Hearing triangles: perceptual clarity, opacity, and symmetry of spectrotemporal sound shapes

In electroacoustic music, the spectromorphological approach commonly employs analogies to non-sonic phenomena like shapes, gestures, or textures. In acoustical terms, sound shapes can concern simple geometries on the spectrotemporal plane, for instance, a triangle that widens in frequency over time. To test the auditory relevance of such triangular sound shapes, two psychoacoustic experiments assessed if and how these shapes are perceived. Triangular sound-shape stimuli, created through granular synthesis, varied across the factors grain density, frequency and amplitude scales, and widening vs. narrowing orientations. The perceptual investigation focused on three auditory qualities, derived in analogy to the visual description of a triangle: the clarity of the triangular outline, the opacity of the area enclosed by the outline, and the symmetry along the vertical dimension. These morphological qualities seemed to capture distinct perceptual aspects, each linked to different acoustical factors. Clarity of shape was conveyed even for sparse grain densities, while also exhibiting a perceptual bias for widening orientations. Opacity varied as a function of grain texture, whereas symmetry strongly depended on frequency and amplitude scales. The perception of sound shapes could relate to common perceptual cross-modal correspondences and share the same principles of perceptual grouping with vision.