# Word-initial voicing in L2 English: Evaluations of /z/ productions before and after perceptual training

Sidsel Holm Rasmussen

► To cite this version:

Sidsel Holm Rasmussen. Word-initial voicing in L2 English: Evaluations of /z/ productions before and after perceptual training. In A. Kirkova-Naskova, P. Humanez-Berral & A. Henderson, (Eds.). Proceedings of the 8th International Conference on English Pronunciation: Issues and Practices, Université Grenoble-Alpes, pp.84-96, 2025, 10.5281/zenodo.16737662. hal-05198884

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<figure><img src="image_1.png" /></figure>
Rasmussen, S. H. (2025). Word-initial voicing in L2 English: Evaluations of /z/ productions before and after perceptual training. In A. Kirkova-Naskova, P. Humanez-Berral, & A. Henderson (Eds.), *Proceedings of the 8th International Conference on English Pronunciation: Issues and Practices* (pp. 84–96). Université Grenoble-Alpes. https://doi.org/10.5281/zenodo.16737662

# Word-initial voicing in L2 English: Evaluations of /z/ productions before and after perceptual training

Sidsel Holm Rasmussen

Aarhus University

## Abstract

This study examines the effect of perceptual training on the production of syllable onset L2 English /z/. L1 Danish participants' productions of English /z/ and /s/ were evaluated in terms of intelligibility, defined as the ability of native English listeners to correctly identify the intended phone in a Two-Alternative Forced-Choice task. Overall, the training group was more intelligible than the control group. After training, trainees' /z/ productions were identified with 61.3% accuracy (gain score of 12 percentage points), whereas the gain score for the control group averaged 4.5 percentage points (from 52.9% to 57.4%). Acoustic measurements were examined from the trainees whose /z/ tokens improved the most after training. Comparison between the two types of evaluations suggests that native listeners' identification of the English voiced alveolar sibilant corresponds to a greater percentage (≥ 25%) of periodicity in the fricative. Methodological considerations concerning different types of production evaluations are discussed.

**Keywords:** English sibilants, evaluation methods, foreign accent, fricative voicing, L2 phonetic training, perception and production

<figure><img src="image_2.png" /></figure>

This chapter is based on the oral presentation given by the author at the 8th International Conference English Pronunciation: Issues and Practices (EPIP 8) held May 8–10, 2024 at the University of Cantabria in Santander, Spain. It is licensed under the Creative Commons Attribution 4.0 International License. To view a copy of the license, please go to: http://creativecommons.org/licenses/by/4.0/.
# 1. Introduction

A number of recent review papers delve into High Variability Phonetic Training (HVPT) as a widely used method for efficiently facilitating the acquisition of novel speech sounds (Barriuso & Hayes-Harb, 2018; Thomson, 2018). This line of research has a long tradition, with early studies demonstrating that exposure to several exemplars of the English /r – l/ contrast improved L1 Japanese listeners’ perception of the non-native speech sounds (Logan et al., 1991; Pisoni et al., 1994). Furthermore, the effects of perception training on L1 Japanese participants’ production of these English consonants were explored using different methods of evaluations which demonstrated that learning (for some trainees) led to improved productions (Bradlow et al., 1997). Meta-reviews have further scrutinized this potential for L2 perception training to lead to improvements in L2 production, and find that small-to-medium production gains are reported in the literature (Sakai & Moorman, 2018; Uchihara et al., 2024). Despite the wide range of results in studies examining the proposed link between perception and production in non-native speech learning, as well as the fact that the two modalities are now generally considered to be co-evolving in a bidirectional manner (Flege & Bohn, 2021), HVPT seems to offer a useful method for learners to receive targeted exposure to specific non-native contrasts. This paper provides results from some of the findings from an HVPT experiment, exploring the efficacy of perception training on L1 Danish participants’ production of English /z/.

