versus accent confound instead report duration, spectral balance and formant frequencies as the most consistent cues in English (Bouchhioua, 2016; van Heuven & Sluijter, 1996).

There has been less prior investigation of the acoustic correlates of stress in production of Arabic stress. Cross-dialectal variation in the acoustic cues to stress is likely, since cross-dialectal variation in phonological stress assignment is well established (Watson, 2011). In addition, some dialects such as Egyptian Arabic (EA) display consistent co-occurrence of stress and accent: the stressed syllable of almost all content words also carries sentence-level accent (Chahal & Hellmuth, 2015).

One of the first studies of the correlates of stress in Arabic was on Jordanian Arabic (JA), and indicated that the cues to stress in JA are duration and F1 (de Jong & Zawaydeh, 1999). In contrast, the correlates of stress reported for Tunisian Arabic are spectral balance and F1, but not duration (Bouchhioua, 2016).

In a previous study we compared the correlates of stress in JA and EA—the two dialects investigated in the present study—and found that both dialects made use of duration, intensity and F0, but not formant frequencies or spectral balance (Almbark, Bouchhioua, & Hellmuth, 2014). The only differences between JA and EA were in the degree to which cues were used: there was greater differentiation of stressed and unstressed syllables by means of duration in EA than in JA, and by means of F0 in JA than in EA. This finding for JA contrasts with that of the earlier study of JA by de Jong and Zawaydeh (1999), which did not fully control for the confound of stress and accent.

2.2. Perception of the correlates of stressTOP

There is also cross-linguistic variation in the relative weighting of acoustic cues to stress in perception, and in the extent to which acoustic cues are relied upon compared to other factors.

Several studies have shown that listeners may rely on only a subset of the available acoustic cues in the signal. A recent study explored the perceptual behavior of English, Russian and Mandarin listeners in a forced choice identification task, in response to disyllabic pseudo-word stimuli in which F0, duration, intensity and F1/F2 of target vowels was systematically varied; vowel quality (F1/F2) had the greatest influence on the choices of listeners from all three language backgrounds, but there was variation in the relative weighting of suprasegmental cues (Chrabaszcz, Winn, Lin, & Idsardi, 2014). F0 was the next strongest cue after F1/F2 for English and Mandarin listeners, but duration and intensity were more important for Russian listeners. Similarly, Standard Mandarin listeners are influenced in their perception of stress minimal pairs, in a sequence recall task, by both duration and F0 cues; this contrasts with Taiwanese Mandarin listeners who attend primarily to F0, reflecting the lack of use of durational cues to word-level prominence asymmetries in Taiwanese Mandarin (Qin, Chien, & Tremblay, 2017). In lexical retrieval tasks, English listeners in fact rely primarily on segmental cues provided by unstressed vowel reduction: the true minimal pair ‘forebear’ (n.) [ˈfɔːbɛə] ~ ‘forebear’ (v.) [fɔːˈbɛə]—in which there are no segmental cues to stress in the form of vowel reduction in the unstressed syllable—is homophonous in perception for English listeners (Cutler, 1986).

Stress perception is also influenced by the phonological status of stress in the listener’s first language (L1). French is a language which does not display word-level stress, and a sequence of studies has shown that although French listeners are able to perceive the acoustic cues to stress in an AX discrimination task, they are unable to discriminate stress minimal pairs in a sequence recall task which requires phonological encoding of those acoustic cues in lexical representations (Dupoux, Pallier, Sebastián-Gallés, & Mehler, 1997); this holds even after long-term exposure to (and advanced proficiency in) Spanish, which is a language with contrastive stress (Dupoux, Sebastián-Gallés, Navarrete, & Peperkamp, 2008).

Finally, perception of stress is not influenced solely by acoustic correlates to stress and their relative weighting or phonological status. Several studies have shown that ‘bottom-up’ phonetic cues are used alongside ‘top-down’ cues such as lexico-semantic information in perception and processing of stress (Cole, Mo, & Hasegawa-Johnson, 2010; Eriksson, Thunberg, & Traunmüller, 2001). Mattys, White, and Melhorn (2005) argue that English listeners rely on different types of cues in a word segmentation task, with cues forming a hierarchy: fine-grained phonetic cues to stress are argued to be lower in the hierarchy than lexical and semantic cues, because phonetic cues are only relied on when performing the task in adverse listening conditions. This may be one strategy which allows listeners to use ‘perceptual normalization’ to recover the hypothesized intended form from non-target-like realizations (Ohala, 1993). In contrast, L2 listeners show less reliance than L1 listeners on ‘top-down’ structural or lexical information in a word-by-word prominence rating task; instead, L2 listeners’ ratings more closely reflected differences in the relative strength of acoustic phonetic cues (Wagner, 2005).

2.3. The interlanguage intelligibility benefitTOP

The term ‘interlanguage’ describes patterns of language use, displayed by second language learners, which fall somewhere between the grammar of the native language and the target language being acquired (Selinker, 1972).

