Second Language Phonetics
Summary and Keywords
The study of second language phonetics is concerned with three broad and overlapping research areas: the characteristics of second language speech production and perception, the consequences of perceiving and producing nonnative speech sounds with a foreign accent, and the causes and factors that shape second language phonetics. Second language learners and bilinguals typically produce and perceive the sounds of a nonnative language in ways that are different from native speakers. These deviations from native norms can be attributed largely, but not exclusively, to the phonetic system of the native language. Non-nativelike speech perception and production may have both social consequences (e.g., stereotyping) and linguistic–communicative consequences (e.g., reduced intelligibility). Research on second language phonetics over the past ca. 30 years has resulted in a fairly good understanding of causes of nonnative speech production and perception, and these insights have to a large extent been driven by tests of the predictions of models of second language speech learning and of cross-language speech perception. It is generally accepted that the characteristics of second language speech are predominantly due to how second language learners map the sounds of the nonnative to the native language. This mapping cannot be entirely predicted from theoretical or acoustic comparisons of the sound systems of the languages involved, but has to be determined empirically through tests of perceptual assimilation. The most influential learner factors which shape how a second language is perceived and produced are the age of learning and the amount and quality of exposure to the second language. A very important and far-reaching finding from research on second language phonetics is that age effects are not due to neurological maturation which could result in the attrition of phonetic learning ability, but to the way phonetic categories develop as a function of experience with surrounding sound systems.
Keywords: second language learning, second language speech production, second language speech perception, cross-language speech perception, foreign-accented speech, intelligibility, comprehensibility, models of second language speech learning, perceptual training, production-perception link
What is second language phonetics? Despite its name, second language phonetics is not just concerned with the perception and production of a second language. This article will use the label “second language” as shorthand for any language learned after the first language is fairly well established, including third, fourth, etc., languages. In fact, research in this area is probably more appropriately described as dealing with the phonetics of a nondominant language, which could include the first language in cases of first language attrition, and with any kind of dynamics between the dominant and nondominant language that unfolds when two or more languages coexist in one mind.
1. Characteristics of Second Language Phonetics
The methods used to study second language (L2) phonetics are largely the same as in general phonetics, with some modifications or limitations necessary in studies of the beginning stages of L2 speech learning. In perception, learners’ lexicons may not be sufficiently developed to use labeling (identification) tasks, so discrimination tasks have to be used, which are largely the same as in the related field of cross-language speech perception. In production, even advanced L2 learners may not be fully proficient with the spelling system of the L2, so any task involving reading may present a confound. Instead, a variety of tasks are used which address the competing demands of ecological validity and experimental control in different ways. Production tasks which aim at ecological validity—that is, emulate language use in a naturalistic environment—include the retelling of comic strips (e.g., Munro & Derwing, 1995a), or tasks in which two speakers negotiate competing visual information as in the map task or in the Diapix task (e.g., Van Engen et al., 2010; see also Anderson et al., 1991). Experimental control, on the other hand, is maximized in the delayed repetition task (e.g., Flege, Munro, & MacKay, 1995), in which speakers repeat a phrase or a word after exposure to intervening speech material.
The vast majority of studies on second language speech production employ indirect methods in that they examine production either through its acoustic consequences (e.g., formant frequencies as a proxy for tongue height and position; see Bohn & Flege, 1992) or in terms of its perceptual consequences (degree of perceived foreign accent, e.g., Major, 1987a, or intelligibility, e.g., Munro, 1998). Probably because of the relatively expensive equipment needed, direct articulatory studies of L2 speech production are quite rare, with some noticeable exceptions, e.g., Zimmermann, Price, and Ayusawa (1984), Flege (1988), Flege and Bohn (1989), Nissen, Dromey, and Wheeler (2007), and Wilson and Gick (2014).
1.2 Second Language Speech Production
1.2.1 Perceptual Studies of Foreign-Accented Speech
Second language speech production, or foreign-accented speech, is characterized by deviations from native norms which can be described as suprasegmental (referring to intonation and stress patterns), segmental (referring to the identity of segments, including deletions, insertions, and substitutions, such as [s] for [θ]), or subsegmental (referring to deviations from the typical realization of a segment, e.g., the nonvelarized [l] for the velarized [ɫ] as in the English word bill). All three aspects of second language speech production are typically examined simultaneously in studies of perceived degree of foreign accent, in which linguistically naive or experienced listeners provide global ratings of degree of foreign accent using Likert scales (e.g., Derwing & Munro, 1997) or direct magnitude estimation (e.g., Brennan, Ryan, & Dawson, 1975; for a comparison of these methods, see Southwood & Flege, 1999). It is of some interest that even linguistically naive listeners can reliably rate different degrees of accentedness (Ryan, Carranza, & Moffie, 1977; Derwing & Munro, 1997; Flege, 1984; Major, 2007; Thompson, 1991); that both the inter- and the intra-reliability of judgments tend to be good (Flege & Fletcher, 1992); that inexperienced raters are stricter than experienced raters, who in turn are more reliable (Thompson, 1991; Huang & Jun, 2015); that native speakers of the L2 are more lenient judges than nonnative speakers (Fayer & Krasinski, 1987; Koster & Koet, 1993); and that that the self-assessment of L2 learners may be flawed in that low-proficiency learners underestimate, and high-proficiency L2 learners overestimate, the strength of their foreign accent (Trofimovich, Isaacs, Kennedy, Saito, & Crowther, 2016).
Studies of perceived degree of foreign accent are typically conducted with different aims, and this section will briefly address the question of which part of the speech signal contributes to foreign accentedness. Other aims will be discussed in section 2 on the consequences of L2 speech (i.e., studies relating degree of perceived accentedness to intelligibility and comprehensibility), and in section 3 on the causes of second language speech, which will outline studies which examined how degree of accentedness relates to learner characteristics such as age at the onset of learning, language use patterns, etc.
Judgments of perceived degree of global foreign accent, in which listeners are presented with phrase-length nonnative utterances, do not directly provide information on what causes the impression of foreign accentedness—nonnative-like production of suprasegmental patterns or individual segments. To examine the contribution of nonsegmental factors to the perception of foreign accentedness, several studies have presented listeners with low-pass filtered foreign-accented speech samples which preserve the intonation contour and the acoustic correlates of stress and rhythm while reducing segmental information considerably (e.g., van Els & de Bot, 1987). These studies all found that suprasegmental properties of foreign-accented speech contribute to the perception of a foreign accent (e.g., Munro, 1995; Trofimovich & Baker, 2006) and that it is possible to identify speech samples as foreign accented on the basis of suprasegmental information alone. However, it does not seem to be the case that suprasegmental deviations contribute more to the perception of a foreign accent than segmental errors (Sereno, Lammers, & Jongman, 2016). One of the very few studies which examined the relation between perceived global foreign accent and local foreign accent, that is, the perceived foreign accentedness of syllable-length stretches of speech, is Major (1987a), who reported a very interesting and, at first sight, counterintuitive finding. Major found the expected positive correlation between Brazilian Portuguese speakers’ degree of perceived global foreign accent in English and the perceived accentedness of their attempts to produce [æt] (excised from the word sat). However, he also reported an unexpected negative correlation between global foreign accent and the perceived accentedness of [ɛt] (excised from bet), meaning that as the strength of the global foreign accent decreased, the strength of the accentedness of [ɛt] increased. The apparently paradoxical results from this study, which was partly confirmed by Bohn and Flege (1992) in a study of English vowel production by German learners, will be discussed in section 1.4, because it points to one of the reasons why even advanced L2 learners cannot avoid a foreign accent.