## 1.1 L2 perception and production of English /s/ and /z/

The pair of English alveolar sibilants is known to cause problems for many learners whose L1 does not have voiced fricatives, and issues related to confusion between the two English phones are reported in both perception and production. For example, it was found that L1 Spanish speakers fail to produce a distinction (as measured acoustically) between word medial and word final English /z/ and /s/ (Mairano et al., 2021). With focused perceptual training, however, L1 Spanish students of English improved both identification and production of /z/ (Fouz-González, 2019). Similarly, in a longitudinal transcription study, Kapranov (2022) found that Norwegian EFL learners consistently misheard /z/ as /s/ even after attending a course in English phonetics. Kapranov takes this result to be indicative of L1 Norwegian learners’ persistent (perceptual) difficulties with the similar sounding non-native /z/. For L1 Swedish speakers of L2 English, McAllister (2007) also reports difficulties related to the /s – z/ contrast in codas; while all productions of English /s/ were judged acceptable by a native English listener, the same was only true for about 22% of the Swedish participants’ /z/ productions, and half of the successful /z/ items were produced by the same two most successful participants. Evidently, many different EFL speakers encounter the same type of problems with the English voiced alveolar fricative.

Acquisitional difficulties for the English /s – z/ contrast have also been explored for L1 Danish participants in previous studies. Examining the contrast in word-final position, Eger and Bohn (2015) found that L1 Danish speakers perceived the contrast less categorically than did native English listeners, yet performed more like native English participants in perceptually distinguishing word-final /s/ and /z/ than they did in production. These findings are in line with those reported by Trapp and Bohn (2002) who trained nine L1 Danish adolescents to perceptually identify coda /s/ and /z/ from minimal pairs. While the trainees' mean identification scores had improved after training, there was no significant improvement in their measured production accuracy. In particular, "it was the production of the word-final /z/ rather than /s/ that created problems for the Danes" (p. 349). Moreover, acoustic analyses demonstrated that L1 Danish trainees' vowel-to-fricative ratio was equally short before and
after training, participants thus failing to employ the duration cue common in native English productions of coda /z/.

Whereas these early studies were concerned with /s/ or /z/ in word final position, Horslund and Bohn (2022) examined how Danes perceive English consonants in initial position. First, they found that initial /s/ and /z/ assimilate to Danish /s/ as a Category Goodness (CG) assimilation type (Best & Tyler, 2007). The authors thus hypothesised that English /s/ is easily identifiable to L1 Danish listeners, while identification of /z/ depends on previous exposure to English. Interestingly, expectations from this hypothesis were not borne out in their following identification study – rather, Horslund and Bohn observed lower identification scores for /s/ than for /z/, and lower sibilant identification scores for the more experienced of two learner groups included. These unexpected results caused the authors to speculate about potential hypercorrection that may lead to such response biases favouring the less familiar phone in a CG pair such as /s/~/z/.

The surprising results from Horslund and Bohn's study (2022), coupled with previous research and reports of other L1 EFL learners' issues, motivates this study of L1 Danish listeners' perception and production of initial English /s/ and /z/ before and after training<fnref n="1" />. The scope of the current paper is concerned only with the productions of the English sibilant contrast in onset position produced by L1 Danish speakers, as the aim of this paper is twofold: First, we examine the extent to which perceptual training of syllable initial English /s/ and /z/ has an effect on non-native production of the same speech sounds. Since Danish does not have any voiced fricatives, both English sibilants are assimilated to the native /s/ (Horslund & Bohn, 2022), and production of the unvoiced English /s/ is expected to be unproblematic (or sufficiently similar to be identified correctly by L1 English listeners), while production of the voiced /z/ is expected to be less nativelike. Secondly, this research also aims to explore the insights we may gain from different types of evaluations to non-native pronunciation. Specifically, this paper addresses the following research questions:

**RQ1:** How accurately can L1 Danish speakers produce the syllable initial sibilant contrast /s - z/ (as judged by native English listeners) before training?

**RQ2:** Do L1 Danish speakers' production of /z/ (the more challenging of the sibilant pair) become more intelligible to native listeners after perceptual training?

**RQ3:** Which production strategies are deployed by the speakers for whom correct /z/ identification by native evaluators is highest/most improved?