The concept of an interlanguage speech intelligibility benefit was proposed by Bent and Bradlow (2003) to explain their findings in a sentence recognition task performed on L1 and L2 English speech samples, by L1 English listeners in comparison to L2 English listeners whose L1 varied. For native English listeners, the native English speech was more intelligible (more keywords accurately recognized) than the L2 English speech; however, for the L2 English listeners, the L1 English and L2 English speech were equally intelligible, regardless of whether the L2 English listener’s L1 background matched that of the L2 English speaker they were listening to.

The two main groups of L2 English listeners in the Bent and Bradlow (2003) study were L1 speakers of Chinese and Korean. Stibbard and Lee (2006) replicated the same study design with L2 English speakers/listeners from more typologically diverse L1 backgrounds, however, and obtained a more nuanced result. They explored the perceptual behavior of L2 English speakers from Saudi Arabia or Korea, at two proficiency levels in English (low and high). In their study, the L1 English listener group showed higher recognition rates than any of the L2 listener groups, but high proficiency L2 English samples were equally well recognized as L1 English samples by both L1 and L2 listeners. The main finding of the replication study was that low proficiency was highly correlated with low intelligibility, as might be expected, but also that there was a matched interlanguage speech intelligibility benefit: low proficiency L2 English speech was better recognized by L2 listeners from the same L1 background as the speaker in the L2 English sample.

In this study we explore whether there is an interlanguage speech intelligibility benefit in respect of L1 versus L2 realization of the phonetic cues to word-level stress.

2.4. The present studyTOP

The main research question of the paper is to determine whether there is an interlanguage intelligibility benefit in perception of English word stress. We use stimuli that were elicited using a paradigm designed to elicit English stress near-minimal pairs in a context in which the target word is realized without a phrase-level accent, thus focusing on listeners’ ability to make use of the phonetic cues to stress in the absence of cues to accent. Since vowel reduction is the primary cue to word stress for native English listeners (as noted in 2.2 above), it was important to use stimuli in which vowel reduction could appear, to determine whether failure to produce target words with appropriate vowel reduction reduces intelligibility, and perhaps differentially so for native versus non-native listeners. We therefore used near-minimal pairs in which vowel reduction in the unstressed syllable provides a segmental cue to stress alongside suprasegmental cues such as duration and intensity. The stimuli were produced by an L1 English native speaker (NE) and two L2 English non-native speakers (L2) from Jordan and Egypt, respectively. The listeners in a forced-choice identification task are L1 English listeners (NE) and L2 English listeners from Jordan and Egypt (L2). The over-arching research question stated in the title of this paper thus breaks down into three sub-questions, which we address in the present study by exploring the interaction of listener language and stimulus language in a single study with a crossed factor design:

Based on Bent and Bradlow’s (2003) findings, we would predict an advantage for NE listeners when listening to NE productions, but no advantage for L2 listeners when listening to other L2 listeners (from any background). Based on Stibbard and Lee’s (2006) findings, however, we predict an overall advantage for NE listeners, but a possible advantage for L2 listeners when listening to L2 listeners. Our interpretation of the results will also consider whether there are differences between NE and L2 listeners in reliance on ‘bottom-up’ phonetic cues versus ‘top-down’ structure-based expectations, by examining possible transfer effects which reflect the different structural properties of stress assignment in listeners’ L1.

3.1. MaterialsTOP

Stimuli which contrast in the position of stress were elicited using the nine English disyllabic near-minimal pairs, listed in Table 1, following Bouchhioua (2008, 2016).

Three further pairs (combine, pervert, and project) were recorded but later excluded from the study, as stress was frequently misplaced due to unfamiliarity with the word in one or both stress positions. Six target-like tokens of the word project (two from each speaker) were used for the training phase of the experiment as outlined further below.

The intended accent status of the target word was varied by using a carrier phrase that either attracts focus to the target word [+accent] or diverts focus away from it [−accent], again following Bouchhioua (2008, 2016), as shown in Table 2. The target word was always elicited in a carrier phrase: ‘say ___ again’. To attract accent onto the target word, a semantically related word preceded the target word in the same carrier phrase. To divert focus away from the target word, two preceding sentences are used to ensure that the target word appears in post-focal position (after the contrastively focused verb ‘SAY’) and is interpreted as old information due to being repeated from the immediately preceding discourse (Cruttenden, 2006; Ladd, 2008). Each sentence ~ context combination was read aloud once; sentences were presented to participants in pseudo-random order on a printed sheet.

The experimental stimuli for the present study were extracted from target-like tokens (as judged to consensus by the first and third authors) produced in −accent condition, as in (1), to investigate the extent to which listeners were able to detect phonetic cues to stress produced by the speakers, in the absence of any additional cues to accent.

The stimuli for the perception experiment were produced by three male speakers, from: Cairo, Egypt (EA); Amman, Jordan (JA); UK (native speaker of British English, NE). The speakers were aged 26, 20, and 39 years, respectively. The Arabic speakers had learned English at school for 12 years but had never resided in an English-speaking country; they were selected from participants in an earlier production study (Almbark et al., 2014). Recordings were made in Cairo, Amman and York, respectively. Recordings were made in .wav format at 44.1 KHz 16 bit, on a Marantz PMD660 with external Shure SM10 headset microphone.

