Synchronically, many Australian languages show a pattern called pre-stopping, where a phonological sequence consisting of a tonic vowel and a nasal or lateral /ˈV(N,L)/ is realized phonetically as [ˈVC_[stop](N,L)] (Dixon, 2002, p. 597; Fletcher and Butcher, 2014, pp. 109–110). While non-contrastive phonetic pre-stopping is common among Australian languages, phonologically contrastive pre-stopping is rare, i.e., languages where /ˈV(N,L)/, /ˈVC_[stop](N,L)/, and /ˈVC_[stop]/ all contrast phonologically with one another are infrequent among Australian languages. Diachronic analyses agree that, in those cases where contrastive pre-stopping does exist, it developed from non-contrastive pre-stopping (Hercus, 1972, 1994, p. 37; Koch, 1997, pp. 276–278; McEntee and Butcher, 2021, pp. 39–44; Simpson and Hercus, 2004, pp. 188–189; Sommer, 1969, pp. 54–55).

Arabana is of particular interest for research on pre-stopping, because it is one of the few Australian languages in which pre-stopping is phonologically contrastive (Harvey et al., 2019); examples illustrating the contrast between pre-stopped and plain nasals in Arabana are provided in Table 1. The change from phonetically conditioned pre-stopping to phonologically contrastive pre-stopped forms is an exhausted sound change in Arabana and is no longer synchronically active in the language; the complex and unpredictable distribution of pre-stopped forms suggests that the change has indeed been inactive for some time. It should be noted that there is no evidence for non-contrastive pre-stopping synchronically in Arabana: Synchronic /#(C)VN/ forms do not have optional pre-stopped realizations, e.g., /amaɲi/ ‘grandmother’ can only be realized as [amaɲi] and does not have [apmaɲi] as an alternative realization. The change operated extensively across the lexicon, but there is one environment where contrastive pre-stopped forms did not develop. This is in the environment */ˈNVN/, where there is a consistent absence of contrastive pre-stopping, i.e., */ˈNVN/ did not develop into /ˈNVC_[stop]N/ but continued as /ˈNVN/. Consistent absence of contrastive pre-stopped forms in the /ˈNVN/ environment holds for all Australian languages with contrastive pre-stopping: Adnyamathanha (McEntee and Butcher, 2021, pp. 32–33), Arabana (Hercus, 1994, pp. 38–39), Arandic languages (Koch, 1997, pp. 276–278), Olkola–Oykangand (Sommer, 1969, pp. 54–55).

Koch (1997) suggests that the most probable explanation for the absence of contrastive pre-stopping in the /ˈNVN/ environment is that the vowel was fully nasalized when the change was taking place elsewhere in the lexicon. Nasalization throughout the vowel is antithetical to a pre-stopped [ˈNVC_[stop]N] form, which requires a raised velum during the oral stop consonant closure in order to generate an increase of intra-oral air pressure. Due to this aerodynamic conflict, phonetic pre-stopping would not have occurred in this environment and, subsequently, did not develop into a contrastive pre-stopped form. By contrast, in an environment such as /ˈ(C)VN/ that did develop contrastive pre-stopping, the hypothesis is that the vowel was realized as oral throughout and, therefore, velum height posed no aerodynamic conflict for a pre-stopped realization.

