1. Introduction
One may intuitively think of second language (L2) intelligibility as how well listeners understand a speaker using an L2. It may be commonly perceived as relating to the understanding and comprehension of speech. Then, this conception of L2 intelligibility has been increasingly popular within the domain of L2 speech as well as language teaching, learning, and assessment. Certainly, the publication of Munro and Derwing’s (Reference Munro and Derwing2011) research timeline on the topic of accent and intelligibility in pronunciation research in Language Teaching has been an important addition to this movement. While we have seen challenges stemming from the limited examinations and the broad yet nebulous scope of applied phonetics and phonology (Munro & Derwing, Reference Munro and Derwing2011), the topic of L2 intelligibility is now undoubtedly recognized as a key topic in the field. In fact, intelligibility is critical for effective communication, which is manifested not only in the context of language classrooms but also in everyday L2 usage. Therefore, it has naturally moved beyond a sole focus on speech signals (Koffi, Reference Koffi2021) and has emerged as both an instructional focus (Levis, Reference Levis2005) and an assessment criterion (Kang et al., Reference Kang, Yan, Kostromitina, Thomson and Isaacs2023). It has also become central to discussion about the globalization of L2 English, particularly in reference to World Englishes (WE) and English as a Lingua Franca (ELF) (Jenkins et al., Reference Jenkins, Cogo and Dewey2011).
However, although a substantial body of literature has attempted to address the concept of L2 intelligibility, the lack of a unitary definition seems to persist. Intelligibility has been defined and measured in many different ways (Jenkins, 2002*; Kang et al., 2018*; Munro, Reference Munro, Edwards and Zampini2008), suggesting that intelligibility is a complex construct affected by multiple dimensions, including perceptual, linguistic, and acoustic features of speech. In 1985, Smith and Nelson synthesized 163 research studies on comprehension and intelligibility published between 1950 and 1985. They divided the broad concept of intelligibility (i.e., international intelligibility) into three sub-categories: intelligibility, comprehensibility, and interpretability. They suggested defining intelligibility more narrowly as the recognition of words or utterances. This distinction was made to address the various interpretations of intelligibility, requiring clearer terminology for more precise analysis and examination.
Forty years later, a need remains to establish a clearer definition of L2 intelligibility, as what constitutes intelligibility varies from context to context. The very features that comprise the intelligibility of specific accented varieties have not yet been fully established (Kang et al., 2020b*). Also, intelligibility measures have been debated from various disciplines, including speech pathology (Liss, Reference Liss, Ball, Müller and Spencer2024; Xue et al., Reference Xue, Van Hout, Cucchiarini and Strik2024) and acoustic phonetics (Babel & Russell, Reference Babel and Russell2015; Pérez-Ramón et al., Reference Pérez-Ramón, García Lecumberri and Cooke2022; Winters & O’Brien, 2013*). Therefore, discussion on speech intelligibility is needed to better understand such a multifaceted construct both conceptually and methodologically.
Following Munro and Derwing’s (1995*) and Derwing and Munro’s (1997*) seminal work, three speech constructs have come to dominate the L2 pronunciation literature: intelligibility, comprehensibility, and accentedness. Intelligibility is referred to as the degree to which a listener actually understands a speaker’s intended message, accentedness as phonological differences between L2 and target language, and comprehensibility as how easy or difficult a speaker is to understand, focusing on processing efforts. In other words, intelligibility can be defined as the accuracy to which a listener can identify the intended utterances produced by a speaker. This distinction has led to a wide acceptance that these constructs are at least partially independent (Derwing & Munro, 1997). That is, an L2 speaker’s speech can be heavily accented but still be largely intelligible, although some research suggests that intelligibility usually decreases as the degree of accent increases (Rogers et al., Reference Rogers, Dalby and Nishi2004). On the other hand, a lack of intelligibility does not necessarily indicate a weak accent. In fact, speakers who are easily understood may still be perceived as having a strong accent (Kang et al., 2018*).