# 2. Method

## 2.1 Participants: Native Danish speakers

Recordings from 45 (F = 34, M = 11) L1 Danish speakers were included in this study. Participants were randomly assigned to the experimental training group (TG, n = 33) or the control group (CG, n = 12). Their profile data are presented in Table 1. Their ages ranged from 20–76 (M = 44.6, SD = 21.9)<fnref n="2" /> and they had started learning English at the mean age of 10.2 (SD = 1.91).<fnref n="3" /> Participants completed a simple language background questionnaire from which

<footnote n="1">¹ More information about the project and our group's research on perceptual training is available on the website: https://cc.au.dk/en/phonetic-flexibility-in-old-age/our-research</footnote>

<footnote n="2">² Note that the participants were recruited to match one of our specified age groups (either 18–35 years or 60+)</footnote>

<footnote n="3">³ Younger participants had on average started learning English earlier (8.7 yrs. old) than the older participants (11.7 yrs.)</footnote>
we confirmed that they had not spent more than 6 months in an English-speaking country. On a scale from 1–5 (poor–good) participants also reported their self-assessed level of “ability to understand spoken English” and their “ability to speak English”.

<caption>**Table 1**</caption>

*Background Information for L1 Danish Participants*

<table><thead><tr><th></th><th>TG (n = 33)<br>M (SD)</th><th>CG (n = 12)<br>M (SD)</th></tr></thead><tbody><tr><td>Age</td><td>44.6 (21.7)</td><td>44.7 (23.4)</td></tr><tr><td>AOL English</td><td>10.2 (1.91)</td><td>10.3 (1.97)</td></tr><tr><td>Score, understand</td><td>4.18 (0.81)</td><td>4.08 (1.08)</td></tr><tr><td>Score, speaking</td><td>4.03 (0.95)</td><td>3.83 (1.53)</td></tr></tbody></table>

## 2.2 Production task

Production of both initial and final /s/ and /z/ were elicited in a delayed repetition task (Flege et al., 1995). Participants first listened to a recording of a native speaker who produced 46 words<fnref n="4" /> separately, inserted in the carrier phrase: *The next word is __*, after which they heard the same voice ask: *What is the next word?* They were then prompted to repeat the word in the carrier phrase: *Let it be __*. Target words examined in the current study were 15 (near) minimal monosyllabic pairs of English words which differed in voicing in the initial fricative (e.g., *sink~zink*). Productions were elicited from participants before the perception tasks (Lab1), and the same production task was completed again after three weeks at a meeting in the lab for post-training test (Lab2). The focus of this paper is limited to the productions of initial /s/ and /z/.

## 2.3 Perception tests and perception training

Perception of the English sibilant contrast was assessed in two separate Two-Alternative Forced-Choice (2AFC) identification tasks in which L1 Danish participants listened to CV and VC syllables, respectively. Token variability was introduced by two different talkers and by various vowel contexts: initial [si], [sa], [su], [zi], [za], [zu], and final [is], [as], [us], [iz], [az], [uz]. In each task participants were asked to determine which sibilant began or ended the syllable. Response buttons were labelled orthographically with “S” and “Z”, and no feedback was provided in the tests. Participants in the experimental group were then trained to perceive onset /s/ and /z/ in a similar identification task, using the same stimuli, but in which corrective feedback was provided after each trial. Trainees were asked to complete a total of 10 training sessions, each of 120 trials, in the course of 3–4 weeks after which the post-training test (identical to the first test) was administered in our lab.

<footnote n="4">⁴ 15 monosyllabic word pairs beginning with /s/ or /z/; 8 word pairs ending with /s/ or /z/</footnote>
## 2.4 Native listener evaluations

Non-native productions of /s/ and /z/ were assessed auditorily by 40 native English judges who were recruited in or near Aarhus, Denmark. Target words were cut from the carrier phrase and presented to listeners who identified the initial sibilant orthographically as either “S” or “Z” in a 2AFC task. Each listening session included 30 English words recorded by three of the L1 Danish participants at Lab1 and Lab2 (30 words x 3 speakers x 2 recordings = 180 items) presented in randomised order, and half of the judges agreed to complete a second session on another day, on which they rated tokens from 3 other speakers. Productions from each of the Danish speakers were evaluated by 4 different native judges, which means that evaluations of each speaker's items are based on the mean identification scores from four listeners. A calculated Intraclass Correlation Coefficient (ICC) of 0.84 indicated good reliability between native listener judges in the production evaluation experiment.<fnref n="5" /> Judges reported using English 50–100% in their daily lives (M = 88.5, SD = 15.3), and they had on average spent eight years in a non-English speaking country (SD = 8.15).

## 3. Results

The native listeners' evaluations of sibilant productions were computed in R studio (R Core Team, 2021) using the Tidyverse package (Wickham et al., 2019). Trials for which the target fricative was heard as intended received the score of 1 while trials in which the sibilant was not identified received the score of 0. Average evaluation scores for each participant were calculated as proportions.