A crucial factor in this hypothesis is, therefore, the height of the velum at the right edge of the vowel interval in a /(C/N)VN/ environment, where a potential pre-stopped realization might arise. In other words, the extent and degree of anticipatory nasalization in Pre-Arabana would determine whether or not an aerodynamic conflict might influence the development of this particular sound change, whereas carryover nasalization would have posed no particular aerodynamic issue.1 Most of the literature on vowel nasalization in Australian languages reports evidence for significant carryover nasalization but very limited evidence for anticipatory nasalization (Butcher, 1999; Butcher and Loakes, 2008, among others), although there are some potential discrepancies. Using dynamic nasal airflow data, Stoakes et al. (2020) show that anticipatory vowel nasalization is restricted in Bininj Kunwok both in timing and in magnitude, but much less so for pre-/ŋ/ vowels compared to vowels preceding other nasal consonants. Tabain et al. (2020) report, however, that there may be acoustic evidence for anticipatory nasalization in Arrernte which may not be obvious to human labellers, and that due to these atypical acoustic characteristics the “impression of a lack of vowel nasalization before nasal consonants may not be entirely true.” (p. 2748). Indeed, if nasal co-articulation was primarily a carryover phenomenon in Pre-Arabana and anticipatory nasalization was limited, then pre-stopping in the */ˈNVN/ environment would not necessarily have been impeded since the degree of nasalization would have been relatively low toward the end of the vowel interval, i.e., precisely the location where a pre-stopped realization might occur. However, the fact that pre-stopping did not occur in the /ˈNVN/ environment suggests that the vowel may have been characterized by a high degree of nasalization throughout.

Initial dropping, where the initial consonant of words is lost, is a common sound change in Australian languages (Dixon, 2002, pp. 589–602). The predominant diachronic pattern in Australian languages was that only singleton consonants occurred in word- and root-initial position historically, whereas both vowels and consonant clusters were largely absent word- and root-initially (Dixon, 2002, pp. 553–555). The diachronic reconstruction for Pre-Arabana follows the predominant pattern: The historical stage immediately preceding the synchronic Arabana that we report here contained only consonants word- and root-initially (Hercus, 1979). However, synchronic Arabana differs from Pre-Arabana in that it does have vowel-initial roots and words, and these synchronic vowel-initial forms derived from their Pre-Arabana counterparts through the process of initial dropping.

There have been two distinct initial dropping changes in Arabana. The first change is the reduction of homorganic semivowel-vowel sequences: */#ji/ > /#i/ and */#wu/ > /#u/. This sound change has operated exhaustively across the lexicon, i.e., there are no roots with initial /#ji/ or /#wu/ in synchronic Arabana, there are only roots with initial /#i/ or /#u/. In a vocabulary of 2142 words (Hercus, 1979), 100 (4.7%) have an initial /i/ and 54 (2.5%) have an initial /u/. Synchronic roots with initial /a/ did not arise via this same process, however, i.e., there were no changes such as */#ja/ > /#a/ or */#wa/ > /#a/. Rather, initial /a/ forms came about diachronically through the second initial dropping change, the reduction of a nasal-vowel sequence: */#ŋa/ > /#a/. This second change differs from the first in that it has not operated exhaustively across the lexicon: Synchronically, there is a contrast between */#ŋa/ and /#a/ in the lexicon with 93 (4.3%) words having an initial /ŋa/ and 119 (5.5%) words having initial /a/ (ibid). Some forms have undergone the */#ŋa/ > /#a/ change since Arabana was first recorded in the late 19^th century (Helms, 1896; Todd, 1886). For example, ‘yes’ is recorded as /ŋaraji/ in the earliest sources, but as /araji/ in later sources (Hercus, 1994, p. 32). The */#ŋa/ > /#a/ change was therefore an actively spreading process when the speakers reported in the current paper acquired Arabana as a first language.

There is no quantitative data on the phonetics of initial dropping as an active process from any Australian language. The unexamined assumption is that initial dropping involves the loss of all temporal and gestural content relating to the initial segment; in the case of the second initial dropping change outlined above, this would imply a loss of all gestural features related to the nasal onset /ŋ/. In the current study, we examine forms with an initial /a/ and show that these vowels are produced with nasal co-articulation patterns that are between those associated with post-oral and post-nasal vowel contexts. In other words, /#a/ forms show nasal co-articulation patterns that are intermediate between /#Ca/ and /#Na/ forms. We interpret this as an indication of a partial maintenance of the effect of co-articulation (i.e., carryover nasalization on the vowel) despite a complete loss of the temporal interval associated with the original source of the co-articulation (i.e., the nasal consonant). In this connection, it may be noted that the reductions of the homorganic semivowel-vowel sequences, *#ji > #i and *#wu > #u, can be analysed as involving loss of temporal content without loss of gestural content.2