Another important characteristic of intelligibility is that it serves a vital communicative function in our everyday interactions, and the success of information transfer depends on the mutual intelligibility between the speaker and the listener. Understanding is an interactive process that requires engagement from both speakers and listeners, rather than relying solely on one party (Smith & Nelson, Reference Smith, Nelson, Nelson, Proshina and Davis2019). Given that communication is a collaborative activity requiring active participation from both speakers and listeners, sharing responsibility for a successful interaction is key to global communication (Clark & Wilkes-Gibbs, Reference Clark and Wilkes-Gibbs1986; Derwing et al., 2014*). Put differently, a speaker’s intelligibility can depend on situational, social, and cultural factors (Smith & Nelson, Reference Smith, Nelson, Nelson, Proshina and Davis2019).
Much research has also examined listeners’ language and linguistic backgrounds that affect L2 intelligibility. Gass and Varonis (1984*) demonstrated listeners’ language experiences (e.g., familiarity with the topic, accent, speaker) were strongly correlated with their judgments of L2 intelligibility. Note that while the authors used the term comprehensibility, their measure was more in line with intelligibility measures. One factor that also contributes to greater tolerance of listeners is known as “the interlanguage speech intelligibility benefit (ISIB)” (Bent & Bradlow, Reference Bent and Bradlow2003). This asserts that L2 listeners may have benefits in interpreting specific acoustic-phonetic features of an L2 that are matched with their own L1s. Although some studies showed mixed results (Major et al., Reference Major, Fitzmaurice, Bunta and Balasubramanian2002) or minimal effects of listeners’ L1 (Munro et al., 2006*), other recent studies have demonstrated consistent shared-L1 effects among L2 listeners with certain L1 backgrounds, i.e., especially among listeners from outer circle countries (e.g., India or South Africa) where English is spoken as an official and second language (e.g., Kang et al., Reference Kang, Thomson and Moran2019, Reference Kang, Yan, Kostromitina, Thomson and Isaacs2023; Shin et al., Reference Shin, Lee and Lidster2021). To explain this, some scholars have argued that shared L1 may not directly influence understanding but may do so in more complex and indirect ways (Miao, Reference Miao2023).
Currently, research on L2 intelligibility seems to be at a turning point, expanding its scope and depth of understanding. Previously, understanding diverse accented speech was a central concern in the globalized world; thus, most research primarily investigated this construct based on human judgments. However, with the rise of interaction with artificial intelligence (AI) chatbots (Labadze et al., Reference Labadze, Grigolia and Machaidze2023), learners or speakers are now at the stage of prioritizing intelligible speech even in their communications with AI (Moussalli & Cardoso, 2020*). Furthermore, the evolution of automatic speech recognition (ASR) capabilities in AI technology has spurred an increase in research focused on ASR-based L2 intelligibility (Inceoglu et al., 2023*). This means that we are entering the initial stage of a new era in speech intelligibility. Accordingly, the current timeline on L2 intelligibility is timely and appropriate at both practical and pedagogical levels. The field stands to benefit from carefully compiled and scrutinized studies, along with their key findings.
Note that the current research timeline stands out distinctly from its predecessor. While Munro & Derwing (Reference Munro and Derwing2011) examined studies concerning classroom pronunciation instruction, with a focus on both accent and intelligibility, the objective of our current timeline is to center the research solely on L2 intelligibility itself. Our aim is to delve into definitions, measures, and constructs of L2 intelligibility which have rapidly evolved over the last four decades. Therefore, our timeline can shed light on the trajectory that L2 intelligibility has followed up to 2024. We further aim to provide readers with insights into potential future directions for L2 intelligibility research and practices. We encourage readers to reflect on how the field has interpreted the notion of intelligibility and its implementation in practice.