Sibilant productions from all 45 native Danish speakers were on average identified as intended in 65.8% of all instances when recorded before perceptual training. These evaluation results were based on 81.4% productions of intended /s/ and 50.3% productions of intended /z/ as summarised in Table 2. About half of the recorded tokens of intended /z/ produced before perception training were identified as /s/ by native listeners. Thirty-three participants trained identification of the /s – z/ contrast auditorily. In the remaining analysis and visualisations, data from the TG and CG are separated and compared.

<caption>**Table 2**</caption>

*Native English (NE) Listeners' Identification of Intended /s/ and /z/ by all Danish (DA) Speakers before Training*

<table><thead><tr><th rowspan="2">Identified by NE</th><th rowspan="2"></th><th colspan="2">Fricative intended by DA speaker</th></tr><tr><th>/s/</th><th>/z/</th></tr></thead><tbody><tr><td></td><th>/s/</th><td>81.4 %</td><td>49.7 %</td></tr><tr><td></td><th>/z/</th><td>18.6 %</td><td>50.3 %</td></tr></tbody></table>

<footnote n="5">⁵ The ICC estimates were calculated using R statistical software (R Core Team, 2021) with the lme4 package (Bates et al., 2015), based on a mean-rating (k = 4), absolute-agreement, one-way random effects model.</footnote>
While identification scores of the intended /s/ productions are consistently high for both
groups at both times of recording (approximately 80% for trainees at Lab 1 and Lab 2; 85–86%
for controls), the /z/ productions are less consistently heard as intended. Boxplots in Figure 1
furthermore reveal great variation among the evaluations of speakers’ recordings of initial /z/
for trainees and controls alike. A logistic mixed-effects model was run using the *lme4* package
(Bates et al., 2015) to examine the effect of training on the evaluation of sibilant production.

Three factors were treatment coded, i.e., the experiment group (TG/CG) with TG as the reference, production session with Lab1 as the reference and intended fricative with “z” as the reference. From Lab1 to Lab2 the evaluated score for /z/ was significantly higher for trainees’ production, $\beta = 0.93$, $z = 5.85$, $p < 0.001$, and a post-hoc z-test comparison using *emmeans* (Lenth, 2025) with Tukey HSD corrections for multiple comparisons shows that that was not the case for the production of /z/ by the control group, $z = -1.99$, $p = 0.49$. The interaction between production sessions and the two groups was not statistically significant, $\beta = -0.34$, $z = -1.02$, $p = 0.31$, suggesting that it is uncertain if the trainees’ production actually does become more intelligible than those of the controls.

**Figure 1**

*Intelligibility of Sibilants as Judged by Native English listeners*

<figure><img src="image_3.png" /></figure>

*Note.* Evaluations (EVAL) of intelligibility for the Training Group – TG (left) and Control Group – CG (right)

**3.1 Individual variation**

Individual intelligibility scores for the 33 speakers in the TG were also calculated before and after training (see Appendix for overview). Figure 2 depicts the individual scores for each speaker, and it clearly demonstrates the varied nature of the data. Results from five selected speakers are highlighted (in yellow) to illustrate this point. For example, the /z/ productions of one speaker (S29) were very consistently identified at both times of testing (top yellow line), while /z/ tokens from another speaker (S02) were hardly ever heard as intended (bottom yellow line). Productions of three speakers improved by at least 35 percentage points after training as represented in the steeper slopes in Figure 2. Intelligibility scores from these five selected speakers are also summarised in Table 3.
Figure 2

Proportion of Correctly Identified /z/ Items

<figure><img src="image_4.png" /></figure>

Note. Intelligibility scores for each individual participant in TG before and after training. The highlighted (yellow) lines represent results from five example speakers.