A small number of studies have attempted to identify the locus of foreign accentedness by relating the degree of perceived foreign accent to various acoustic measurements. Trofimovich and Baker (2006) examined the contribution of suprasegmental properties to native Korean speakers’ foreign accent in English and reported that pause duration and speaking rate were more likely than stress timing and peak alignment to contribute to foreign accent, partly confirming earlier studies which reported an association between slow speaking rates and accentedness (Munro & Derwing, 1998, 2001). The conflicting results from studies which attempted to relate global foreign accent to segment properties are difficult to interpret because of widely different methodologies and numbers of participants. For example, Major (1987b) reported highly significant correlations between the global foreign accent ratings of 53 native Brazilian Portuguese speakers of English and the voice onset time (VOT) of their English voiceless stop productions, whereas Magen (1998), who related eight segment properties in the speech of her two participants to their global foreign accent, reported that the global ratings were not affected by voicing differences. The difference between the results of these two studies is surprising, because native Portuguese and native Spanish speakers, who share the prevoiced versus short lag voicing contrast, should have identical problems with the English voicing distinction, which is short lag versus long lag. Probably the most detailed study relating global foreign accent scores to both the intelligibility and the acoustic properties of L2 vowel production is Flege (1992), which examined native Dutch speakers’ success in producing the vowels of English. Flege reported that the degree of foreign accent was not uniformly related to the intelligibility of all L2 English vowels or to the degree with which L2 English vowels were acoustically closer or more distant from natively produced English vowels. Rather, it was the vowel /æ/, which does not exist in Dutch, whose production accuracy was related to global accentedness (better /æ/ production—milder accent), whereas the production of vowels that Dutch and English share was largely unrelated to degree of global foreign accent.
Perceptual studies examining degree of foreign accent are useful to determine the relative degree of global foreign accent of various speaker groups (differing in, e.g., age at the onset of learning; see section 3) and to evaluate whether and how the degree of foreign accent has social or linguistic communicative consequences (see section 2). An additional use of perceptual judgments of foreign-accented speech samples can be found in studies which examine the intelligibility of nonnative speech, whether as a means to identify L2 production problems (e.g., Bohn & Bundgaard-Nielsen, 2009), to complement acoustic analyses (e.g., Levy & Law, 2010; Flege, Bohn, & Jang, 1997), or to assess the effect of L2 experience on L2 production (Flege, Takagi, & Mann, 1995). In the Bohn and Bundgaard-Nielsen (2009) study, native English listeners identified L1 Danish speakers’ attempts to produce the vowels of English, which resulted in intelligibility scores ranging from 17% to 99% correct identifications for individual vowels. In the Flege et al. (1997) study, the results for the intelligibility of English /i, ɪ, ɛ, æ/ as produced by native speakers of four different languages mirrored the acoustic analyses. Flege, Takagi, and Mann (1995), who provided their judges with graded scales to indicate how certain they were that L1 Japanese speakers had produced English /r/ or /l/ in words that could be part of minimal pairs, reported that Japanese adults can learn to produce English /r/ and /l/ correctly.
1.2.2 Acoustic Studies of L2 Speech Production
Perceptual studies of foreign-accented speech can be informative on various levels, but they cannot provide detailed information on the fine-grained acoustic differences between the speech of native and nonnative speakers. Studies examining the acoustic properties of foreign-accented speech are typically motivated by an interest in how one of three factors (or a combination of them) affect L2 speech production: previously acquired languages (or L1s for short), the age of the L2 speaker at the onset of learning (AOL), and the quality and quantity of L2 experience. The influence of previously acquired languages on L2 production is massive and easily detectable, especially in the early stages of L2 learning, and is probably one of the most obvious and best-documented facts about L2 phonetics, as can be seen from the speech accent archive (Weinberger, 2014) with currently ca.1,800 sound files of the same short text read by nonnative speakers with many different language backgrounds. Acoustic studies examining the influence of L1s on L2 production are typically conducted to assess the influence of L2 experience on learner groups with different amounts of L2 experience, as in the Flege et al. (1997) study.
The phonetic phenomena that have been studied most frequently in acoustic studies of L2 speech production are L2 vowel production and the implementation of the voicing contrast in initial or final position, probably because languages differ greatly in their vowel inventories and in the ways in which they implement voicing contrasts, and because important acoustic correlates of vowel production and of voicing are fairly easy to measure. These studies have provided valuable and precise information on three central issues in L2 speech production: what causes AOL to be negatively correlated with the perceived degree of foreign accent (why is younger better?), what enables and what limits adult learners’ ability to learn nonnative speech sounds, and what is the level on which L2 phonetic learning takes place—the phoneme or the context-sensitive allophone?
The role of AOL in L2 speech production can be summarized by quoting from Guion’s (2003) study on Quichua and Spanish vowel production in L1 Quichua speakers who had started to learn Spanish at different ages: “The results of the acoustic analyses indicate that the earlier in life one is exposed to a language, the greater the chance of acquiring the vowels of that language and producing them in a native-like fashion” (Guion, 2003, p. 118). Several studies, especially those by Flege and his collaborators, suggest that the likelihood of establishing separate categories for the sound categories of the two(+) languages of L2 speakers diminishes with increasing AOL. Unlike child learners, who may be able to establish separate categories and keep them separate as in the Flege, MacKay, and Meador (1999a) study, adults’ L2 productions are often best characterized as being based on merged categories which represent compromises between the implementation of categories of the L1 and the L2, as in Flege (1991). This study showed that L1 Spanish speakers with an English AOL of five to six years produced English /t/ accurately (i.e., with English-like VOT values), whereas L1 Spanish speakers who had started to learn English as adults produced English /t/ with VOT values that were in between the long-lag English /t/ and the short-lag Spanish /t/. A range of studies have observed that adult learners produce some L2 sounds with values that are intermediate between the values of monolingual speakers of the languages involved, for example, Flege (1987), who reported that both highly experienced L1 French speakers of English and highly experienced L1 English speakers of French produced the /t/s in French and in English with VOT values that were in between those of monolingual speakers of French (short-lag VOT) and English (long-lag VOT).