The overall distribution of nasalization within vowels in Arabana is therefore a particular question of interest, since data related to synchronic productions of vowels in nasal environments may help clarify these remaining uncertainties about the absence of pre-stopped /ˈNVSN/ forms and the gestural nature of initial dropping involving */#ŋa/ > /#a/. However, there is no quantitative data on synchronic vowel nasalization in Arabana, data which could provide a database for the reconstruction of vowel nasalization and assessment of its potential interactions with sound change. The current paper addresses this shortcoming.

The analysis reported here is based on two datasets. Dataset 1 is experimental materials recorded from Sydney Strangways, a first-language speaker of Arabana born in 1932. These materials were recorded in four field trips between July 2017 and September 2020. Dataset 2 is audio data from Laurie Stuart, another first-language speaker of Arabana born in 1913, now deceased. These audio materials were not recorded under experimental conditions. Rather, they were originally recorded to accompany a set of Arabana teaching materials already in print (Wilson, 2004). In order to provide comparison data with Dataset 1, we cross-referenced both data sets for the same vowel environments. As the audio materials from Dataset 2 were produced for pedagogical purposes, however, the prosodic environment of the data varied from token to token. Given the difference in age between the two speakers, Laurie acquired Arabana as a first language within a larger cohort of speakers than Sydney. Laurie also acquired Arabana at an earlier stage in the active spread of the change */#ŋa/ > /#a/ across the lexicon.

For each token in both data sets, we annotated the first vowel along with its neighboring consonants. Word-initial vowel environments were included in order to observe patterns related to initial dropping, yielding the six vowel environments previously mentioned: /CVC/, /NVN/, /NVC/, /CVN/, /#VC/, and /#VN/.4 Annotation was carried out via visual inspection of the audio waveform and broadband spectrogram in Praat (Boersma and Weenink, 2021). The boundaries between nasals and vowels were determined by waveform amplitude and appearance of formant structures particularly in higher frequencies. Nasals have reduced waveform amplitude and different formant positions and attenuated formant structures compared with surrounding vowels. Stops were identified through a break in formant structures, i.e., closure, followed by turbulence in the waveform and a spike in high frequencies in the spectrogram, i.e., bursts. Visual examples of annotation boundaries for vowels, nasals, and stops are shown in Figure 2.

In the absence of physiological measures of nasality, we must infer the degree of nasalization acoustically. A traditional approach to estimating the degree of nasalization from vowel acoustics is to choose (generally) a single acoustic correlate in an implicit, top-down manner. Acoustic correlates that are commonly used as metrics of vowel nasality are A1-P0, A1-P1, or formant bandwidths (in particular, F1 bandwidth), due to the general tendency for these features to pattern with changes in nasality (Fujimura, 1961; Maeda, 1993; Feng and Castelli, 1996; Chen, 1997; Stevens, 2000). One issue with this approach is the assumption that the relationship between the chosen acoustic correlate and the degree of nasalization is similar across languages and speakers. However, single-metric acoustic analyses of vowel nasalization have been shown to be inconsistent in their ability to estimate the degree of nasalization in a way that is robust to linguistic, inter-speaker, and intra-speaker variation (Styler, 2017), due in part to the complex acoustic interactions between the oral and nasal cavities, which can be idiosyncratic and difficult to predict accurately and consistently (Carignan, 2018). An alternative approach is to ascertain the mapping between acoustic features and the degree of nasalization in an explicit, bottom-up manner through speaker-specific statistical learning of acoustic patterns that characterize the presence of nasality in the signal. An application of this approach is described in Carignan (2021), where the generalized method is referred to as Nasalization from Acoustic Features (NAF). In that study, speaker-specific machine learning of the acoustic realization of nasalization was carried out using principal components regression. In the current study, we extend the NAF method to use gradient-boosted decision trees to learn speaker-specific patterns of the relationship between acoustic variation and the height of the velum.

