Investigating phonological theories with crowd-sourced data: The Inventory Size Hypothesis in the light of Lingua Libre (2022)

Abstract: Data-driven research in phonetics and phonology relies massively on oral resources, and access thereto. We propose to explore a question in comparative linguistics using an open-source crowd-sourced corpus, Lingua Libre, Wikimedia’s participatory linguistic library, to show that such corpora may offer a solution to typologists wishing to explore numerous languages at once. For the present proof of concept, we compare the realizations of Italian and Spanish vowels (sample size = 5000) to investigate whether vowel production is influenced by the size of the phonemic inventory (the Inventory Size Hypothesis), by the exact shape of the inventory (the Vowel Quality Hypothesis) or by none of the above. Results show that the size of the inventory does not seem to influence vowel production, thus supporting previous research, but also that the shape of the inventory may well be a factor determining the extent of variation in vowel production. Most of all, these results show that Lingua Libre has the potential to provide valuable data for linguistic inquiry.

## introduction one of the main challenges in data-driven research on the phonetics-phonology interface is the access to reliable, exploitable oral resources in sufficient amounts. while linguists working on other linguistic levels such as semantics or syntax can use written data as a proxy for language production, phoneticians and phonologists are limited to oral data, thus relying on audio recordings for vocal languages or video recordings for signed languages. accessing massive amounts of such data is difficult enough, especially for studies in language comparison, that require such amounts in not one, but at the very least two languages. to overcome this challenge, researchers developed two strategies. on the one hand, they can collect their own corpora, e.g., the cmu wilderness corpus @xcite or its emanation, the voxclamantis corpus @xcite , or other types of language-specific laboratory recordings such as the timit database for english @xcite or nccfr for french @xcite . on the other hand, they can gather audio recordings from other sources such as tv or radio shows, as was done for instance in the framework of the international project oseo quaero ( www.quaero. org/ ), or from audio books, as exemplified by the librispeech corpus for english @xcite . both options have the disadvantage of being overly costly, both in money and human resources, and sometimes not freely accessible to the community. a third path has been recently explored: crowd-sourced data, recorded by volunteers and therefore much less costly in time and money and generally opensource. the project common voice @xcite , https://commonvoice.mozilla.org ) for instance was launched in 2017 by mozilla for the intended purpose of creating a free database for the development of speech recognition software. in march 2022, it contains ∼18,000 hours of speech, 14,000 of which have been validated by other speakers, in 87 languages. in the present paper, we explore a similar project: lingua libre, a participatory linguistic media library developed by wikimedia france ( https: //lingualibre.org ). it was launched in 2015, and, in march 2022, it counts ∼700,000 recordings in 148 languages across 775 speakers. this database is interesting to explore because it differs from common voice in the fact that its aim is not primarily the development of new technologies, or even linguistic inquiry in general, but patrimonial conservation of languages. lingua libre was used only once for academic purposes, i.e., to automatically estimate the transparency of orthographies in 17 languages @xcite . with this study, we aim to show that such data can be easily processed and useful to answer phonological questions in linguistic typology. in this proof of concept, we explore the realization of vowels by comparing two romance languages: italian and spanish. the outline of the paper is as follows. in section 2, we describe our research question to justify our choice of languages. in section 3, we present our corpus and methodology. in section 4, we provide an analysis of the vowels in italian and spanish. section 5 concludes and discusses the results. ## the inventory size hypothesis vs the vowel quality hypothesis in this paper, we offer to use lingua libre to tackle the question of vowel production with regards to vowel inventory. our research question stems from various theories regarding the shape of vowel inventories in the world's languages. our study however focuses on synchronic phonetic variation with regards to phonological systems (on the phylogeny of vowel systems in the languages of the world, see @xcite and references therein). the original vowel dispersion theory (liljencrants and @xcite and a few years later the adaptive dispersion theory @xcite , stem from the h&h ("hypoand hyperspeech") model of communication, that assumes that speakers tend toward minimal and sufficient perceptual contrast, i.e., operate a trade-off between articulatory economy (hypospeech) and perceptual distinctiveness (hyperspeech). in the original works, these theories are the foundation for phylogenetic research on the distribution of vocalic categories in the languages of the world, for instance to explain why three-vowel systems usually display /a, i, u/ and not, say, /a, y, u/. phoneticians however have particularly focused on one hypothesis that emerges from this model: the more vocalic categories the language has in its phonemic inventory, the less phonetic variation the corresponding vowel realizations will display. this is the hypothesis we ourselves focus on in the present paper, to which we will refer as the inventory size hypothesis, henceforth ish. this hypothesis has been tested in a number of studies, with contradictory results. @xcite on american english, @xcite on french and two dialects of arabic and larouche and steffann (2018) on quebec french and inuktitut support the ish while bradlow (1995) on english and spanish, @xcite on english, spanish and french, @xcite on 5 dialects of catalan, @xcite on 5 dialects of chinese and @xcite on 3 german languages, do not provide evidence in favor of the ish, which can be due, for the last three at least, to the genetic and geographical closeness of the languages and possible bilingualism of the speakers. studies on larger sets of languages however tend to invalidate the hypothesis: @xcite found inconclusive results on 7 languages across 6 language families; @xcite on 28 languages, gendrot and adda-decker (2007) on 8 languages across 4 families, and salesky et al. (2020) on 38 languages across 11 families, found no evidence for an effect of inventory size on the global acoustic