However, while studies like the ones just mentioned suggest that advanced L2 learners produce some of the sounds of the L2 (and the L1, see section 1.4) as compromises between L1 and L2 categories, there is also clear evidence that adult learners can do more than just modify existing categories to accommodate both native and nonnative production values. A case in point is the Garibaldi and Bohn (2015) study, which reported that highly experienced adult L1 Spanish speakers of Danish had learned to produce a Danish-like /y/ and a long-lag /t/ in Danish. Because Spanish does not have an /y/ vowel, and because the /t/ of Danish is very long-lag and affricated (as opposed to the short-lag /t/ of Spanish), this finding adds to several previous studies which indicate that speech learning ability for adults is intact for some sounds of the L2, but may be compromised for others. The reasons for this differential speech learning ability, and the theory-based approaches to account for these findings, will be discussed in section 3, which focuses on the causes and factors that shape second language phonetics.
Finally, acoustic studies of L2 speech have contributed importantly to an understanding of the unit of L2 speech learning—the abstract phoneme or the context-sensitive allophone. These studies have clearly shown that L2 speech production is best analyzed at the level of the context-sensitive allophone. Correct production of L2 sounds by L2 learners in one position does not guarantee correct production in other positions, as in the Colantoni and Steele (2007) study of L1 English speakers’ L2 French /r/ productions, or in the Yavaş (1997) study of devoicing of L2 English final voiced stops by L1 speakers of Japanese, Mandarin, and Portuguese. In addition, what L2 learners are bringing to the task of L2 learning is L1-based expectations about coarticulation, which may not be appropriate for the L2, as documented for the /u/ vowels of English and French by Oh (2008), and by Levy and Law (2010) for the consequences of language-specific coarticulation patterns for L1 English speakers of French.
1.3 Second Language Speech Perception
The one overarching question in all research on L2 speech perception is: How malleable are the speech perception habits that listeners acquired for the rapid and efficient processing of their L1? The background to this question is one of the most important findings from infant speech perception research, namely, that humans initially are universal perceivers who can discriminate phonetic contrasts not found in their ambient language, such as the English [pa] – [pha] distinction, which two-month-old Kikuyu-learning infants discriminate even though this contrast does not exist in Kikuyu (Streeter, 1976). Similar results have been observed for many consonant contrasts which are absent in the infant’s ambient language, but which very young infants nevertheless discriminate well (e.g., Trehub, 1976; Aslin, Pisoni, Hennessy, & Perey, 1981; Eilers, Morse, Gavin, & Oller, 1981; Werker, Gilbert, Humphrey, & Tees, 1981). During the second half of the first year of life, however, infants’ speech perception becomes attuned to the ambient language, which is evident from a dramatic decline in the ability to discriminate nonnative contrasts (Werker & Tees, 1984a). This very useful attunement to the L1 can be dysfunctional in L2 acquisition, when L2 learners encounter nonnative speech sounds which are characterized and differentiated by cues which are not used, or used differently, in the L1. What Strange (2011) described as Selective Perceptual Routines which nonnative speakers use for L1 speech sound processing may be inappropriate for L2 speech perception.
The very obvious and important question for L2 speech perception research is whether the abilities that all learners once had (as infants) are lost for good once they tune in to their L1, or whether these abilities are dormant and can be relearned later in life. The response, based on abundant and clear evidence, is neatly summarized by Werker (1994) with the title statement “Developmental change does not involve loss.” The research which has shown that speech perception development involves a reversible shift in attention, not some kind of neurologically based loss, is an important piece of counterevidence against the hypothesized existence of a maturationally limited language learning ability, as proposed by Lenneberg’s (1967) Critical Period Hypothesis, which assumes a developmental loss of neural plasticity.
Cross-language speech perception research, which studies the perception of nonnative speech sounds by listeners with no L2 experience, has shown that nonnative speech sounds are mostly, but not exclusively, perceived through what Trubetzkoy (1939, p. 47) described as the “sieve” of the native language. One of the effects of this sieve is an L1-dependent warping of the perceptual space, as shown by Iverson et al. (2003) for the distance between English /r/ and /l/ as perceived by American English, German, and Japanese listeners. This warping is due to a stretching of perceptual distances between sounds that are distinctive in the L1, and a shrinking of distances between sounds that are not, as first shown by Butcher (1976) for different vowel contrasts in German, French, and English (see Bohn, 1995). The level on which the L1 sieve operates is not just the abstract phoneme (as originally postulated by Trubetzkoy) but rather the position- and context-sensitive allophone, as documented by a range of studies which have shown that L2 perception is affected by the vowel and consonant context in which sounds appear (e.g., Gillette, 1980; Strange & Dittmann, 1984; Schmidt, 1996; Lambacher, Martens, Nelson, & Berman, 2001). Additional studies indicate that listeners use their L1-based expectations about coarticulation (e.g., fronted variants of English /u/ in alveolar contexts) in nonnative speech perception (e.g., Strange, Bohn, Nishi, & Trent, 2005; Bohn & Steinlen, 2003).
Evidence supporting the claim that these and other native language biases are malleable comes from at least three sources. First, there are studies which have shown that the sensory ability to detect phonetic differences that are irrelevant in the L1 but relevant in the L2 does exist in adult L2 learners. Miyawaki et al. (1975) showed that even though L1 Japanese listeners discriminated the American English /r/-/l/ contrast at much lower levels than L1 American English listeners, they performed as well as the L1 American English listeners in a discrimination task on nonspeech stimuli consisting of the isolated F3 component which is the main perceptual cue for the /r/-/l/ contrast. This study, as well as a range of additional studies (e.g., Werker & Tees, 1984b; Werker & Logan, 1985; Kewley-Port, Bohn, & Nishi, 2005), has clearly shown that nonnative listeners retain the peripheral sensory ability to perceive acoustic cues differentiating nonnative contrasts, and that perceptual problems exist at a central linguistic level.
Laboratory training studies and studies of naturalistic L2 acquisition have shown that these sensory abilities are available for linguistic purposes so that L2 learners can reorganize their L1-shaped perceptual habits to develop perceptual patterns that are appropriate for the processing of their L2. Perceptual training studies are typically conducted with at least two groups which differ only in that the control group participates in a pretest and a posttest (to establish whether simple retesting affected perceptual performance) and the experimental group receives training between pre- and posttest. Training regimes may take on many different forms (for methodological aspects of perceptual training studies, see Logan & Pruitt, 1995), and there is broad agreement in the field that robust learning through perceptual training is best achieved through a high variability training technique in which trainees are exposed to the full range of variability within each category, thus emulating what learners will encounter naturalistically in a new language environment. The conclusion from the large number of studies which have employed high variability training is the same as in a review of studies on lexical tone training conducted by Wang and her collaborators: “the adult brain retains a high degree of plasticity” (Sereno & Wang, 2007, p. 257; see also Bradlow, Pisoni, Akahane-Yamada, & Tohkura, 1997; Bradlow, Akahane-Yamada, Pisoni, & Tohkura, 1999; Lively, Logan, & Pisoni, 1993; Lively, Pisoni, Yamada, Tohkura, & Yamada, 1994; Logan, Lively, & Pisoni, 1991, 1993; Pisoni & Lively, 1995; Trapp & Bohn, 2002; Wang, Jongman, & Sereno, 2003; Iverson, Hazan, & Bannister, 2005; Jongman & Wade, 2007; Rato, 2014). This conclusion is further supported by studies of naturalistic learning outside the laboratory, which have shown that the speech perception habits of adults are malleable given sufficient L2 experience. (e.g., Best & Strange, 1992; Bohn & Flege, 1990; Flege et al., 1997).
