Gradual or abrupt? The phonetic path to morphologisation

•Non-segmental analysis of /uːl/ with high-speed ultrasound (20 speakers).•Statistical analysis of dynamic formant change.•In SBE, /uː/ fronting is limited before morpheme-final coda /l/.•Sound change can be affected by morphological structure from early-on.

While some sound changes occur in environments defined in purely phonological terms, others may become sensitive to morphological boundaries. In this paper, we investigate the phonetic nature of this latter diachronic development: does it happen through small gradient increments, or is there a categorical shift from one allophone to another? We focus on goose-fronting and /l/-darkening in Southern British English, the interaction of which is sensitive to morphological boundaries. Relatively retracted realisations of the vowel and dark realisations of the /l/ appear before a morpheme boundary, even when a vowel follows (e.g. fool-ing), whereas in monomorphemic words (e.g. hula), there is more /uː/-fronting, and the /l/ is relatively lighter. We analyse the phonetic realisation of such pairs as hula vs. fool-ing in 20 speakers of Southern British English using both acoustic and articulatory (ultrasound) instrumental methods. All the speakers express the morphological contrast in some way, although effect sizes vary dramatically. For some speakers, the contrast involves subtle articulatory differences without any clear acoustic consequences, whereas other speakers show robust differences employing multiple phonetic correlates. We therefore argue that the hula~fool-ing contrast would be misrepresented if analysed in terms of a small number of either /uː/ or /l/ allophonic categories. Instead, we interpret the results as supporting the predictions of phonological frameworks that incorporate phonetically-gradient morphologisation.