Abstract The increased vocal effort associated with the Lombard re- flex produces speech that is perceived as louder and judged to be more intelligible in noise than normal speech. Previous work illustrates that, on average, Lombard increases in loud- ness result from boosting spectral energy in a frequency band spanning the range of formants F1-F3, particularly for voiced speech. Observing additionally that increases in loudness across spoken sentences are spectro-temporally localized, the goal of this work is to further isolate these regions of maximal loud- ness by linking them to specific formant trends, explicitly con- sidering here the vowel formant separation. For both normal and Lombard speech, this work illustrates that, as loudness in- creases in frequency bands containing formants (e.g. F1-F2 or F2-F3), the observed separation between formant frequencies decreases. From a production standpoint, these results seem to highlight a physiological trait associated with how humans in- crease the loudness of their speech, namely moving vocal tract resonances closer together. Particularly, for Lombard speech, this phenomena is exaggerated: that is, the Lombard speech is louder and formants in corresponding spectro-temporal regions are even closer together.