Table 3

Intelligibility Scores percentage for /z/

<table><thead><tr><td>Speaker number</td><td>Lab1 score</td><td>Lab2 score</td><td>Gain score</td></tr></thead><tbody><tr><td>S29</td><td>98.3</td><td>100</td><td>1.67</td></tr><tr><td>S30</td><td>56.7</td><td>95</td><td>38.3</td></tr><tr><td>S08</td><td>45</td><td>80</td><td>35</td></tr><tr><td>S01</td><td>1.69</td><td>48.3</td><td>46.7</td></tr><tr><td>S02</td><td>1.67</td><td>1.67</td><td>0</td></tr></tbody></table>

3.2 Exploring the acoustic properties of /z/

To further probe the nature of successful /z/ productions, tokens from the five highlighted speakers in Figure 2 were analysed for duration of and periodicity in the fricative portion. The three speakers with the highest gain scores (S30; S08; S01) were those whose tokens were most improved in terms of target like pronunciation. Tokens from the two speakers with consistently highest (S29) and lowest (S02) intelligibility scores were also included. Altogether, acoustic measurements were obtained for 150 tokens (15 /z/ words x 2 times of recording x 5 talkers) by selecting the fricative portion and using the voice report in Praat (Boersma & Weenink,
2022) to tally the fraction of locally unvoiced frames, which was then inverted. The results are summarised in Table 4.

<caption>**Table 4**</caption>

*Duration and Percent of Periodicity of Initial Fricatives in Intended /z/ Productions by Five Speakers*

<table><thead><tr><th rowspan="2">speaker_id</th><th colspan="2">Lab1<br>(pre-training recordings)</th><th colspan="2">Lab2<br>(post-training recordings)</th></tr><tr><th>/z/ duration<br>in ms<br>M (SD)</th><th>percent periodicity<br>M (SD)</th><th>/z/ duration<br>in ms<br>M (SD)</th><th>percent periodicity<br>M (SD)</th></tr></thead><tbody><tr><td>S29</td><td>201 (11.6)</td><td>62.5 (30.3)</td><td>216 (26.5)</td><td>90.3 (18.7)</td></tr><tr><td>S30</td><td>207 (20.1)</td><td>10.8 (16.8)</td><td>190 (44.1)</td><td>45 (28.1)</td></tr><tr><td>S08</td><td>204 (23.6)</td><td>10.3 (16.5)</td><td>217 (27.8)</td><td>30.3 (30.8)</td></tr><tr><td>S01</td><td>280 (46.4)</td><td>1.85 (2.13)</td><td>212 (45.9)</td><td>8.84 (12.2)</td></tr><tr><td>S02</td><td>284 (42.6)</td><td>4.6 (8.09)</td><td>269 (29.7)</td><td>4.45 (6.25)</td></tr></tbody></table>

*Note.* Each summary statistic denotes the mean (M) and the standard deviation (SD) of 15 tokens.

Most notably, the mean proportion of periodicity in the initial fricative increases for all speakers after training, except for S02, whose /z/ productions were consistently misidentified by native listeners. Conversely, the more successful speaker S29 produces /z/ with the largest proportion of periodicity, which also increases after training. Plotting the intelligibility evaluations from native listeners against the measured periodicity proportion (see Figure 3, left panel), it becomes clear that native listeners are most likely to identify /z/ when voicing is present in more than 25% of the initial sibilant segment. Average fricative duration decreases for three speakers after training, but in its current form (not normalised for speaking rate) it is uncertain to what extent this measurement reflects alterations to trainees' fricative productions, although there is a weak trend in the expected direction of shorter fricatives heard more consistently as voiced by the native listeners (see Figure 3, right panel).

**4. Discussion**

This study found that perceptual training of the English /s – z/ contrast coincides with small improvements in productions, in agreement with findings from other training studies (Sakai & Moorman, 2018). Specifically, non-native productions of /z/ are more intelligible to native listeners after training (61.3 %) than before (49.3%), but the intelligibility scores for productions by each individual speaker in the experimental (training) group also demonstrates that there are major differences in how much, and indeed if, speakers' /z/ tokens become more
<caption>Figure 3</caption>

*Average Intelligibility Score and Acoustic Measurements*

<figure><img src="image_5.png" /></figure>

*Note.* The mean intelligibility score for each intended /z/ item from the five example speakers (rated by four native listeners) are plotted against periodicity in the sibilant (left) and against its raw duration in ms (right).

