The Acoustic Features and Didactic Function of Foreigner-Directed Speech: A Scoping Review

Abstract

Purpose : This scoping review considers the acoustic features of a clear speech register directed to nonnative listeners known as foreigner-directed speech (FDS). We identify vowel hyperarticulation and low speech rate as the most representative acoustic features of FDS; other features, including wide pitch range and high intensity, are still under debate. We also discuss factors that may influence the outcomes and characteristics of FDS. We start by examining accommodation theories, outlining the reasons why FDS is likely to serve a didactic function by helping listeners acquire a second language (L2). We examine how this speech register adapts to listeners' identities and linguistic needs, suggesting that FDS also takes listeners' L2 proficiency into account. To confirm the didactic function of FDS, we compare it to other clear speech registers, specifically infant-directed speech and Lombard speech. Conclusions: Our review reveals that research has not yet established whether FDS succeeds as a didactic tool that supports L2 acquisition. Moreover, a complex set of factors determines specific realizations of FDS, which need further exploration. We conclude by summarizing open questions and indicating directions and recommendations for future research.

Phonetic accommodation in non-native directed speech supports L2 word learning and pronunciation

“…NNDS has also been referred to as “L2 speech accommodation” because it is assumed to be the result of the speaker’s accommodation to the listener’s low L2 proficiency and learning needs (see 2 – 5 for theoretical frameworks). In line with this, NNDS results in clear speech, and it is proposed to serve a didactic function by assisting learners to better understand, perceive, and pronounce their L2 6 – 8 . Piazza et al 6 proposed that the didactic function of NNDS comprises two related aspects: a didactic purpose and a didactic impact .…”

“…Compared with NDS, such adaptations lead to the production of several acoustic features that enhance clarity of NNDS and potentially support L2 learning. The most typically studied features of NNDS are speech rate reduction and acoustic exaggeration of vowels, i.e., vowel hyperarticulation 6 . Vowel hyperarticulation is assumed to be the key acoustic feature that serves a didactic function in NNDS because it results in a clearer and more distinctive representation of vowel categories 9 .…”

“…Vowel hyperarticulation is assumed to be the key acoustic feature that serves a didactic function in NNDS because it results in a clearer and more distinctive representation of vowel categories 9 . These features together are proposed to support speech perception, comprehension, and even production 1 , 6 , 10 – 12 . Indirect evidence that NNDS supports speech comprehension is provided by studies showing that listeners rate the intelligibility of NNDS higher than that of NDS.…”

“…Conversely, L2 learners have been reported to understand NNDS better than both NDS and Lombard speech, which is a register produced to contrast background noise during native-native interactions 17 , 18 . Lombard speech shares some acoustic features with NNDS, but manifested to different extents 6 . For instance, NNDS highlights phoneme differences to a greater extent than Lombard Speech 15 , 19 .…”

Studying Child-Directed Speech

“…However, to ascertain what role features that distinguish CDS from ADS may play in language development it is necessary to examine their effects on learning gains, as we will discuss in Section 3. A deeper understanding of the function of CDS also benefits from comparing it to ADS sampled from a greater variety of contexts and situations (Weinstein & Baldwin, 2022), and to other forms of speech directed at different addressees like foreigner-directed speech (e.g., Piazza et al, 2022), pet-directed speech (e.g., Burnham et al, 2002) or elderspeak (e.g., Kemper & Harden, 1999).…”