2. Coding process
Our initial examination relied on Google Scholar and library database searches as the primary sources for pertinent studies. Throughout this search process, key terms such as “L2 intelligibility” and “speech intelligibility” were employed. It is important to note that our initial search was restricted to publications in academic journals, conference proceedings, and book chapters. Theses and dissertations were excluded. Moreover, only documents published in English were investigated due to length constraints and practicality considerations. As a result, a total of 117 studies were primarily identified. Subsequently, specific criteria were applied to refine and finalize the selection of the studies.
To begin with, our timeline exclusively incorporated primarily empirical investigations that had been published in peer-reviewed journals, meaning that conceptual or argumentative manuscripts were excluded. Additionally, a decision was made that conference proceedings and book chapters be omitted from this process to ensure that all included studies underwent rigorous peer review. In terms of the selection of journals, we confined our choice to those indexed in the Social Sciences Citation Index (SSCI) by Clarivate Analytics (https://mjl.clarivate.com/home). Consequently, articles from non-indexed journals focusing on L2 intelligibility were omitted. However, there were three exceptions to this criterion: the journals Speech Communication, Journal of the Acoustical Society of America, and Journal of Second Language Pronunciation were included despite the first two belonging to the Science Citation Index Expanded (SCIE) and the last one to the Emerging Sources Citation Index (ESCI), respectively. This choice was made because SCIE is considered to correspond to SSCI, and the latter journal holds significance in the field of L2 speech in spite of its indexing status. In the case of review papers, a few were retained under two conditions: (1) they must contain empirical research, and (2) must have been published in peer-reviewed journals. Conversely, non-empirical review papers, particularly those that mainly discuss theoretical aspects, were removed. Moreover, our selections were limited to papers that encompassed pedagogical implications due to their relevance to the journal of Language Teaching’s aim and scope. As a result, we have narrowed down our timeline to 50 studies.
3. The current research timeline
During the coding process, we noticed an increase in interest in L2 intelligibility in recent years. The current research timeline includes a total of 50 studies, with 16 conducted before 2011 and 34 after 2012. Surprisingly, 21 of these studies have been conducted since 2020 alone. This trend highlights the growing attention to L2 speech intelligibility within the field of Applied Linguistics.
Drawing from the discussions outlined above, we identified the major themes of our research timeline. These themes are categorized into three main areas based on their focus: what (i.e., the research focus), which (i.e., the perspectives from which intelligibility is approached), and how (i.e., the way intelligibility is defined and operationalized). The first theme pertains to research focus (F), which signifies what the studies have investigated. This theme is particularly pertinent to research questions. Within this theme, there are three sub-themes. One of these sub-themes revolves around the relationship between intelligibility and other speech constructs such as comprehensibility and accentedness (F-SpC). While earlier studies predominantly focused on the interplay among speech constructs (e.g., Derwing & Munro, 1997*; Munro & Derwing, 1995*), more recent studies have addressed comprehensibility and accentedness alongside other aspects of intelligibility such as intelligibility-related phonetic features or contextual factors (e.g., Ali, 2023*; Huensch & Nagle, 2021*, 2023*; Jułkowska & Cebrian, 2015*; Kennedy & Trofimovich, 2008*, Matsuura et al., 1999*). Studies falling under the second sub-theme examined the variables that affect intelligibility (F-Var). These variables may include factors related to the listener (e.g., Field, 2005*; Kang et al., 2020a*; Kennedy & Trofimovich, 2008*; Matsuura, 2007*; Munro et al., 2006*), the speaker (e.g., Field, 2005*; Hahn, 2004*; Munro et al., 2006*; Setter, 2006*; Zielinski, 2008*), or specific speech features (e.g., Deterding & Kirkpatrick, 2006*; Emara & Shaker, 2024*; Kang et al., 2020b*). It is noteworthy that the majority of timeline studies encompass this sub-theme, indicating a strong desire within the field to examine intelligibility and explain the characteristics of intelligible speech. The last sub-theme is directly linked with L2 intelligibility itself (F-Int). This sub-theme includes studies whose primary objective is to define, refine, or explore L2 intelligibility as a construct. Such studies may involve conceptual discussions of intelligibility (e.g., Smith & Rafiqzad, 1979*), empirical efforts to establish thresholds that distinguish intelligible from unintelligible speech (e.g., Kang et al., 2020b*), or comparisons of various methods used to measure intelligibility (e.g., Brodkey, 1972*; Kang et al., 2018*).
