Documenting and analyzing dialect variation is traditionally the domain of dialectology and sociolinguistics. However, modern approaches to acoustic analysis of dialect variation have their roots in Peterson and Barney's [(1952). J. Acoust. Soc. Am. 24, 175–184] foundational work on the acoustic analysis of vowels that was published in the Journal of the Acoustical Society of America (JASA) over 6 decades ago. Although Peterson and Barney (1952) were not primarily concerned with dialect variation, their methods laid the groundwork for the acoustic methods that are still used by scholars today to analyze vowel variation within and across languages. In more recent decades, a number of methodological advances in the study of vowel variation have been published in JASA, including work on acoustic vowel overlap and vowel normalization. The goal of this special issue was to honor that tradition by bringing together a set of papers describing the application of emerging acoustic, articulatory, and computational methods to the analysis of dialect variation in vowels and beyond.

The history of research on dialect variation in the Journal of the Acoustical Society of America (JASA) follows closely on Peterson and Barney's (1952) ground-breaking work on vowel variation, although a smaller number of papers in JASA examine dialect variation in consonants and suprasegmentals. Further, despite Peterson and Barney's (1952) convincing demonstration of acoustic vowel variation across talkers and Ladefoged and Broadbent's (1957) explicit acknowledgment of the role of social factors, such as dialect background, in generating this kind of variation, most of the scholarship on dialect variation that has been published in JASA has appeared in the last 25 years.

The legacy of Peterson and Barney's (1952) work on the analysis of dialect variation is most obvious in a series of papers by Hillenbrand et al. (1995), Hagiwara (1997), and Clopper et al. (2005). Hillenbrand et al. (1995) conducted a replication of Peterson and Barney's (1952) study with the explicit goal of addressing some of the shortcomings of the original work, including the issue of regional dialect. Whereas Peterson and Barney (1952) recorded talkers from all over the United States, and even a small number of talkers who were born outside of the United States, Hillenbrand et al. (1995) selected their participants from the Northern Midwestern United States to constrain the regional dialect of their talkers to the Northern variety of American English. Hagiwara (1997) conducted a second replication of Peterson and Barney's (1952) study with talkers from Southern California, with the explicit goal of comparing his results to those obtained by Peterson and Barney (1952) and Hillenbrand et al. (1995) to consider the effects of regional dialect on vowel production. Hagiwara (1997) observed substantial differences in the vowel systems of the talkers in each of the three studies, including not only differences in the acoustic properties of particular vowels but also fundamental differences in the shape of the acoustic vowel space across dialects. Hagiwara (1997) concluded with the suggestion that other researchers contribute to our understanding of regional vowel variation through similar replications of their own. Clopper et al. (2005) heeded this proposal and furthered the comparison of vowel systems across dialects through an analysis of the vowel systems of talkers from six regional dialects of American English (New England, Mid-Atlantic, North, Midland, South, and West). We found further evidence of the variation that Hagiwara (1997) had noted, as well as some novel variation that had not been reported previously in JASA or elsewhere.

Beyond American English, descriptions of the vowel systems of regional varieties of a number of other languages have found a publication home in JASA since Hillenbrand et al. (1995) resurrected interest in this area among speech scientists. These studies include acoustic analyses of the vowel systems of Israeli Arabic (Amir et al., 2014), Dutch in the Netherlands and Belgium (Adank et al., 2004a, 2007), British English (Williams and Escudero, 2014), Saterland Frisian (Schoormann et al., 2017), European and Brazilian Portuguese (Escudero et al., 2009), and Iberian and Peruvian Spanish (Chládková et al., 2011). In all of these studies, the basic methodological approach follows Peterson and Barney's (1952) proposal. Thus, Peterson and Barney's (1952) pioneering work on the acoustics of vowels provided the methodological foundation for numerous substantive descriptive contributions to the literature on acoustic-phonetic dialect variation.