The data smooths in Figures 4, 5, 6 were created using the ggplot2 R package (Wickham et al., 2021) with locally weighted smoothing (“LOESS” smoothing). These smooths display the respective means and 95% confidence intervals bands (i.e., standard error of the mean) of the six vowel environments, each of which is denoted by a separate color and line style. The pattern for C_C is as expected: The NAF values are low throughout the entire vowel interval, suggesting that the velum remains raised throughout the vowel. The pattern for N_N is precisely the opposite: The NAF values are high throughout the entire vowel interval, suggesting that the velum remains lowered throughout the whole vowel. The patterns for the N_C and C_N environments are practically mirror images of one another: The degree of nasalization (as inferred from the NAF values) decreases in a linear manner throughout the vowel interval in the N_C environment and increases in a linear manner throughout the vowel interval in the C_N environment. This cline-like linear change is generally characteristic of phonetic (rather than phonological) vowel nasalization (Cohn, 1990), and the pattern for the C_N environment suggests not only that anticipatory nasalization does indeed occur in Arabana but that it is fairly substantial both in magnitude and in temporal extent, reaching 50% magnitude halfway through the vowel interval.

The patterns for the two word-initial vowel conditions are perhaps the most surprising. On the one hand, the degree of nasalization reaches a relatively high level at the end of the vowel interval in the #_N environment and a relatively low level at the end of the vowel interval in the #_C environment, suggesting that velum height is indeed conditioned by anticipatory co-articulatory effects in these environments. On the other hand, rather than the beginning of the vowel being oral in word-initial position (as might be expected for what is ostensibly a phonologically oral vowel), the degree of nasalization is already at a moderate level from the very start of the vowel interval in both the #_N and #_C environments, suggesting a moderate degree of vowel nasalization word-initially. In other words, the time-varying pattern of vowel nasalization for #_N is intermediate between those of C_N and N_N, while the pattern for #_C is intermediate between those of C_C and N_C. Since the 10% time points in these environments were not included in the model training, it is possible that these patterns involving the left edge of the vowel interval are simply the result of under-fitting of the model for these cases. We will explore these word-initial environments in greater detail as a post-hoc exploration in Section 4.3.

The distributions of all NAF values within the target vowels—i.e., all values combined, independent of the time course of the vowel interval—are displayed as both probability density functions and horizontal box plots in Figure 7, with the separate vowel environments denoted by color. The distributions for the N_N and C_C environments are as expected: A left-skewed distribution centered on 1 for the N_N environment, suggesting that most of the samples display a high degree of nasalization but with a range of samples displaying moderately less; and a right-skewed distribution centered on 0 for the C_C environment, suggesting that most of the samples display a low degree of nasalization but with a range of samples displaying moderately more. The results for the N_C and C_N environments are also unsurprising. The NAF values span the entire range for both environments, due to the time-varying patterns observed above: In the N_C environment the NAF values start high, end low, and pass through the entire range of values in between, and in the C_N environment the NAF values start low, end high, and pass through the entire range of values in between. There is, however, a difference between the two environments with regard to the average of the range of NAF values: The average is slightly higher in the N_C compared to the C_N environment. Finally, the results for the word-initial vowel environments mirror the time-varying qualitative results: In word-initial position, vowels generally display a moderate degree of nasalization. However, there is a large range of NAF values in the word-initial position, especially for the #_C environment.