The well-established fact that perceptual learning mechanisms are intact in L2 learners does not mean that, given sufficient experience with the L2, all L2 learners will develop perceptual habits that are indistinguishable from native speakers of the L2. On the contrary, many L2 learners have persistent perceptual problems in spite of extensive L2 experience, and the difficulty or ease with which nonnatives perceive L2 sounds may differ greatly for different sound types of the L2. This variation in perceptual difficulty has been observed for consonants contrasts (e.g., Polka, 1991, 1992; Bohn, Best, Avesani, & Vayra, 2011), for vowels (e.g., Polka, 1995; Polka & Bohn, 1996; Best, Hallé, Bohn, & Faber, 2003), and for tones (Hallé, Chang, & Best, 2004; Burnham & Mattock, 2007; So & Best, 2010, 2014).
The well-documented perceptual problems of even advanced L2 learners can be classified as being associated with either the perceptual relation of the sounds of the L1 and the L2 (see Section 3) or with universal (i.e., not L1-dependent) perceptual preferences for salient cues which may or may not be appropriate for the perception of L2 sounds. For example, nonnative place-of-articulation contrasts have been shown to be more difficult to perceive than nonnative voicing contrasts. Tees and Werker (1984) reported that their English-speaking participants could successfully be trained to perceive a Hindi voicing contrast (voiceless aspirated vs. breathy-voiced stops), but that training on a nonnative place of articulation (Hindi retroflex vs. dental stops) was not successful. One possible explanation for this is the assumed salience of the relatively long-duration cues signaling voicing differences, as opposed to the short-duration cues signaling place of articulation differences. Polka (1991) further examined native English listeners’ discrimination of the Hindi retroflex versus dental stop contrast in four different voicing contexts and reported that discriminability was partly related to the acoustic salience of place cues in the different voicing contexts.
Another case in point is vowel duration, which Bohn (1995) hypothesized to be used by L2 learners whose L1 experience has desensitized them to spectral differences, such as L1 Spanish speakers, whose L1 has just one vowel, /i/, in an area of the vowel space where English has two, /i/ and /ɪ/. Flege et al. (1997) examined how L1 Spanish speakers identified stimuli from a synthetic /bit/-to-/bɪt/ continuum varying in spectral quality between the /i and /ɪ/ endpoints as well as in vowel duration (long-medium-short). Their responses were based almost exclusively on the duration, not the spectral quality of the stimuli, even though Spanish does not use duration to differentiate vowels. This unexpected result gave rise to the Desensitization Hypothesis, which states that “whenever spectral differences are insufficient to differentiate vowel contrasts because previous linguistic experience did not sensitize listeners to these spectral differences, duration differences will be used to differentiate the nonnative vowel contrast” (Bohn, 1995, pp. 294–295). The Desensitization Hypothesis has been tested and supported in a range of studies involving English, Dutch, and German as L2s, with native speakers of Mandarin, Portuguese, Russian, Polish, Catalan, and Turkish, who all preferred to use duration to differentiate nonnative vowel contrasts even though their L1 does not have vowel duration contrasts (Flege et al., 1997; Rauber, Escudero, Bion, & Baptista, 2005; Escudero & Boersma, 2004; Morrison, 2005; Kondaurova & Francis, 2008; Bogacka, 2004; Cebrian, 2006; Escudero, Benders, & Lipski, 2009; Darcy & Krüger, 2012).
An additional universal preference is captured by the Natural Referent Vowel (NRV) framework developed by Polka and Bohn (2003, 2011, see also Bohn & Polka, 2014). Polka and Bohn found in several infant perception experiments (e.g., Polka & Bohn, 1996; Bohn & Polka, 2001) that vowels that are relatively more peripheral in the vowel space (such as /i/ as opposed to /e/) have a special status in vowel perception: if presented in a change/no change discrimination task as background vowels with less peripheral vowels as foreground vowels, they cause a decrease in discriminability, whereas the opposite direction of presentation (less peripheral as background – more peripheral as foreground) increases discriminability. This perceptual asymmetry, which points to a bias favoring peripheral vowels, is lost if both vowels are functional in the learners’ ambient language, such as /u/ and /y/ for learners of German. However, this bias is maintained in the absence of specific experience, as for L1 English speakers, for whom /u/ is a Natural Referent Vowel in the perception of the nonnative /u/-/y/ contrast. Importantly, the defining characteristic of an NRV is not that it exists in the learner’s L1, but that it is the relatively more peripheral vowel in a vowel contrast. An exciting recent development is presented by research reporting perceptual asymmetries for consonants in infants (Nam & Polka, 2016), native listeners (Bundgaard-Nielsen, Baker, Kroos, Best, & Harvey, 2015) and nonnative listeners (Lai, 2009), which motivated Bundgaard-Nielsen et al. (2015) to propose the Natural Consonant Referent Hypothesis, in which apical consonants are perceptually privileged (i.e., are referents) among coronal consonants. The conclusion from research supporting universal perceptual biases as expressed by the Desensitization Hypothesis or Natural Referent frameworks for consonant and vowel perception is that the relation between the segment inventories of the L1 and the L2, as important as it may be, does not fully account for L2 speech perception and the perceptual problems that L2 learners may encounter.
This section briefly summarized important aspects of research on auditory L2 perception, but it should be mentioned that the contribution of the visual channel to L2 speech perception has also been studied to address the questions of whether visual speech cues can be exploited by L2 learners and whether these cues can be used to increase the efficiency of perceptual training (e.g., Werker, Frost, & McGurk, 1992; Hardison, 1999, 2003, 2005; Hazan, Sennema, Iba, & Faulkner, 2005; Hazan et al., 2006; Navarra & Soto-Faraco, 2007; Wang, Behne, & Jiang, 2008; Chen & Massaro, 2008). This line of research can be summarized as follows. Visual speech gestures can enhance L2 perception, as in the Navarra and Soto-Faraco (2007) study on L1 Spanish speakers’ perception of the L1 Catalan /e/-/ɛ/ contrast, but whether L2 learners make use of visual information depends on the L1 (Hazan et al., 2006). Training with audiovisual stimuli can be more effective than training with auditory stimuli alone, but the effectiveness of visual information seems to depend on the salience of the viseme (Hazan et al., 2005).