In contrast to this growing body of work on vowel acoustics within and across dialects, studies in JASA on the topic of dialect variation in consonants have been limited primarily to descriptive analyses of stop consonants, despite speech scientists' long history of conducting research on consonant acoustics and variation. The studies published in JASA on consonant variation include work on stop releases in American English (Byrd, 1992), pre-nasalization of voiced stops in Greek (Kong et al., 2012), and stop bursts in Greek (Themistocleous, 2016). Studies in JASA describing suprasegmental dialect variation are similarly fairly limited, but reflect a wide range of phenomena, including speaking rate in American English (Byrd, 1992), rhythm in German (Dellwo et al., 2015), overall pitch level in Mandarin (Deutsch et al., 2009), and intrinsic f0 in American English (Jacewicz and Fox, 2015). Thus, the descriptive contributions in JASA to dialect variation research are beginning to advance beyond vowel variation to other linguistic domains, in which speech scientists also have methodological and theoretical expertise.

In parallel with the renewed interest in the last 2 decades among speech scientists in documenting acoustic-phonetic dialect variation, the perception of this variation has also received increasing attention in JASA in recent years. Together, this body of work has revealed the critical role of dialect experience in shaping perceptual dialect similarity (Clopper et al., 2006, on American English), cross-dialect vowel perception (Chládková and Escudero, 2012, on European and Latin American Spanish and Portuguese; Dufour et al., 2007, 2010, 2016, on French; Evans and Iverson, 2004, on British English; Jacewicz and Fox, 2012, on American English; Lengeris, 2016, on Greek), cross-dialect consonant perception (Tuinman et al., 2011, on American vs British English), and the perception of suprasegmental variation (Brunelle, 2012, on register in Cham; Kirby, 2010, on tone in Vietnamese). Thus, the contributions in JASA to our understanding of dialect variation go beyond descriptions of acoustic-phonetic variation to demonstrations of the effects on speech processing of this variation and listeners' experience with it.

Over the last 2 decades, these acoustic and perceptual descriptive contributions have been accompanied by several influential methodological advances in the acoustic analysis of vowel variation. The most impactful of these recent contributions is the thorough evaluation of acoustic vowel normalization algorithms by Adank et al. (2004b), who provided a solution to a long-standing problem in the analysis of acoustic vowel variation: how to minimize the effects of physiological variation without losing critical social information. The recommendations of Adank et al. (2004b) regarding acoustic vowel normalization have been broadly embraced by the field, as reflected in the nearly 100 citations to their work in venues including the Journal of Sociolinguistics and Language Variation and Change, two of the leading journals for sociolinguistic research.

A second set of methodological advances incorporates temporal information as a dimension of variation, extending our understanding of vowel variation beyond midpoint or steady-state formant frequencies. One of these methods involves quantifying the degree of acoustic overlap of neighboring vowel categories to explore how temporal and spectral information combine to produce acoustic vowel category distinctions across dialects (e.g., Brubaker and Altshuler, 1959, on American English; Fridland et al., 2014, on American English; Wassink, 2006, on American and Jamaican English). This approach also has obvious applications for analyzing vowel category mergers and the structure of the overall vowel system, further expanding on Hagiwara's (1997) consideration of vowel space shape. Another one of these methods considers the trajectories of formant movement over the course of the vowel and has revealed variation in vowel-inherent spectral change, including the shape, direction, and magnitude of formant trajectories, in both monophthongs and diphthongs across dialects (e.g., Elvin et al., 2016, on Australian English; Fox and Jacewicz, 2009, on American English; Williams and Escudero, 2014, on British English).