The results of the Tukey contrast tests for the LME model are shown in Table 3. Significant differences at the Bonferroni-adjusted α level are marked with asterisks. All of the pair-wise differences are significant according to the Tukey contrast tests, and thus the ranking of the average degree of nasalization in the six vowel environments is as follows: C_C < #_C < C_N < N_C < #_N < N_N. There are two particularly important aspects of these results that we would like to note. First, the quantitative results support the qualitative observations that #_C patterns between C_C and N_C, while #_N patterns between C_N and N_N. Second, the greater degree of nasalization for N_C compared to C_N supports the claim of a greater degree of carryover compared to anticipatory vowel nasalization in Australian languages (Butcher, 1999; Butcher and Loakes, 2008; Stoakes et al., 2020), although the difference between the two environments in our data is small—indeed, this pair-wise difference yields the second smallest estimate magnitude of all 15 contrast tests. This suggests that, even though carryover nasalization is greater than anticipatory nasalization in Arabana, anticipatory nasalization is nonetheless fairly substantial.

The qualitative assessment in Section 4.1 suggested that word-initial vowels are moderately nasalized by both of the Arabana speakers, even in the ostensibly oral #_C environment, and the quantitative assessment in Section 4.2 confirmed these patterns. As previously noted, since the 10% time points in these word-initial environments were not included in the model training, it is possible that these patterns associated with the left edge of the vowel are simply the result of model under-fitting. In other words, since the model was never explicitly trained on acoustic features at the beginning of the vowel interval in word-initial contexts, it may be the case that the model predictions in these contexts are simply ambiguous (i.e., neither oral nor nasal, according to the model), and therefore fall in the middle of the 0–1 scale. In this section, we investigate the patterns of nasalization that may be inferred by two of the features, both of which are commonly used as single-metric acoustic correlates of vowel nasalization: (formant-compensated) A1-P0 and F1 bandwidth.

When the nasal cavity is acoustically coupled to the oropharyngeal cavity during the production of vowel nasalization, additional poles (spectral resonances) associated with the side-branching nasal cavity are introduced to the combined acoustic transfer function (Maeda, 1993; Stevens, 2000). Chen (1997) proposed two measures to capture the relationship between the amplitudes of oral and nasal poles, with A1-P0—i.e., the difference between the amplitude of the most prominent F1 harmonic and the amplitude of the harmonic estimated to correspond to the lowest-frequency nasal pole—being the most robust measure. Correction functions based on the frequencies and bandwidths of nearby formants were also proposed to help make the measure even more robust. Thus, (formant-corrected) A1-P0 has often been used as an acoustic correlate of nasalization, with a decrease indicating an increase in the degree of nasalization.

The increased surface area of the acoustically-coupled vocal tract and the soft tissues of the nasal cavity absorb acoustic energy more than occurs in the oral cavity alone (i.e., during oral vowel production), resulting in a global reduction in formant amplitude and widening of formant bandwidths (Stevens, 2000, p. 193). The widening of formant bandwidths is predicted to be most evident in lower frequencies, due to the relatively close proximity of oral and nasal poles. Thus, F1 bandwidth has often been used as an acoustic correlate of nasalization, with an increase indicating an increase in the degree of nasalization.

In order to directly compare the results for A1-P0 and F1 bandwidth, we use here P0-A1 instead of A1-P0; thus, an increase in either P0-A1 or F1 bandwidth is inferred as an increase in the degree of nasalization. Figure 8 displays the results for formant-compensated P0-A1 and Figure 9 displays the results for F1 bandwidth. Both of these measures suggest that the word-initial environment is characterized by a high degree of nasalization, even in the #_C environment (which is presupposed to be both phonologically and phonetically oral). In fact, based on these two measures alone, the degree of nasalization in these word-initial contexts is suggested to be even higher than when immediately adjacent to a nasal consonant, i.e., in N_N and N_C. Although this latter observation would be phonetically puzzling if indeed accurate, these results do at least support the findings from the primary NAF analysis that word initial contexts are nasalized in Arabana. A potential explanation for this pattern is discussed in Section 5.2.