1.4 Second Language Phonetics: The Influence of L2 on L1
As mentioned at the beginning of this entry, a proper understanding of second language phonetics includes the influence of the L2 on the L1. One would expect such an influence when language dominance shifts so that the L2 becomes the dominant language, with the extreme result of L1 loss. The consequences of a shift in language dominance have been observed on the segmental level, as in Major’s (1992) study of the influence of L2 Portuguese VOT values (with a prevoiced–short-lag contrast) on L1-English attrited speakers (English has a short-lag vs. long-lag contrast). These consequences have also been observed on the global foreign accent level, as in a study by Yeni-Komshian, Flege, and Liu (2000), who reported that L1 Korean speakers who had started to learn L2 English at a young age and who had resided in the United States for at least 15 years, were perceived to speak their L1 with a foreign accent. Similarly, de Leeuw, Schmid, and Mennen (2010) found that L1 German speakers had developed a foreign accent in their L1 after long-term exposure to either English or Dutch. A particularly interesting case was reported by Mayr, Price, and Mennen (2012), who compared two monozygotic twins, one of which had stayed in the L1 Dutch-speaking environment, whereas the other had changed her L1 accent and evidenced signs of L1 attrition after 30 years of residence in an L2 English-speaking environment.
The influence of the L2 phonetic system on the L1 has also been observed in bilinguals who have not shifted language dominance, as in a study by Ulbrich and Ordin (2014), who reported that L1 German speakers, whose L1 does not have postvocalic /r/, used postvocalic /r/ in their L1 as a consequence of long-term exposure to an English accent with postvocalic /r/. The interaction of two phonetic systems in the minds of bilinguals is amply demonstrated, as in Flege (1987), who found that both highly experienced L1 French speakers of English and highly experienced L1 English speakers of French produced the /t/s in French and in English with VOT values that were in between those of monolingual French speakers (short-lag VOT) and monolingual English speakers (long-lag VOT). However, these phonetic compromises (see also Section 1.2.2) are not the only way in which bilinguals deal with the phonetic systems of their two(+) languages. Guion’s (2003) study of Quichua-Spanish bilinguals found that acquiring L2 Spanish vowels could affect the production of L1 Quichua vowels in that the Quichua vowels were produced higher by bilinguals than by Quichua monolinguals. Guion proposed that this vowel reorganization serves to enhance the perceptual distinctiveness between the vowels of the combined L1 and L2 system. Both processes, assimilation resulting in phonetic compromises (as in Flege, 1987) and dissimilation resulting in enhancement (as in Guion, 2003) account for the fact that “the bilingual is not two monolinguals in one person” (Grosjean, 1989), that is, the sound system of a bilingual will necessarily differ from the sound systems of monolingual speakers of the two languages. The dissimilation of phonetic categories as observed by Guion (2003) is also the most likely reason why the /ɛ/ vowel, which is shared by English and Portuguese, was produced in a less English-like fashion by experienced than inexperienced native Brazilian Portuguese speakers (Major, 1987a; see also section 1.2.1 and Bohn & Flege, 1992). Apparently, the experienced learners in both the Major (1987a) and the Bohn and Flege (1992) studies produced an English /ɛ/ vowel that was polarized away from the newly established English /æ/ vowel, resulting in productions of English /ɛ/ which were higher in the vowel space than the fairly low L1 English /ɛ/ productions.
Interestingly, the mutual influence of the L1 on the L2 and the L2 on the L1 has not just been documented for L2 learners with long-term experience with the nonnative language, but also after quite brief amounts of exposure. Sancier and Fowler (1997) reported that a stay of a few months’ length in either the United States or in Brazil affected the VOT values of an L1 Portuguese speaker every time she traveled between the two countries. Chang (2012) found that a six-week course in Korean (in Korea) affected monolingual English speakers’ production of English consistently in terms of assimilation to phonetic properties of Korean, “at segmental, subsegmental, and global levels, often simultaneously.” These studies are of great interest not just for an understanding of L2 phonetics (showing how little exposure is sufficient to change phonetic systems), but for psycholinguistics in general, as they demonstrate how sensitive and adaptive speakers’ phonetic systems can be.
1.5 The Relation Between L2 Speech Perception and Production
Current models of L2 phonetics, such as Flege’s (1995) Speech Learning Model, postulate that the production problems of L2 learners are perceptually based. The implications of this assumption are that correct perception of L2 sounds is a necessary (but not sufficient) prerequisite for correct production, and that L2 learners should initially be better at perceiving L2 sounds than at producing them. The problem with any attempt to examine claims about the relation between perception and production is that there is no common metric for perceptual and production abilities except perhaps measures of nativelikeness of nonnative perception and production. Keeping this in mind, the picture derived from research on this relation in L2 phonetics is quite confusing. On the one hand, there is evidence from perceptual training studies and from studies of naturalistic L2 learning suggesting either a general perception-production link in L2 speech or a situation in which improved perceptual abilities cause improved L2 speech production. For example, Flege et al. (1999a) reported a significant correlation between the measures of L1 Italian speakers’ L2 English vowel production and perception, and Crowther and Mann (1992) found a “concordance” between the use of vowel duration in production and sensitivity to vowel duration in perception in a study of English final consonant voicing by L1 Mandarin and L1 Japanese speakers. Several training studies also support the perception-production link in L2 speech learning, because successful perceptual training alone led to improved production (e.g., Bradlow et al., 1997; Wang et al., 2003). Additional support for the view that L2 learners are better able to perceive than to produce L2 sounds comes from studies which examined the nativelikeness of perception and production of a nonnative contrast (e.g., Eger & Bohn, 2015) and from studies which examined the ability of L2 learners to detect a foreign accent. For example, Neufeld (1980) reported that L1 English speakers who produce L2 French with a foreign accent could detect a foreign accent in French speech (see also Højen, 2002 for a similar result showing L1 Danish speakers’ ability to detect a Danish accent in English).
On the other hand, there are several studies which report accurate production of L2 sounds by L2 learners who do not accurately perceive the same sounds (e.g., Briere, 1966; Goto, 1971; Sheldon & Strange, 1982; Tees & Werker, 1984; Gass, 1984; Flege & Eefting, 1987; Takagi & Mann, 1995; see also Ladefoged, 1967). The finding that “perceptual difficulties often persist for those . . . who have already learned to produce the phonemes appropriately” (Tees & Werker, 1984, p. 589) appears paradoxical and has not yet found a satisfactory explanation. A better understanding of this phenomenon may come from studies which examined the production-perception relationship at different stages of L2 learning, e.g., Slawinski and MacNeil (1994) and Bohn and Flege (1997). The Slawinski and MacNeil study suggests that it may take learners time to align L2 production and perception and that a mismatch between production and perception abilities is typical of the early stages of L2 learning. Sheldon (1985) offers as an explanation for the fact that productive abilities may be better than perception abilities in L2 speech learning that mispronunciations are more noticeable and more stigmatizing than non-nativelike perception, which provides learners with a stronger motivation to improve performance in production. A possible conclusion from these studies is that the units for production and perception are different in the early stages of acquisition (de Jong, Hao, & Park, 2009), but become aligned later in advanced L2 learners.