The second major theme, “which,” refers to the various perspectives (P) from which the concept of intelligibility is approached. Given that intelligibility is explored across diverse disciplines and viewpoints, this theme provides a comprehensive framework for understanding its conceptual scope. In this timeline, intelligibility can be interpreted from six different angles as follows: (1) an instructional perspective (P-Inst) (e.g., Yenkimaleki & Van Heuven, 2022*), where intelligibility is viewed as an essence of the intelligibility principle, which prioritizes effective communication and listener understanding. This contrasts with the nativeness principle, which emphasizes native-like pronunciation as the ultimate goal (Levis, Reference Levis2005). In addition, intelligibility can be explicated in relation to (2) technology (P-Tech), (3) World Englishes (P-WEng), and (4) social perception (P-SocP). To be specific, technology such as ASR is often utilized to assess intelligibility (e.g., Emara & Shaker, 2024*; Inceoglu et al., 2023*; Mroz, 2018*, 2020*), while it can also be construed in terms of international intelligibility (Sewell, 2013*), and listeners’ negative perceptions of L2 intelligible speech (e.g., Hendriks et al., 2023*; Lee & Bailey, 2023*; Rubin, Reference Rubin1992). As a concept related to speech, intelligibility can also be analyzed from either (5) a phonetics perspective (P-APhon) – encompassing acoustic, articulatory, and auditory phonetics – or (6) an L2 pronunciation perspective (P-Pron). Although these perspectives may appear similar, they differ in focus and methodology. The former views intelligibility as a physical phenomenon, analyzing segmental features (e.g., vowels, consonants) and measurable speech properties using acoustic and articulatory analysis, including parameters like F0 (e.g., Aoyama et al., 2023*; Jin & Liu, 2014*; Kawase et al., 2014*; Winters & O’Brien, 2013*). Conversely, the latter defines intelligibility as a perceptual construct, prioritizing listener-based assessments over acoustic measurements. While it also considers pronunciation features, it focuses more on how speech is interpreted and judged by listeners beyond purely phonetic dimensions (Yates, Reference Yates2017). Many applied linguistics studies (e.g., Field, 2005*; Hahn, 2004*; Kennedy & Trofimovich, 2008*) fall into this category.
Our third major theme focuses on the operationalization of L2 speech intelligibility (O), exploring how it is defined and implemented in each study. As previously mentioned, there is no singular, universally accepted definition of intelligibility. Consequently, researchers in the field have employed various measurement methods, reflecting the absence of standardized and widely accepted approaches. Considering that the operationalization of L2 intelligibility is based on its definition, we identified the final major theme as examining how scholars operationalize intelligibility. In other words, regardless of the research focus (F) or perspective (P), if intelligibility is operationalized in a specific way, the corresponding O theme should also be applied. For example, a study that investigates intelligibility thresholds (F-Int) and uses transcriptions as the measurement method should be coded as both F-Int and O-Tran. In many studies in our timeline, L2 intelligibility was operationalized by using a transcription method (O-Tran). This approach involves measuring intelligibility based on the number of words or sentences listeners correctly understand. Alternatively, intelligibility could be operationalized through listeners’ scalar ratings (O-Scal). In this case, intelligibility is assessed by listeners’ subjective judgments of their own understanding of L2 speech (e.g., Lascotte & Tarone, 2022*; Moussalli & Cardoso, 2020*; Murphy, 2014*; Saito & Van Poeteren, 2012*; Yenkimaleki & Van Heuven, 2021*). If the studies employed alternative intelligibility measures (e.g., Aoyama et al., 2023*; Deterding & Kirkpatrick, 2006*; Jin & Liu, 2014*; Kawase et al., 2014*) or incorporated a combination of different methods (e.g., Emara & Shaker, 2024*; Moussalli & Cardoso, 2020*), we coded them as O-Othr.
