Between Bouba and Kiki Lies Grougi: Modelling the Rough Middle Ground of Sound–Shape Correspondences

Cross-modal correspondences between audition and vision have been extensively studied in relation to pitch and loudness, but timbre remains more challenging to capture empirically and model acoustically due to its multidimensional nature. Nevertheless, recent research has shown that timbral qualities such as brightness, roughness, and clarity can be reflected in the visual properties of images. Building on the well-known Bouba/Kiki effect, the present study extends this framework to musical timbres and investigates visual forms that span a continuum ranging from full roundness to high angularity. We hypothesised not only that visually induced roughness would occupy the perceptual middle ground between these two extremes, but also that this categorisation would be meaningfully reflected in auditory perception. To test this, 27 participants were asked to associate 30 short sound stimuli with visual shapes using a morphing interface that enabled continuous transformation from a circle to a jagged, starlike form. The results revealed strong, non-random associations between sounds and the three shape categories (smooth, rough, and sharp). Acoustic modelling further supported these distinctions, showing that established models of auditory roughness and sharpness effectively predicted participants' assignments of the sound stimuli within these categories. Together, these findings offer empirical backing for perceptually grounded mappings between sound and image, with potential applications in artistic creation, education, and interactive media.