2. Consequences of Second Language Phonetics
This section presents an overview of the consequences of producing nonnative speech sounds with a foreign accent. These consequences can be primarily linguistic–communicative (e.g., reduced intelligibility) or they can affect the social relationship between nonnative speaker and listener (e.g., through stereotyping). However, this is not a clear-cut dichotomy, as reduced intelligibility can affect the social relationship between speaker and listener (Giles, 1970), and social stereotypes about nonnative speakers can affect comprehensibility (Rubin & Smith, 1990).
2.1 Linguistic–Communicative Consequences
Research on the consequences of foreign-accented speech supports the everyday experience that nonnative speech may be less intelligible than native speech, which can be due to segmental errors (e.g., Bohn & Bundgaard-Nielsen, 2009), errors in stress placement (e.g., Hahn, 2004; Field, 2005), the temporal patterning of speech (e.g., Tajima, Port, & Dalby, 1997), or nonnative prosody in general (e.g., Bannert, 1984). However, there are two important lines of research which have shown that it is not the case that a foreign accent per se reduces intelligibility. The first of these is associated with a series of studies by Munro and Derwing, who examined the relationship between foreign accentedness, intelligibility (the degree to which a message is heard as intended), and comprehensibility (the degree of effort perceived by the listener in attempting to understand the message). This research has clearly shown that the three dimensions accent, intelligibility, and comprehensibility are related but quasi-independent: “Even heavily accented speech is sometimes perfectly intelligible” (Munro & Derwing, 1995a; see also Derwing & Munro, 1997; Munro, 2008). In addition, Munro and Derwing (1995b) showed that processing time for foreign-accented statements (which can be interpreted as an objective measure of subjective comprehensibility) was not related to degree of foreign accent. An important practical implication from Derwing and Munro’s research is that accent reduction does not automatically entail improved intelligibility.
The second line of research that has addressed the relation between foreign accentedness and intelligibility is concerned with the mutual intelligibility of foreign-accented speech in listeners with different L1s or with the same language background as the speakers. Bent and Bradlow (2003) examined how well native speakers of “Chinese,” Korean, and English understood English utterances produced by native speakers of these languages. They reported not just a “matched interlanguage speech intelligibility benefit” (e.g., the intelligibility of Korean-accented English was highest for L1 Korean listeners), but also a “mismatched interlanguage speech intelligibility benefit” (the L1 “Chinese” listeners found Korean-accented English more intelligible than native-accented English). However, the existence and the nature of the interlanguage speech intelligibility benefit (ISIB) is still a matter of debate, which suggests that a potential benefit may depend on the proficiency of the speakers and on whether the benefit is examined for speakers or listeners. Stibbard and Lee (2006), for example, reported a mismatched interlanguage speech intelligibility detriment for low-proficiency L1 Korean and Saudi speakers of English. Hayes-Harb et al. (2008) presented evidence of a matched ISIB for listeners (i.e., native Mandarin listeners were more accurate than native English listeners at identifying Mandarin-accented English), but failed to find evidence of an ISIB for talkers (i.e., native Mandarin listeners did not find Mandarin-accented English speech more intelligible than native English speech), just like Munro, Derwing, and Morton (2006), whose L1 Cantonese, Japanese, Mandarin, and English listeners did not exhibit an intelligibility benefit for English speech produced in their own accent. A possible reason for the partly conflicting results regarding the existence of a matched or mismatched ISIB could be the differences in materials used (e.g., extemporaneous narratives in Munro et al., 2006 vs. read sentences in Bent & Bradlow, 2003).
2.2 Social Consequences of Foreign-Accented Speech
The fact that foreign-accented speech may have negative social consequences is very well documented (for overviews, see Lippi-Green, 1997; Munro, 2003; Lindemann, 2003). Foreign-accented speech may cause negative stereotyping (e.g., Ryan et al., 1977), typically on the social dimension of status and less on the solidarity dimension (e.g., Brennan & Brennan, 1981; Hosoda, Stone-Romero, & Walter, 2007), which can affect the general employability (Carlson & McHenry, 2006), the specific salary (Davila, Bohara, & Saenz, 1993), and the credibility (Lev-Ari & Keysar, 2010) of the nonnative speaker.
Research by Kinzler and colleagues suggests that the bias against a nonnative accent is learned early in life (Kinzler, Dupoux, & Spelke, 2007; Kinzler, Shutts, DeJesus, & Spelke, 2009). Top-down processes clearly contribute to this bias, as in the Babel and Russell (2015) study, in which the presentation of a visual prime (a picture of an ethnic Chinese or “White” speaker) affected the accentedness ratings for auditorily presented sentences (see also Rubin & Smith, 1990; Rubin, 1992). However, there is also evidence that the signal alone may drive the social evaluation of foreign-accented speech, especially on the level of prosody, as in studies which reported on the emotive impact of nonnative intonation (e.g., Cunningham-Andersson & Engstrand, 1989; Holden & Hogan, 1993).
Interestingly, there are also studies which suggest that foreign-accented speech does not always have negative social consequences. Albrechtsen, Henriksen, and Færch (1980) reported no effect of accentedness on the judgment of speaker personality for their L1 Danish participants, and Cargile (2000) presented evidence from “Chinese”-accented English that contradicts the generalization that nonnative accents cause its speakers to be judged less suitable for high status jobs and more suitable for low-status jobs (as, for instance, in the Kalin & Rayko, 1978 study). These studies, like the Cargile and Giles (1998) study on the social evaluation of Japanese-accented English, suggest that top-down processes which are based on non-linguistic stereotypes about speaker groups may play an important role in social evaluation of nonnative speech, at least for relatively high-proficiency nonnative speakers.
3. Causes of L2 Phonetics
This section presents an overview of the most important factors that shape L2 speech perception and production, namely, the L1of L2 speakers, their age at the onset of learning (AOL), and the quality and quantity of experience with their two(+) languages, including language use patterns. A range of additional potential factors has been claimed to affect the perception and production of L2 speech but will not be reviewed here because they are much less important (for a more detailed review of factors affecting degree of foreign accent in an L2, see Piske, MacKay, & Flege, 2001). An important exception is musical ability or musical training, which has been shown to facilitate the perception and production of nonnative tones (e.g., Gottfried, 2007) as well as other unfamiliar speech contrasts (e.g., Slevc & Miyake, 2006). One of the factors that has been discussed in great detail in general L2 learning research is motivation (e.g., Gardner & Lambert, 1972), but research on L2 speech learning has shown a negligible contribution of integrative and other such motivational variables (Purcell & Suter, 1980; see also Oyama, 1976; Thompson, 1991; Flege et al., 1995, 1999b; Yeni-Komshian et al., 2000).