intelligible (cf. Figure 2). A brief examination of samples of the more intelligible /z/
productions suggests that periodicity in the fricative portion is an important cue to native
listeners' identification of the accented /z/ items. For the three speakers whose /z/ productions
improve the most, the measured percent periodicity increases from Lab1 to Lab2. Is this
perhaps due to them becoming increasingly sensitive to this cue as a result of their perception
training? In order to connect the dots between the evaluations of native listeners' judgments
and the actual realisations from our non-native speakers, more detailed acoustic analyses of all
recordings would be required (Mairano et al., 2021). Doing so could also elucidate the phonetic
details of /z/ tokens in speakers who fail to improve after training.<fnref n="6" /> Furthermore, inspections
into potential lexical biases might reveal a skewed set of target words in which most /s/ items
are much more frequent than the /z/ competitor, from which we would thus also suspect a
preference for the “S” response by native English listeners. Finally, greater familiarity with
Danish-accented English may cause some listeners to more accurately identify the intended
target sounds, so the experimental design means some speakers' tokens may have been
evaluated by more lenient listeners. However, since each native listener only judged
productions from a few speakers it is not possible to meaningfully explore this point from the
current data.

These results raise the question of how we can meaningfully measure (altered) productions
of challenging non-native target phones from highly controlled laboratory recordings.

**4.1 Evaluations of L2 production**

Introducing a framework for conceptualising methods of evaluating the effects of L2
pronunciation instruction, Saito and Plonsky (2009) acknowledge that “[t]here is no single best
assessment method; rather, each instrument carries with it a set of tradeoffs” (p. 654).
Production may be assessed more globally, through “listeners’ holistic judgements of L2

<footnote n="6">⁶ I thank the anonymous reviewer who suggested this argument.</footnote>
speakers' speech samples" and concepts such as "perceived fluency", "accentedness", and "intelligibility" may be grouped within this holistic construct (p. 656). Relying on human intuitions, such global measures of production proficiency are obviously very subjective in nature, whereas acoustic investigations may provide more objective measures. However, even so, in determining which acoustic properties to examine, subjective decisions and presumed knowledge about target productions cannot fully be avoided. A third type of pronunciation measurement seems to straddle the methods of human-intuition-based and acoustic-based evaluations of specific features, i.e., expert raters who are asked to attend carefully to specific phonetic features (pp. 658–60). This method thus still relies on human evaluations but focuses listeners' attention to providing a more nuanced measure of the target phones.

In the current training experiment, we defined sibilant target intelligibility in L2 production as a binary judgment from native listeners who were specifically required to evaluate the onset sibilant as either /s/ or /z/<fnref n="7" />. In this procedure we follow the definition of target phone "intelligibility" used by Munro and Derwing (2008), and thus attain a similar method of production evaluation based on the perceptual identification of native listeners. Key differences in methodology, however, involve the number of possible response categories (i.e., resulting in a different number of competitors in the assessment task), the linguistic experiences of the native listeners (e.g., phonetically trained or not, and more or less familiar with speakers' L1), and the scope of stimuli exposure (listeners evaluating all items or a subset thereof). Such differences are important to acknowledge and to keep in mind when comparing research results across studies, but also in fact within studies, when comparing L2 abilities in different modalities. In the present training experiment, the primary aim was to test the potential effect of perceptual training in the same (perceptual) modality, while a secondary aim was to examine the potential effect of perception training on another modality, production. In an attempt to mitigate what has been identified as important discrepancies in the typically obtained measurements of the two modalities (Thomson, 2022), production was here assessed in a way that emulated the perception task (2AFC identification). It should be acknowledged, however, that there are additional methodological steps to be mindful of in future research, e.g., controlling for lexical frequency of target words or having L2 speakers produce nonsense words (Eger & Bohn, 2015). Other useful ways to obtain knowledge about potential pronunciation improvements involve scalar ratings, in which listeners evaluate the extent to which a spoken item matches a target (Munro & Derwing, 2008; Munro et al., 1996). Obtaining multiple different types of evaluative measures on the same production data helps us better understand the specific aspects that may impact L2 speech.

**5. Conclusion**

L2 production of specific non-native features can be assessed in many different ways, some more nuanced and fine-grained than others. The production data from L1 Danish speakers presented in this paper were collected before and after perceptual training of the English /s – z/ contrast, and the evaluative measure of intelligibility was the ratio of correctly identified sibilants by native listeners. Notwithstanding potential biases in the data, the intelligibility evaluations demonstrate that the group of speakers who trained perception of /s/ and /z/ also produced the contrast more intelligibly after training than was the case for an age matched control group. Intelligibility scores of /z/ tokens in selected speakers were compared to the proportion of periodicity in the fricative, and comparisons revealed that fricatives with at least

<footnote n="7">⁷ In this sense, we directed evaluators' attention to specific phonetic details. Listeners were not specifically selected for their meta-linguistic knowledge, and they do not meet the literature criteria for being considered expert raters.</footnote>
25% pulsing were consistently identified as voiced. Computationally extracted measurements of additional acoustic detail applied to the entire dataset of Danish accented sibilants could perhaps give further insights into the productive L2 development of this particularly challenging English contrast.