space. building on these negative results, we suggest that it may not so much be the number of categories but their actual quality that influences the vowel's realizations. for instance, between two imaginary languages a and b displaying /a, e, i, o, u/ vs /a, e, i, y, o, u/ respectively, it is also possible that not all the categories in language b will display less variation than those in language a: only [i] and possibly @xcite , which compete with /y/ in b but not in a, would show less variation in b than in a. we propose to refer to this restatement of the original hypothesis, as the vowel quality hypothesis, henceforth vqh. in this paper, we aim to test this alternative: either the ish is valid, and all the vowels of the system will be affected by the size of the inventory, or the vqh is more accurate, and only some vowels or some acoustic parameters will be affected depending on the other vowels comprised in the system. the third possible outcome is that neither the ish nor the vqh is accurate. to test our hypothesis, we focus on the f1 and f2 values of the vowels in two romance languages: spanish and italian. spanish has a limited vowel inventory, with only 5 categories /a, e, i, o, u/ while italian has 7: /a, e, e, i, o, o, u/. their inventories differ only in the number of degrees of aperture (spanish has open, mid and closed vowels while italian has open, mid-open, mid-closed and closed vowels), which manifest as variation on the first frequency, f1. if the ish is valid, we expect vowel productions from each language to differ in both f1 and f2, while if the vqh is valid, we expect spanish and italian vowels to differ only in f1. ## materials and methodology as a crowd-sourcing tool, lingua libre allows any speaker to log in, fill in a profile with basic metadata for themselves of for other speakers, and record themselves or their guests reading lists of words in their language. the device detects pauses, which allows for the recording to end when the word has been read and the next recording to start automatically after, therefore effortlessly generating relatively short audio files for each word. each audio file is supposed to be titled on the same template of 'language -speaker -item'. for example, for the recording 'spa.-marreromarcosolucionar.wav', the language is spanish ('spa'), the speaker id is 'marreromarco', and the recorded item is 'solucionar', 'solve'. all audio files are under a creative commons licence, i.e., open-source. first, the recordings are scrapped from the lingua libre database. in the present study, we extract a subsample of 500 items for /a, e, i, o, u/ in each language, to counter the fact that both languages have different amounts of data points and to also control for number of speakers (5) in each language. in total, we have 500 occurrences for each of the 5 vowels in both italian and spanish, which results in 5000 tokens. to avoid a potential sample bias, the sampling of tokens is conducted 10 times. we also took care to limit our investigation to the european variety of spanish, to avoid any mismatch with the more limited geographical expansion of italian. second, the recordings are segmented and aligned using webmaus @xcite , the online open-access version of the maus software @xcite . maus creates a pronunciation hypothesis graph based on the orthographic transcript of the recording (extracted from the name of the audio file) using a grapheme-to-phoneme converter. during this process, the orthographic transcription is converted to the speech assessment methods phonetic alphabet (sampa). the signal is then aligned with the hypothesis graph and the alignment with the highest probability is chosen. experiments have shown that the maus-based alignment is 95% accurate compared to human-based alignments @xcite . third, the selected vowels are extracted from the recordings and analyzed in terms of formants. for each recording of each vowel, the mean f1 and f2 of the entire sound are calculated. the mean formants are considered to attenuate the effect of co-articulation with the left and right contexts. ## results: shape of the inventory, more than size, influences vowel production we focus on the f1 and f2 values for the 5 vowels that spanish and italian have in common, /a, e, i, o, u/. our hypothesis is that, if the ish is valid, we will find variation in both f1 and f2 for all vowels, while if the vqh is valid, we will find variation only in f1, especially in /a/, /e/ and /o/, which are in direct competition with /e/ and /o/. as general information, figure 1 provides the mean values for f1 (top tier) and f2 (bottom tier) in italian (left brackets) and spanish (right brackets) for all 5 vowels of interest. it shows that f1 is significantly lower in spanish for all 5 vowels, while f2 is statistically higher only for back vowels. to test our hypotheses, however, we are less interested in f1 and f2 values in general than in their variation. figure 2 shows the variation coefficient (standard deviation divided by the mean) of f1 (top tier) and f2 (bottom tier) for each replication of each vowel category in italian (left brackets) and spanish (right brackets). each point represents the variation coefficient of a formant and a vowel for a replication. these results show that there is significantly less variation in f1 in italian /a/, /e/, /o/ and /u/ than in spanish, thus supporting the vqh. the difference between f2 variation coefficients is also significant but inverted for /e/, /i/, and /u/ where we observe more variation for italian than for spanish, thus invalidating the ish. these results are also supported by the linear mixed models we conducted (in both bayesian and non-bayesian versions) based on the 500 data points from each of the 10 replications. first, ## conclusion and discussion we used crowd-sourced data to test two competing hypotheses in language typology: the production of vowels is influenced either by the size of the inventory, or by its shape. our proof-of-concept on italian and spanish shows that the size of the inventory does not influence the realization of vowels, but the exact quality of the vowels at hand does. our study also points to several caveats. first, all audio files were not properly labeled and were thus unusable. moreover, from a human point of view, it should be noted that crowd-sourced data heavily rely on the participants' good will and that researchers have no choice but to trust the provided metadata. one possible solution to that last problem would be for lingua libre to propose a verification tool, as does common voice, to improve the reliability of the data and metadata. however, crowd-sourced data proved to be a promising tool for linguistic inquiry, especially to investigate language universals, and could thus be tested on more substantial sets of languages.