In addition to evidence for substantial carryover nasalization, which is typical for Australian languages, we have also observed evidence for substantial anticipatory nasalization in Arabana, even though the overall degree of nasalization in the vowel is marginally less than observed for carryover nasalization. As noted in Section 1.1, a pre-stopped realization and anticipatory nasalization are antithetical to one another: The velum cannot be both high (for a pre-stopped production) and low (for nasalization) at the same time, i.e., at the right edge of the vowel. In order to resolve this aerodynamic conflict, either nasalization is lost and a stop can arise, or nasalization is maintained and a stop does not arise.8 Pre-stopping is a completed sound change process in Arabana, and our results show evidences of both of these outcomes: Vowels preceding oral consonants do not exhibit anticipatory nasalization (regardless of the historical origin of the consonant), and vowels preceding nasal consonants do exhibit anticipatory nasalization.

How, then, can we account for the one environment where pre-stopped forms did not arise historically in Arabana and where nasalization at the right edge of the vowel is synchronically preserved, i.e., /NVN/? Our results suggest that, synchronically, anticipatory nasalization is present in all pre-nasal contexts: /NVN/, /CVN/, and /#VN/. Each of these thus poses a potential aerodynamic conflict for the realization of a pre-stopped form, since the degree of nasalization is high at the right edge of the vowel. If we reconstruct similar patterns for Pre-Arabana, then the question remains: Why would pre-stopping occur in the /CVN/ and /#VN/ contexts, but not in /NVN/? We propose that the answer is to be found in the suggestion made by Koch (1997): The vowel was fully nasalized when the sound change was taking place.

Our results indicate a high degree of nasalization by both speakers throughout the entire vowel duration in the /NVN/ environment. This pattern would be at odds with co-articulatory planning that is prevalent in one direction (carryover) but restricted in another (anticipatory), as has been claimed for other Australian languages (Butcher, 1999; Butcher and Loakes, 2008; Stoakes et al., 2020). Our results suggest that anticipatory nasalization is not restricted in Arabana, however: Both carryover co-articulation and anticipatory co-articulation are substantial in both magnitude and temporal extent. When carryover and anticipatory planning are both active within a single vowel segment (such as in the /NVN/ environment) then consistent nasalization throughout the vowel is not at odds with co-articulatory planning but is, rather, expected. Thus, our findings of both extensive anticipatory nasalization in all pre-nasal contexts, as well as consistent nasalization throughout the vowel interval in specifically the /NVN/ context, are congruent with one another. We therefore reconstruct */NVN/ in Pre-Arabana as having the same realization as we have observed here, and we propose that it is precisely the consistent nasalization throughout the vowel interval that resisted the development of pre-stopping in this particular environment.

Our results show evidence of maintenance of vowel nasalization in environments where deletion of vowel nasalization might reasonably be predicted. With the sound change */#ŋa/ > /#a/, it is predicted that the loss of the timing and gestural coordination targets for the initial */ŋ/ would be accompanied by the loss of the carryover nasalization from this consonant onto the following vowel, i.e., the loss of the nasal consonant would involve the complete loss of its gestural content. Therefore, on general grounds, the prediction is that the degree of vowel nasalization in the pair #CVN and #VN should pattern similarly, while the degree of vowel nasalization in the pair #CVC and #VC should also pattern similarly. Rather, according to the NAF metric used this study, #VC patterns intermediately between #CVC and #NVC for both speakers, while #VN patterns intermediately between #CVN and #NVN for Sydney and similarly to #NVN for Laurie.