The results of acoustic analysis of the selected stimuli for duration, F0, intensity, F1/F2 and two measures of spectral tilt (H1.H2 or H1.A3), comparing properties of the vowel in the initial syllable (only), in stressed and unstressed condition, are illustrated in Figures 1–2. We used a normalized vowel duration measure to control for inter-speaker variation in speech rate, by calculating vowel duration as a proportion of the whole word. The acoustic properties of the stimuli were explored in a series of linear mixed models (LMM) using lme4 (Bates, Maechler, Bolker, & Walker, 2015) in R (Core Team, 2014), with each acoustic measure in turn as dependent variable, speaker (EA ~ JA ~ NE) and stress (stressed ~ unstressed) and their interaction as fixed factors, and a random intercept for item.

Figure 1: Median and interquartile range for values of (from left to right) maximum F0, peak intensity, normalized vowel duration and two measures of spectral emphasis H1–H2 and H1A3, in the first vowel of experimental stimuli produced by the native speaker of Egyptian Arabic (EA; top row), Jordanian Arabic (JA; middle row), and English native speaker (NE; bottom row), grouped by stress condition (whether the vowel in which measurements was taken was stressed or unstressed).

Figure 2: F1/F2 plot of the first vowel in experimental stimuli produced by the native speaker of Egyptian Arabic (EA), Jordanian Arabic (JA) and English (NE), where the vowel is stressed (black dots) or unstressed (white dots).

The acoustic analysis shows that F0 differentiates stressed and unstressed syllables in the L2 English productions of the JA speaker, but not in those of the EA or NE speakers. Similarly, although intensity is somewhat higher in stressed syllables than unstressed syllables for all three speakers, including the EA speaker (for whom intensity is the strongest cue to stress on average), nevertheless it is only in the JA speaker’s production that this difference is significant. In contrast, neither vowel duration nor spectral tilt (H1.H2 or H1.A3) is used to differentiate stressed and unstressed syllables by any of the speakers. Finally, both F1 and F2 differentiate stressed and unstressed syllables to a significant extent in the NE speaker’s productions, but not in the productions of the EA speaker and JA speaker.

The differences among the three speakers in the observed cues to stress in the experimental stimuli match the generalizations reported for the full set of speakers who participated in the study from which the stimuli were extracted (Almbark et al., 2014), which also reports the phonetic realization of stress in L1 EA and JA by the same speakers.

3.2. ParticipantsTOP

Participants were recruited by email invitation among the friends and family of graduate students of linguistics from Egypt and Jordan, and among students at the University of York. A total of 42 listeners meeting our inclusion criteria (by native language/dialect, excluding early bilinguals) completed the online perception experiment on a voluntary basis. From these a balanced subset of 36 was selected at random to yield three listener groups by native language: EA, JA or English (NE), with six male and six female listeners in each group. The Arabic-speaking listeners had all studied English for at least 12 years; six had English medium schooling (two EA, four JA); one JA listener was in the UK at the time of taking test.

3.3. ProcedureTOP

The experiment was run using an online survey tool (SurveyGizmo, 2019). Participants first read an information sheet and provided their informed consent to participate; they then completed a questionnaire about age, sex, native language and dialect, and, for L2 listeners, number of years of study of English.

Participants were familiarized with the test paradigm in a training phase; a selection of English stimuli were presented, which differed in stress position as in the main test, using the target word ‘project’ [ˈpɹɒdʒɛkt] ~ [pɹəˈdʒɛkt]. Participants were asked to answer the following question for each word they heard: “Was it PROject (first syllable) or proJECT (second syllable)?”. Feedback was given as to whether the provided answer was correct or incorrect.

After the training phase, in the first test phase the 36 sound files produced by the two L2 English speakers (9 target words × 2 stress conditions × 2 speakers = 36) were presented in randomized order. Each sound file was shown on a separate page with the question “Is it ___ (first syllable) or ___ (second syllable)?” and two answers (e.g., “SUBject with stress on the first syllable” or “subJECT with stress on the second syllable”) to choose from, in a binary forced choice. Then, in the second test phase, the 18 sound files produced by the L1 English speaker were presented, following the same procedure as for the first test phase.

We presented all L2 speech in one block, then all L1 speech in a separate block, to restrict the listeners’ task to word recognition. Randomisation of tokens extracted from L1 and L2 speech in one block might have drawn listeners’ attention to evaluation of the degree of foreign accent rather than the intelligibility (i.e., recognition) of the utterances as intended.

3.4. AnalysisTOP

Each response was coded for accuracy: responses which matched the intended form of the word as elicited were coded as correct, otherwise as incorrect. Results were explored using binomial generalized linear mixed models (GLMM) using lme4 (Bates et al., 2015) in R (Core Team, 2014), with accuracy as the dependent variable, using likelihood ratio tests to identify the best fit model. The predictions of the model were extracted using lsmeans (Lenth, 2016) and plots were produced using ggplot2 (Wickham, 2009).