3.1 The Native Language (L1)
Clearly the most important and conspicuous factor in L2 phonetics is the speaker’s L1 or, rather, the relation between the sound systems of the L1 and the L2. It is probably safe to say that any study that has compared the perception and/or production of L2 speakers with different L1s has found differences that are attributable to the L1s, such as Flege et al. (1997), which compared the perception and the production of English front vowels by L1 speakers of German, Spanish, Korean, and Mandarin, or Bohn and Best (2012), which compared the perception of American English approximants by L1 speakers of Danish, German, French, and Japanese. Models of L2 speech attempt to predict the perception and production of L2 sounds based on the perceived relation between the sounds of the L1 and the L2. The most widely used models are Flege’s (1995, 2003) Speech Learning Model (SLM) and Best’s (1995) Perceptual Assimilation Model (PAM) and its extension to L2 speech learning, PAM-L2 (Best & Tyler, 2007), which have both been tested in a large number of studies on L2 phonetics. For other more recently developed models which have not been tested as extensively as the SLM and PAM(-L2), see Strange (2007, 2011) on the Automatic Selective Perception Model (ASP), Kuhl et al. (2008) on the Native Language Magnet theory-expanded (NLM-e), Escudero and Boersma (2004) on the Second Language Linguistic Perception Model (L2LP), and Major and Kim (1996) on the Similarity Differential Rate hypothesis (SDRH).
The SLM is originally a model of L2 speech production; however, it can be and has been extended to L2 speech perception. The basic assumption of the SLM is that “the mechanisms and processes used in learning the L1 sound system . . . remain intact over the life span, and can be applied to L2 learning” (Postulate 1 of the SLM: Flege, 1995, p. 239). Learning problems of L2 learners can be predicted from the perceptual relationship between the sounds of the L1 and the L2. This relationship is arranged along a continuum between identical sounds (e.g., [m] in English and in German), similar sounds (e.g., English [ʉ] and German [u]), and new sounds (e.g., English [ɻ] for L1 Japanese speakers). L2 sounds that are perceived to be similar to L1 sounds will not be learned correctly, because they are subject to equivalence classification, which blocks category formation. However, the formation of categories for sounds of the L2 is possible for those sounds that evade equivalence classification because they do not have easily identifiable L1 counterparts. The SLM rejects maturational accounts of age effects and attributes these effects instead to the increasing likelihood with which L2 sounds will be perceived as similar, because L2 learners become more familiar with pronunciation variants of the sounds of their L1 as they become older, increasing the range of tokens that are accepted as members of a given category.
The PAM was originally developed as a model of cross-language speech perception by naive listeners, that is, listeners with no L2 experience. It aims to account for why the degree of perceptual difficulty varies considerably across nonnative contrasts by assuming that listeners assimilate L2 phones to L1 phonological categories that are closest in terms of phonetic similarity. The model describes six assimilation types and predicts that difficulty or ease of cross-language speech perception, as reflected in discrimination levels for nonnative contrasts, depends on how contrasting phones are assimilated. For instance, if English [w] and [ɻ] are assimilated to German /v/ and /ʁ/, respectively, this “Two Category” assimilation type will result in excellent discrimination of the English contrast by L1 German speakers, as in Bohn and Best (2012). On the other hand, the “Single Category” assimilation of Mandarin [tʰ] and [tsʰ] to just one L1 category, Danish /tsʰ/, with no difference in goodness-of-fit ratings for [tʰ] and [tsʰ], will result in poor discrimination, as in Rasmussen and Bohn (2017). Other assimilation types are predicted to result in discrimination levels in between those for the Two Category and Single Category assimilation types. PAM has been extended by Best and Tyler (2007) to PAM-L2, which aims to account for the perceptual changes that occur in L2 learning. An important characteristic of PAM-L2 is that it considers the L2 learner’s aim to learn the higher-order invariants (phonemes) of the L2, which are the building blocks of the L2 lexicon.
Obviously, an encyclopedia entry cannot provide a comprehensive description of the SLM and PAM (or other models), so the interested reader is referred to original sources. The two models briefly described in this section have inspired a large number of studies on L2 phonetics, which have provided considerable support for the basic assumptions of these models, based on experiments examining the L2 phonetics of a broad range of segment types, such as different consonants (e.g., Flege, 1991; Best & Strange, 1992; Hallé, Best & Levitt, 1999; Guion, Flege, Akahane-Yamada, & Pruitt, 2000; Best, McRoberts, & Goodell, 2001; Harnsberger, 2001; Bohn & Best, 2012; Bohn et al., 2011; Garibaldi & Bohn 2015), vowels (e.g., Flege, 1987; Bohn & Flege, 1990; Polka & Bohn, 1996; Flege et al., 1997; Flege et al., 1999a; Guion, 2003; Bundgaard-Nielsen et al., 2011; Garibaldi & Bohn, 2017), and tones (e.g., Hallé et al., 2004; Sereno & Wang, 2007; So & Best, 2010, 2014; Tyler et al., 2014). Several of these studies have directly compared the predictive power of hypotheses generated by the SLM and PAM, e.g., Guion et al. (2000) and Bohn and Best (2012), which confirmed the basic assumption of these models but also pointed to necessary modifications of, or additions to, these models in the light of new data.
Models of L2 speech learning are primarily concerned with the influence of the L1 at the level of the individual segment (SLM) or phonological contrast (PAM). However, L1 influence has also been documented at more general levels, such as the overall size of the L1 vowel inventory, which was positively correlated with identification accuracy for L2 vowels in a study by Iverson and Evans (2007), or the linguistic distance between L1 and L2, which Wieling, Bloem, Baayen, and Nerbonne (2014) found to be a significant predictor of the degree of foreign accentedness using an adaptation of a distance metric developed by Levenshtein (1966). Other systemic influences of the L1 on L2 perception have been reported by Pajak and Levy (2014), who found that experience with contrastive duration for one segment type (e.g., vowels) aids the perception of a duration contrast for a nonnative segment type (e.g., consonants). Bohn and Best (2012) hypothesized that the consistently high discrimination of the American English /w/-/j/ contrast by native speakers of Danish, German, and French (whose discrimination accuracy was better than that of L1 English listeners) could be due to the use of contrastive rounding for vowels in Danish, German, and French, noting that /w/ and /j/ differ importantly in terms of lip rounding.
Finally, it should be pointed out that there are a number of phenomena in L2 phonetics that are not easily explained by referring to L1 influence, whether on the segment, contrast, feature, or inventory level. For instance, Wode (1977) reported that L1 German learners initially substituted [w] for English [ɻ], even though German has no [w]. Interestingly, English-learning children likewise substitute [w] for [ɻ] (Edwards, 1971). Further instances of what has been described as developmental (versus transfer) processes are the motivation behind Major’s (1986) Ontogeny Model. In addition, the Desensitization Hypothesis (Bohn, 1995), the Natural Referent Vowel framework (Polka & Bohn, 2011), and the Natural Consonant Referent Hypothesis (Bundgaard-Nielsen et al., 2015), which were presented in section 1.3, are all motivated by L2 speech phenomena that cannot be accounted for in terms of L1 influence but are likely universal.
