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We combined acoustic pharyngometry (APh) and speech data from 42 adults to benchmark eight widely used formant-based VTL estimators against incisors-to-glottis length and to test an interpretable two-stage bias-corrected linear estimator. Across more than 400\u2009000 central frames with valid F1\u2013F4, traditional quarter-wave, odd-harmonic, and dispersion-type estimators correlated with VTLAPh but showed poor out-of-sample anatomical recovery and strong calibration compression. Re-estimated one-stage linear models reduced mean absolute error (MAE; median \u22481.0\u2009cm) but still overestimated shorter tracts and underestimated longer tracts. A two-stage model markedly improved calibration and agreement, outperforming one-stage linear and nonlinear alternatives (median per-vowel MAE 0.39\u2009cm, median out-of-sample R2=0.83). Front and front-rounded vowels were the most informative. Speaker-level 95% limits of agreement were about \u00b10.9\u2009cm, indicating that the method is better suited to aggregated tract-scale estimation than to direct anatomical measurement. These results identify calibration bias as a central limitation of standard formant-based VTL estimators and provide a practical, interpretable route to tract-scale estimation from similarly processed labeled-vowel data under matched conditions.<\/jats:p>","DOI":"10.1121\/10.0044192","type":"journal-article","created":{"date-parts":[[2026,6,22]],"date-time":"2026-06-22T12:11:48Z","timestamp":1782130308000},"page":"5650-5666","update-policy":"https:\/\/doi.org\/10.1063\/aip-crossmark-policy-page","source":"Crossref","is-referenced-by-count":0,"title":["Vocal-tract length estimation from vowel formants benchmarked against acoustic pharyngometry"],"prefix":"10.1121","volume":"159","author":[{"ORCID":"https:\/\/orcid.org\/0000-0003-1102-4916","authenticated-orcid":false,"given":"Daniel","family":"Friedrichs","sequence":"first","affiliation":[{"name":"Zurich Forensic Science Institute 1 , 8004 Zurich,","place":["Switzerland"]},{"name":"Department of Computational Linguistics, University of Zurich 2 , 8050 Zurich,","place":["Switzerland"]}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Urs","family":"Guerrini","sequence":"additional","affiliation":[{"name":"Department of Computational Linguistics, University of Zurich 2 , 8050 Zurich,","place":["Switzerland"]}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-6739-0838","authenticated-orcid":false,"given":"Axel","family":"Ekstr\u00f6m","sequence":"additional","affiliation":[{"name":"Speech, Music & Hearing, KTH Royal Institute of Technology 3 , 114 28 Stockholm,","place":["Sweden"]},{"name":"Centre for Cultural Evolution, Department of Psychology, Stockholm University 4 , 106 91 Stockholm,","place":["Sweden"]}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-8494-6025","authenticated-orcid":false,"given":"Volker","family":"Dellwo","sequence":"additional","affiliation":[{"name":"Department of Computational Linguistics, University of Zurich 2 , 8050 Zurich,","place":["Switzerland"]}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-3969-6549","authenticated-orcid":false,"given":"Steven","family":"Moran","sequence":"additional","affiliation":[{"name":"Linguistic Research Infrastructure (LiRI), University of Zurich 5 , 8050 Zurich,","place":["Switzerland"]},{"name":"Institute of Biology, University of Neuch\u00e2tel 6 , 2000 Neuch\u00e2tel,","place":["Switzerland"]}],"role":[{"vocabulary":"crossref","role":"author"}]}],"member":"231","published-online":{"date-parts":[[2026,6,22]]},"reference":[{"key":"2026062208114419700_c1","first-page":"2102","article-title":"Estimation of the vocal tract length of vowel sounds based on the frequency of the significant spectral valley","year":"2018"},{"key":"2026062208114419700_c2","doi-asserted-by":"publisher","first-page":"5588","DOI":"10.3758\/s13428-023-02288-x","article-title":"A practical guide to calculating vocal tract length and scale-invariant formant patterns","volume":"56","year":"2023","journal-title":"Behav. 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