These results suggest that the sound change */#ŋa/ > #/a/ has involved the loss of the oral constriction associated with /ŋ/—as indicated by the loss of its temporal slot in speech production—but not a complete loss of the velum gesture. The end result is thus a partial maintenance of the effect of co-articulation (i.e., carryover nasalization on the vowel) even though the original source of the co-articulation (i.e., the nasal consonant) has been lost. As discussed in Section 1.2, the change */#ŋa/ > /#a/ was actively spreading through the lexicon when the speakers reported here acquired Arabana as a first language. This may be an important factor in the maintenance of some velic gestural content, even though the lingual gestural content of /#ŋ/ has been lost. The fact that Laurie acquired Arabana 19 years earlier than Sydney and therefore earlier in the active spread of the initial dropping change may also be relevant to the fact that Laurie’s #VN realizations are similar to his #NVN realizations whereas Sydney’s #VN realizations are intermediate between his #CVN and #NVN realizations.

We would like to note that these patterns occur in an environment where a prosodic boundary might have some affect on the degree of nasalization. Given that the prosodic context of the target words was not controlled for in the current study, these results for the word-initial environment should be approached with an appropriate degree of circumspection. This being said, domain-initial effects have generally been shown to result in the reduction of vowel nasality, not in its enhancement (Jang et al., 2018; Cho et al., 2017).

The results from this study support the notion that individual speech sounds are composed of an ensemble, or constellation of autonomous gestures—as posited by, e.g., Articulatory Phonology (Browman and Goldstein, 1986; Saltzman and Munhall, 1989). The findings for initial dropping of */#ŋa/ > /#a/ suggest that the separate lingual and velar gestures involved in the production of [ŋ] were decoupled at some diachronic stage, leaving only the velar gesture as a synchronic vestige of the nasal consonant when the lingual gesture was lost. Once decoupled, the temporal interval of the velar gesture would no longer be time-locked to the timing slot of /ŋ/ in */#ŋa/. Rather, the independent velar gesture, which was once part of the gestural constellation of /ŋ/, would then be “free” to shift to the temporal interval of the vowel, i.e., vowel nasalization. Decoupling of gestural sub-components of speech segments has been argued to play a key role in mechanisms of sound change, since such decoupling might permit independent temporal realignment (Beddor, 2009) and/or kinematic alteration (Carignan et al., 2021) of one of the decoupled gestures.

This has further implications for possible mechanisms of sound change. The development of contrastive nasal vowels arising from loss of a following nasal consonant is typologically common, e.g., Latin vinum [winum] ‘wine’ > Old French vin [vĩn] > Modern French vin [vɛ̃]. In these cases, the preceding vowel can be reinterpreted as nasal when the oral constriction of a nasal coda is reduced or lost. Our results suggest that any mechanism that is responsible for the emergence of contrastive vowel nasality from regressive coarticulation and subsequent loss of consonant nasality, might also function in the same manner for the emergence of contrastive vowel nasality from perseverative coarticulation and subsequent loss of consonant nasality. Of course, the development of vowel nasalization in these environments is not inevitable; rather, vowel nasality has been shown to be diachronically malleable (Sampson, 1999), even over relatively short timescales (Zellou and Tamminga, 2014). The between-speaker differences that we observe for the word-initial environments might provide a reason for such malleability. The differing patterns for Laurie’s and Sydney’s word-initial productions suggest that there may be some independence associated with the temporal and magnitudinal characteristics of the decoupled velar gesture: Although both speakers exhibit extensive temporal nasalization in word-initial environments, there is a marked difference between the two speakers with regard to the magnitude of nasalization. While Laurie exhibits high magnitude of nasalization word-initially, Sydney—who is only 19 years Laurie’s junior—exhibits much less so. Thus, a diachronic weakening of the magnitude of the velar gesture, even while the temporal extent of the velar gesture is maintained, may be one possible reason why contrastive vowel nasality has not developed in Arabana, despite partial maintenance of vowel nasalization in the sound change */#ŋa/ > /#a/.