3.2 Age at the Onset of Learning
For L2 learners with the same language background, age at the onset of learning (AOL) is the single most important determinant of how well the sound system of an L2 will be mastered. However, research on L2 phonetics provides no support for maturational accounts of age-related changes in the ability to perceive or produce the sounds of the L2 correctly. Maturational accounts inspired by Lenneberg’s (1967) Critical Period Hypothesis predict a clearly defined age (or age range) which separates a “before” with full phonetic learning abilities from an “after” with reduced phonetic learning abilities. Additionally, the age at which the difference between “before” and “after” emerges should be accompanied by neurophysiological changes which cause this difference. The clear evidence against this assumption comes primarily from two kinds of production studies. The first of these are large-scale studies by Flege and his collaborators on the relation between AOL and degree of perceived global foreign accent (Flege & Fletcher, 1992; Flege et al., 1995, 1999b; Yeni-Komshian et al., 2000). These studies have not shown a sharp increase in foreign accentedness, which would signal the passing of a critical period, but rather a gradual age-related decline of the ability to avoid a foreign accent, which is already observable at an AOL of five to eight years and continues to decline into the third decade of life. While these and other studies suggest that extensive and early experience with an L2 does not completely override L1-shaped phonetic habits (see also Sebastián-Gallés & Soto-Faraco, 1999; Hopp & Schmid, 2013), other studies have demonstrated that at least some adults can achieve nativelike pronunciation, either for specific segments like English [ɻ] and [ɫ] for Japanese adults (Flege, Takagi, & Mann, 1995), for segmental properties like VOT and vowel duration (L1 English learners of French in Birdsong, 2007), or in terms of global foreign accent (Bongaerts, Van Summeren, Planken, & Schils, 1997; see also Bongaerts, 1999). However, the large majority of all L2 learners do not perceive or produce the L2 in a nonnative fashion. The most appropriate conclusion from studies on the age factor in L2 speech learning is in line with the basic assumptions of the SLM: even though all humans possess the same general capability for speech learning, irrespective of whether this is the L1, L2, or L3, the ability to learn L2 sounds typically depends on the extent of L1 development. That is, AOL is very closely related to the likelihood of discerning differences between L1 and L2 sounds, and it is this likelihood, not biological age, that modulates the human speech-leaning ability.
3.3 L2 Experience
An easily applied but very coarse index of L2 experience is length of residence (LOR), which has been used in several studies to classify learners as “experienced” or “inexperienced” (e.g., Bohn & Flege, 1990, 1992). Keeping in mind that the time spent in a nonnative environment is not necessarily a valid index of the more relevant measures, quality and quantity of experience with the L2 and the L1, studies that have examined the effect of LOR on L2 phonetic abilities have yielded at least two interesting general insights on L2 speech learning. The first of these is that even after many years of LOR (15 years in the Flege et al., 1995 study, and 19 years in the Derwing, Munro, Foote, Waugh, & Fleming, 2014 study), phonetic learning still continues, which suggests that ultimate attainment in L2 learners is best characterized as an asymptote of learning, not a fossilized state. The second important insight derives from studies which have shown early experience with the sound system of a language, followed by an extended period of no experience with that language (as in international adoptees), provides a benefit in phonetic competence as revealed by, e.g., rapid reacquisition, suggesting that phonetic knowledge, once acquired, will never be lost completely (Au, Knightly, Jun, & Oh, 2002; Knightly, Jun, Oh, & Au, 2003; Oh, Jun, Knightly, & Au, 2003; Singh, Liederman, Mierzejewski, & Barnes, 2011; Choi, Cutler, & Broersma, 2017).
Language use patterns, which are likely to change over time, are arguably a more valid measure of L2 experience than LOR. The effect of language use patterns, especially of L1 maintenance, has been studied in great detail by Flege and his collaborators (e.g., Flege, Frieda, & Nozawa, 1997; MacKay, Meador, & Flege, 2001; Piske, Flege, MacKay, & Meador, 2002). An important result from these studies is that the positive effect of an early AOL on L2 phonetic abilities will be attenuated by continued L1 use: “If the L1 remains strong and active, it will influence the identification of at least some phonetic segments in an L2, even if the L2 is learned in childhood” (MacKay et al., 2001, p. 121). The importance of language use patterns is further highlighted in a study by Derwing and Munro (2013), who examined whether and how changes occurred in two nonnative speaker groups’ L2 English comprehensibility, fluency, and accentedness. Even though both L1 Mandarin speakers and L1 Slavic (Ukrainian and Russian) speakers had an LOR of seven years in Canada, they differed greatly: “The Mandarin L1 speakers showed no change over time on any of the dimensions, while the Slavic language L1 speakers improved significantly in comprehensibility and fluency” (Derwing & Munro, 2013, p. 163). The authors suggest that this striking difference is at least partly due to their L1 vs L2 language use patterns.
4. Conclusion: The Past and the Future of L2 Phonetics
This article has aimed to give an overview of what is known about the characteristics, the consequences, and the causes of L2 speech. Interest in these research areas dates back from at least the late 19th century (e.g., Hale, 1885; Boas, 1889, Viëtor, 1894), with some sporadic interest in the first half of the 20th century (e.g., Polivanov, 1931), and with a slowly but steadily rising interest in the second half of the 20th century (e.g., Briere, 1966; Stevens, Liberman, Studdert-Kennedy, & Öhman, 1969). Presently, the field of L2 phonetics has become one of the most active and productive areas of both psycholinguistics and phonetics, as witnessed by the proportion of articles on L2 phonetics in leading journals of these fields and by the constantly increasing number of participants at the New Sounds conference series, which is dedicated to L2 phonetics (http://conferences.au.dk/newsounds2016/).
The large body of research that has accumulated on L2 phonetics has not merely provided important insights into the characteristics, consequences, and causes of L2 speech, it has also contributed importantly to psycholinguistics and phonetics in general, for instance by providing overwhelming evidence that human language learning ability is not limited by some neurologically based critical period. However, many important questions which are not of interest only to those working in the field of L2 phonetics either remain unanswered or require more in-depth research. Without any attempt to be exhaustive or systematic, the questions concern:
• the perception-production relationship in L2 learning;
• the most valid way to establish cross-language similarity (which is an important predictor of speech learning success/failure);
• the continued search for universal biases in (L2) speech perception (in addition to those expressed by, e.g., the Natural Referent Vowel framework);
• the identification of higher-order influences on L2 phonetics, and the integration of these influences in models of L2 phonetic learning; and
• L2 speech learning in mature populations, to test the claim that speech learning mechanisms and processes remain intact over the life span. (This article has reviewed the convincing reasons for this claim, which, however, has only been supported in studies with young adults under the age of 40 years.)
An update of this entry on L2 phonetics in 10 years is likely to contain new insights on these and other questions, as well as new questions that will continue to attract bright minds from linguistics, psychology, and neighboring areas such as education, engineering, and neuroscience.
Work on this article was in part supported by the Carlsberg Foundation and Inge Lehmanns Legat af 1983. I thank Camila L. Garibaldi, Anna B. Jespersen, Sidsel Rasmussen, and the reviewers for valuable comments on an earlier draft.
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