A third set of methodological advances expands the scope of vowel variation research beyond acoustic comparisons across dialects to consider how dialect variation interacts with other sources of acoustic-phonetic vowel variation and with vowel perception. First, several studies have extended descriptions of vowel variation beyond isolated CVC contexts to consider the effects of segmental (Chládková et al., 2011, on Iberian and Peruvian Spanish), lexical (Clopper and Tamati, 2014, on American English), and semantic (Clopper and Pierrehumbert, 2008, on American English) context on dialect variation. This work highlights the complexity of vowel variation, especially in the context of natural speech, which exhibits far more variability with respect to these factors than the lab-based speech that has typically been examined by speech scientists. Second, although the previous perceptual work has relied primarily on traditional methods in speech science, such as identification (e.g., Jacewicz and Fox, 2012), discrimination (e.g., Kirby, 2010), speeded classification (e.g., Brunelle, 2012), and lexical decision (e.g., Dufour et al., 2007), Evans and Iverson (2004) developed a novel best-exemplar vowel perception task to explore the effects of perceptual adaptation and listener experience on vowel perception. This method is particularly well-suited to studies examining the connections between the perception and production of variation, allowing for a more complete understanding of the nature of the cognitive representations of dialect variation. All of these recent methodological contributions to the study of dialect variation that have appeared in JASA reflect the strengths of speech scientists in theoretical acoustic-phonetics, quantitative analysis, and behavioral perceptual methods. The papers in this special issue continue this tradition of finding novel ways to apply the analytical tools of speech science to the study of acoustic dialect variation.

The 12 papers in this special issue present novel applications to the study of dialect variation of emerging articulatory methods and a range of computational and statistical techniques. The papers also exhibit breadth in their coverage of linguistic phenomena. Together, they explore variation in vowels and consonants in a range of human languages, including Dutch, German, Catalan, and Englishes from around the world, as well as in humpback whale song.

Four of the papers in the special issue, by Strycharczuk and Scobbie, by Mielke et al., by De Decker and Mackenzie, and by Ximenes et al., illustrate how traditional acoustic methods and modern articulatory methods, including ultrasound and electromagnetic articulography (EMA), can be combined to advance our understanding of several well-documented dialect variants. Strycharczuk and Scobbie compared acoustic measures of vowel fronting (F2) with ultrasound measures of tongue position in their examination of back-vowel (/u, ʊ/) fronting in Southern British English. Their inclusion of a range of consonantal contexts among their stimulus materials further allowed for an examination of segmental effects on the realization of these variants in both the acoustic and articulatory domains. Mielke et al. compared acoustic measures of /æ/ raising (F1) with ultrasound measures of tongue height across varieties of North American English. Mielke et al. further applied a principal components analysis to the ultrasound data to allow for a consideration of temporal dynamics in articulation. De Decker and Mackenzie compared acoustic measures of /l/ velarization in syllable initial and final positions with ultrasound measures of tongue retraction in Newfoundland English. Their focus on a consonantal variable (/l/) demonstrates the potential impact of the combined acoustics-articulatory approach for non-vowel variables. Finally, Ximenes et al. compared acoustic measures of vowel height (F1) and backness (F2) with EMA of tongue height and tongue position for ten monophthongs in Australian and North American English. The EMA data further permitted an investigation of lip rounding, allowing Ximenes et al. to consider a third articulatory dimension in the interpretation of the acoustic results. In addition to their individual methodological contributions, all four of these studies revealed mismatches between the acoustic data and the articulatory data, suggesting that articulatory data may be critical for understanding acoustic-phonetic variation within and between dialects.

The other two papers in the special issue that involved articulatory methods, by Rodriguez and Recasens and by Wieling and Tiede, respectively, demonstrate the potential for articulatory measures to reveal other dimensions of dialect variation. First, Rodriguez and Recasens used ultrasound to explore lingual coarticulatory resistance across ten consonants and five vowels in Catalan. Similar to the work by Adank et al. (2004b) on acoustic vowel normalization, Rodriguez and Recasens compared six different measures of coarticulatory resistance that have been proposed in the literature and made recommendations regarding the most promising measures for exploring dialect variation in this domain. Second, Wieling and Tiede used EMA to explore baseline articulatory settings during pauses in two regional varieties of Dutch. Their results extend previous work using ultrasound by Wilson and Gick (2014) demonstrating different articulatory settings across languages in bilingual speakers to an EMA-based analysis of between-speaker regional variation in articulatory settings. Thus, the papers by Rodriguez and Recasens and by Wieling and Tiede provide the methodological foundation for exploring two additional sources of phonetic dialect variation beyond segmental variables.

