A key challenge in the design of Virtual Musical instruments (VMIs) is finding expressive, playable, learnable mappings from gesture to sound that progressively reward practice by performers. Designing such mappings can be particularly demanding in the case of non-contact musical instruments, where physical cues can be scarce. Unaided intuition works well for many instrument designers, but others may find design and evaluation heuristics useful when creating new VMIs. In this paper we gather existing criteria from the literature to assemble a simple set of design and evaluation heuristics that we dub articulacy. This paper presents a design case study in which an expressive non-contact finger-tracking VMI, Sound Spheres, is designed and evaluated with the support of the articulacy heuristics. The case study explores the extent to which articulacy usefully informs the design of a non-contact VMI, and we reflect on the usefulness or otherwise of heuristic approaches in this context