## Acknowledgements

I am grateful to my colleagues for valuable help and insights: I thank Birgitte Poulsen for sharing her data with me for this work and Birgitte Poulsen and Ocke-Schwen Bohn for experimental design and discussions. I also thank Jonas Villumsen for his resourceful assistance in statistical analyses. Any errors are mine.

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## Appendix

### Individual intelligibility scores for /z/ productions

<table><thead><tr><td>speaker</td><td>Lab1 score</td><td>Lab2 score</td><td>Gain score</td></tr></thead><tbody><tr><td>S01</td><td>1.67</td><td>48.3</td><td>46.7</td></tr><tr><td>S30</td><td>56.7</td><td>95</td><td>38.3</td></tr><tr><td>S08</td><td>45</td><td>80</td><td>35</td></tr><tr><td>S19</td><td>65</td><td>93.3</td><td>28.3</td></tr><tr><td>S21</td><td>43.3</td><td>70</td><td>26.7</td></tr><tr><td>S13</td><td>13.3</td><td>38.3</td><td>25</td></tr><tr><td>S34</td><td>36.7</td><td>61.7</td><td>25</td></tr><tr><td>S25</td><td>26.7</td><td>50</td><td>23.3</td></tr><tr><td>S06</td><td>45</td><td>66.7</td><td>21.7</td></tr><tr><td>S07</td><td>18.3</td><td>36.7</td><td>18.3</td></tr><tr><td>S10</td><td>8.33</td><td>25</td><td>16.7</td></tr><tr><td>S11</td><td>55</td><td>68.3</td><td>13.3</td></tr><tr><td>S20</td><td>6.67</td><td>20</td><td>13.3</td></tr><tr><td>S26</td><td>75</td><td>88.3</td><td>13.3</td></tr><tr><td>S28</td><td>85</td><td>98.3</td><td>13.3</td></tr><tr><td>S31</td><td>86.7</td><td>98.3</td><td>11.7</td></tr><tr><td>S17</td><td>28.3</td><td>38.3</td><td>10</td></tr><tr><td>S35</td><td>73.3</td><td>81.7</td><td>8.33</td></tr><tr><td>S23</td><td>90</td><td>96.7</td><td>6.67</td></tr><tr><td>S03</td><td>96.7</td><td>100</td><td>3.33</td></tr><tr><td>S24</td><td>21.7</td><td>25</td><td>3.33</td></tr><tr><td>S36</td><td>78.3</td><td>81.7</td><td>3.33</td></tr><tr><td>S05</td><td>40</td><td>41.7</td><td>1.67</td></tr><tr><td>S29</td><td>98.3</td><td>100</td><td>1.67</td></tr><tr><td>S32</td><td>85</td><td>86.7</td><td>1.67</td></tr><tr><td>S02</td><td>1.67</td><td>1.67</td><td>0</td></tr><tr><td>S09</td><td>96.7</td><td>96.7</td><td>0</td></tr><tr><td>S14</td><td>53.3</td><td>53.3</td><td>0</td></tr><tr><td>S15</td><td>23.3</td><td>23.3</td><td>0</td></tr><tr><td>S12</td><td>28.3</td><td>25</td><td>-3.33</td></tr><tr><td>S22</td><td>91.7</td><td>88.3</td><td>-3.33</td></tr><tr><td>S33</td><td>43.3</td><td>40</td><td>-3.33</td></tr><tr><td>S04</td><td>8.33</td><td>3.33</td><td>-5</td></tr></tbody></table>

## About the author

Sidsel Holm Rasmussen is a postdoc at the English Department of Aarhus University. She is interested in a wide variety of topics within the fields of speech, hearing, communication and language teaching. Her current research focuses on phonetic and audio-visual training, speech perception and second language acquisition.

sidsras@gmail.com

sira@cc.au.dk