In sum, the themes in our timeline have been coded as follows:
F: Research focus (what the studies investigated)
F-SpC: relationship with other speech constructs (i.e., comprehensibility, accentedness)
F-Var: variables/features affecting intelligibility
F-Int: intelligibility itself (e.g., measures, thresholds)
P: various perspectives from which the concept of intelligibility is approached
P-Inst: instructional approach (intelligibility principle vs. nativeness principle)
P-Tech: technology-related (ASR)
P-WEng: World Englishes
P-SocP: social perception (social discrimination, bias)
P-APhon: acoustic phonetics
P-Pron: pronunciation
O: operationalization of L2 intelligibility (how it is defined and operationalized)
O-Tran: transcription methods (includes word/sentence transcription)
O-Scal: scalar rating
O-Othr: other methods
In addition to the primary themes, our timeline encompassed an additional supplementary category. Given that our timeline focuses on L2 speech, each study naturally had a designated target language for intelligible speech. Therefore, we divided the themes into TL-Engl (English as the target L2) and TL-Othr (other languages as the target L2).
TL: target language
TL-Engl: English is a target L2
TL-Othr: other languages are the target L2
In terms of the coding process, the second author, serving as the primary coder, coded all themes, while the third author independently coded 12.24% of the timeline studies. Interrater agreement was calculated as the proportion of shared codes to total codes across the double-coded sample (29/30). A 97% interrater agreement was achieved, ensuring reliability in the coding process. When disagreements occurred, the authors met and discussed to reach a consensus. Table 1 presents an overview of the timeline studies.
Table 1. Overview of the timeline studies
* Totals may not add up to 50, as some studies employed more than one method. Indicates that the complete reference is listed in the following timeline.
The present timeline aims to comprehensively incorporate prominent contributions to L2 speech intelligibility, providing our readers with an overview of its change and offering deeper insight into how perspectives, definitions, and measures of L2 intelligibility have evolved. Therefore, it should be noted that our annotations in the timeline specifically set out to focus on L2 intelligibility and its operationalization.
Okim Kang is Professor of Applied Linguistics and Director of the Applied Linguistics Speech Lab at Northern Arizona University (NAU), Flagstaff, AZ. Her research interests are speech production and perception, L2 pronunciation and intelligibility, L2 oral assessment and testing, automated scoring and speech recognition, World Englishes, and language attitudes. She has published multiple books, and numerous journal articles and book chapters. She has also delivered over 200 invited talks and conference presentations, and obtained a number of intramural and extramural grants from various federal and testing agencies. She is currently Associate Editor for Applied Linguistics.
Yuna Bae is a Ph.D. student in Applied Linguistics at Northern Arizona University, where she specializes in second language speech research. Her main research interests include speech perception and production, with an emphasis on artificial intelligence (AI) applications, speech technology, language assessment, and prosody in second language acquisition. She is particularly interested in how AI systems perceive and process diverse accented speech and the implications this has for second language learning and teaching.
Yongzhi (Vito) Miao is Assistant Professor of Applied Linguistics at Texas Tech University. His teaching and research interests include second language speech, particularly in relation to: (1) second language acquisition, exploring the inclusion of diverse accent varieties in language classrooms and its impact on students’ linguistic development; (2) language assessment, investigating the integration of diverse accents in high-stakes language proficiency tests; (3) research methodology, with a focus on measuring speech constructs as latent variables; and (4) intercultural communication, aiming to facilitate communicative success in English as a lingua franca context by improving listener perception and comprehension of diverse accent varieties.
1 Authors’ names are shown in small capitals when the study referred to appears elsewhere in this timeline.
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