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Though one way to examine talkers’ speech enhancement behavior is to compare their productions between plain- and clear-speaking styles (e.g., Ferguson & Kewley-Port, 2002; Ferguson 2004; Granlund et al., 2012; Picheny et al., 1986), other studies suggest that speech enhancements are not uniform phenomena (e.g., Gilbert, Chandrasekaran, & Smiljanić, 2014; Hazan & Baker, 2011; Scarborough & Zellou, 2013; Tuomainen &

Hazan, 2018). That is, talkers' efforts to enhance acoustic-phonetic characteristics of speech can be implemented differently depending on the types of task that they engage in to produce speech enhancements. Particularly, studies examining native talkers’ speech enhancements in different contexts suggest that there are differences in acoustic

characteristics of speech enhancements produced in read speech with explicit instructions to speak clearly vs. in spontaneous speech during a conversation (e.g., Hazan & Baker, 2011; Scarborough & Zellou, 2013). For example, native talkers’ speech enhancements elicited in read speech, using the instruction to speak clearly as if talking to someone who is hearing impaired, result in more extreme changes in some acoustic-phonetic

characteristics (e.g., pitch range, speaking rate, vowel duration, vowel space) than speech enhancements elicited in spontaneous speech (e.g., elicited using a fill-in-the-blank

worksheet in a map task: Scarborough & Zellou, 2013; using ’spot the difference’ picture tasks with noise: Hazan & Baker, 2011). The acoustic-phonetic modifications in the speech produced for an imaginary hard-of-hearing listener (as compared to the speech produced for a real listener) also involved reduced coarticulation (i.e., less overlap between vowels and nasal consonants; Scarborough & Zellou, 2013).

Variations in the degree of acoustic-phonetic modifications are observed within different types of spontaneous speech as well. For example, Hazan and Baker (2011) demonstrated that talkers make different types of acoustic modifications depending on the type of noise that their listeners are experiencing. In the study, talkers engaged in a “spot the difference" picture task with a partner who heard the speech in different masking conditions, including listening to speech through a three-channel noise-excited vocoder, or with multi-talker babble. When interacting with a partner who was listening to their speech in the multi-talker babble condition, talkers made greater changes in terms of F0, intensity, and vowel formants compared to when they were interactions with a partner who was in the vocoder condition, suggesting that talkers modified their speech differently depending on the type of communicative barrier that their listeners were experiencing. The presence of an actual listener also impacts talkers’ acoustic modifications in spontaneous speech. When producing foreigner-directed speech, native talkers employed more extreme changes in durations and vowel space when giving instructions to an imagined non-native listener in a map task, compared to when talking to a real non-native listener (present in the room: Scarborough et al., 2007). Thus, these studies demonstrate that the characteristics of speech enhancements can be greatly influenced by the methods of eliciting the speech.

they are instructed to speak clearly for an imagined listener but also when listeners’

communicative needs are signaled in the communication context. This is further supported by the findings that talkers are able to enhance acoustic features of the speech in a

contextually-relevant way. For example, when a listener misunderstands a particular part of an utterance (e.g., a specific word), talkers selectively enhance that part of the utterance to correct the misunderstanding (e.g., Maniwa, Jongman, & Wade, 2009; Ohala, 1994; Oviatt, Levow, Moreton, & MacEachern, 1998; Schertz, 2013; Stent, Huffman, & Brennan, 2008). Specifically, when native English talkers spoke to a simulated speech recognizer and received a feedback that the utterance was misunderstood (e.g., the talker says “pit” but the computer guesses “bit”), the talkers enhanced the misunderstood contrast by manipulating a relevant acoustic feature (e.g, VOTs of the /p/ and /b/) in the second repetition (Schertz, 2013). This type of targeted error correction did not occur when the talker received an open-ended request for repetition (e.g., “???”). Such targeted segmental enhancements in response to listeners’ feedback have also been found for a temporal aspect of a vowel contrast (English /i/-/ɪ/: Schertz, 2013) as well as for temporal and spectral aspects of English fricative contrasts (Maniwa et al., 2009).

Furthermore, talkers make contextually-relevant speech enhancements even without feedback from the listener. For example, in a communicative task involving conveying information to a listener, native English talkers exaggerated differences in VOTs of English word-initial consonants (e.g., /p/-/b/) when a target word to communicate (e.g., pill) was displayed with another word that is minimally different (e.g., bill), compared to when it was not (Baese-Berk & Goldrick, 2009; Buz, Jaeger, & Tanenhaus, 2014; Buz, Tanenhaus, & Jaeger, 2016). Similar types of contextually-relevant hyperarticulation have been

observed for an English word-final fricative voicing contrast (e.g., dose vs. doze: Seyfarth, Buz, & Jaeger, 2016). Further, it has been suggested that contextually-relevant

hyperarticulation of a target word may only occur in the context of other words that are sufficiently similar to the target word (e.g., one major phonological feature away: Kirov & Wilson, 2012). The researchers showed that native English talkers exaggerated VOTs of word-initial voiceless stop consonants (e.g., cap) when a word differing in place of articulation (e.g., tap) was contextually co-present, but not when a word differing by both place and manner of articulation (e.g., kilt vs. hilt) was contextually co-present (Kirov & Wilson, 2012). Though the investigation of such contextually-relevant hyperarticualtion has mostly been limited to native talkers’ productions, one study demonstrated that highly proficient non-native talkers exaggerated a non-native contrast (e.g., /æ/-/ɛ/) when a target word (e.g., sat) was placed next to a similar word (e.g., set) in a word-communication task (Hwang, Brennan, & Huffman, 2015). Thus, these studies have demonstrated that

experienced talkers (i.e., native talkers and highly proficient non-native talkers) are able to make targeted speech enhancements based not only on listeners’ feedback but also on potential communication difficulty signaled in the context.

In sum, these studies have demonstrated that talkers’ speech enhancements can be elicited differently using a variety of tasks, ranging from a reading task with explicit instructions to speak clearly, to a communication task where talkers produce unscripted spontaneous speech. Given that characteristics of the elicitation task influence the way native talkers make speech enhancements (e.g., Hazan & Baker, 2011), it is possible that the nature of task influences the types of acoustic-phonetic enhancements made by non- native talkers of different proficiency levels. Thus, in order to better understand how talkers

of different linguistic backgrounds implement speech enhancement strategies, we investigate native and non-native talkers’ speech enhancements produced in different contexts, including in clear speech, where they read materials based on explicit instructions to speak clearly, as well as in a more communicative context, where listeners’ needs for enhanced speech intelligibility are signaled rather implicitly in the interaction.