One limitation of the current study is the assumption we make about our acoustic metric of nasality, the NAF prediction values. Single acoustic metrics of nasality such as A1-P0 or F1 bandwidth assume that the relationship between the metric and the degree of nasalization holds true for all situations: All languages, all speakers, all phonetic contexts, all recordings, etc. If this relationship fails for any number of reasons, then the reliability of these metrics in accurately capturing the degree of nasalization diminishes. Moreover, if some other, independent phonetic phenomenon affects these metrics in a systematic manner, then the resulting measurement may be artificially inflated due to the confound. For example, breathy voicing, like vowel nasality, is characterized by lower A1-P0 values and higher F1 bandwidth values (Chen, 1997; Simpson, 2012; Styler, 2015). The acoustic similarity between vowel nasality and breathy voicing may indeed be the cause for their perceptual similarity (Imatomi, 2005; Ohala and Amador, 1981) and also their co-occurrence in synchronic (Carignan, 2017; Garellek et al., 2016) and diachronic (Ohala, 1975) patterns. Ultimately, this means that phonetic effects that are independent of nasality (such as breathy voicing) can influence single-metric approaches in ways that can lead to incorrect interpretations about the degree of nasalization present in the acoustic signal.

Here, we make a different set of assumptions about the metric of nasality used in the study. We do not make the assumption that the relationship between a single given acoustic feature and the degree of nasalization holds for all situations; indeed, a key assertion of the NAF approach is that there is no single metric for which this is true, and so a better approach is to statistically learn the relationship between a conglomerate of different features and the degree of nasalization for each use case, e.g., each individual speaker. However, the assumptions we make instead regard the physiological state of the velum: We assume that time points immediately adjacent to oral consonants will coincide with a raised velum and time points immediately adjacent to nasal consonants will coincide with a lowered velum. When assigning the numeric value “0” to ostensibly oral contexts and “1” to ostensibly nasal contexts, we are drawing an interpretation that numeric values on this 0–1 scale correspond to lesser or greater degrees of nasality. It is important to remember what these values actually represent, however: Numeric predictions that observations are more or less similar to values that characterize the training data. Thus, we assume that NAF values near 0 correspond to “oral” when, more accurately, they correspond to “observations with acoustic features that are characteristic of a vowel adjacent to an oral consonant.” Likewise, we assume that NAF values near 1 correspond to “nasal” when, more accurately, they correspond to “observations with acoustic features that are characteristic of a vowel adjacent to a nasal consonant.”

Another limitation of the study is the small number of speakers. Exploratory research with only two speakers is merely a case study, and it is difficult to generalize from such a case study to the language as a whole. However, when working with critically endangered languages such as Arabana, it is often impossible to obtain data from more speakers or even to obtain more data from the same speakers, e.g., when one of the speakers has died since the original data were collected. It is in extreme cases like these when methodologies such as machine learning and data re-sampling allow researchers to make full use of the limited data available to them. We are heartened that, when using such methodologies, similar patterns emerge for both of the Arabana speakers included in this study. These similarities lend support to generalizing from our limited data to the Arabana language more broadly.

A final limitation of the study is the lack of physiological data (e.g., nasal airflow, nasometry, or physical estimates of velum height) to serve as ground truth for comparison with the NAF predictions, especially as a basis for validating the temporal patterns that we observe. All things being equal, more direct estimates of nasality (such as aerodynamic measures) are preferable to less direct estimates of nasality (such as acoustic measures). However, all things were not equal in the current study, namely: (1) physiological equipment were not available for use when the data collection originally took place, and (2) some of the data were collected from a speaker who has since passed away. However, in the original proof of concept of the NAF method (Carignan, 2021), nasometric measures were used as ground truth to compare with NAF predictions. The results indicated not only that the NAF method provides an accurate estimate of the magnitude of nasalization (an average correlation coefficient of 0.92 [SD = 0.05] with the ground truth), but also that the NAF method provides an accurate estimate of the timing of nasalization (a 95% Bayesian credible interval that contained the ground truth). Those findings support the use of the NAF method as a way of estimating both relative magnitude and timing of vowel nasalization in cases where physiological measurements are not practical or even possible, e.g., in the current study.