The remaining six papers in the special issue are more diverse in their methodological approaches, but together they involve a range of statistical and computational techniques for quantifying and interpreting acoustic dialect variation. Three of the papers, by Jannedy and Weirich, by Renwick and Olsen, and by Brown and Wormald, provide illustrations of novel applications of existing analytical techniques to dialect variation. First, Jannedy and Weirich used a spectral moments analysis and a discrete cosine transformation (DCT) analysis, which has previously been used to analyze vowel variation across dialects (e.g., Watson and Harrington, 1999; Williams and Escudero, 2014), to explore variation in sibilant fricatives in three varieties of German, including two varieties spoken in Berlin. Jannedy and Weirich further supported their claims about the usefulness of DCTs over spectral moments with perceptual data. Second, Renwick and Olsen's study drew on previous work using historical corpora (e.g., McCarthy, 2010) and on Hall-Lew's (2010) proposal to apply Pillai scores obtained from MANOVAs in the interpretation of vowel mergers. Renwick and Olsen used these techniques to look for evidence of the Southern Vowel Shift and African American English features in recordings of speakers from a small area in southeastern Georgia from the Linguistic Atlas of the Gulf States. Finally, Brown and Wormald investigated the usefulness of the ACCDIST model of automatic accent recognition (Huckvale, 2004) for addressing questions about dialect variation itself, beyond simply identifying the varieties. They observed that the model output could be used to reveal similarities within and between two ethnic varieties in each of two regions in the United Kingdom, as well as the relevant linguistic features that distinguish among the varieties. Thus, all three of these papers demonstrate the benefits of extending and combining existing methods to obtain novel insights into dialect variation, even in relatively small geographic areas.

The remaining three papers in the special issue, by Kiefte and Nearey, by Fox and Jacewicz, and by Garland et al., respectively, all describe novel extensions of previous computational methods for the analysis of dialect variation. Kiefte and Nearey built a model of consonant-independent vowel-inherent spectral change, following previous work in this area based on fixed consonantal contexts (Broad and Clermont, 2014). They tested their model on spontaneous Canadian Maritimes speech, demonstrating that it successfully predicted variation in formant trajectories as a function of neighboring consonants and thereby confirmed its potential for future research assessing dialect variation in vowel-inherent spectral change based on spontaneous speech. Fox and Jacewicz extended their own previous research on dialect variation in vowel space area (Fox and Jacewicz, 2008) by proposing a novel method of estimating formant density within the vowel space. Their formant density measure combines intuitions from previous work about the potential for dialect variation to emerge in acoustic vowel overlap (e.g., Wassink, 2006) and in formant trajectories (e.g., Fox and Jacewicz, 2009) to generate a more complete representation of how the vowel space is used within and across dialects. Finally, unlike the other papers in the special issue, which all focus on human speech, Garland et al. examined dialect variation in humpback whale song. Their analysis was based on Levenshtein distances, which have also been applied in previous studies of human dialect variation (e.g., Heeringa et al., 2009). However, Garland et al. developed a weighted Levenshtein distance metric that allows for the quantification of whale song similarity at multiple levels of hierarchical structure. This hierarchical approach has potential applications in the study of human dialect variation in the domain of suprasegmentals, where hierarchical structure related to syllables and phrases of various sizes may vary within and across dialects.

Together, the 12 papers in this special issue present a range of advances to our methodological toolkit for analyzing dialect variation. They demonstrate the value of combining methods from acoustics, articulation, and perception to strengthen our interpretation of the variation we observe in each domain individually. They also point to some further methodological advances that are required in the study of dialect variation and make concrete suggestions for how to proceed, including expanding our focus beyond vowels to include consonant and suprasegmental variation, extending our acoustic analyses beyond midpoint or steady-state measures to include an assessment of temporal dynamics, and considering a wider range of spoken language materials in our analyses, including spontaneous speech and historical corpora.

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