Acoustruments (low-cost, passive, power-less mechanisms, made from plastic) can bring rich, tangible functionality to handheld devices. Through a structured exploration, the research team identified an expansive vocabulary of design primitives, providing building blocks for construction of tangible interfaces utilizing smartphones’ existing audio functionality. By combining design primitives, familiar physical mechanisms can all be constructed from passive elements. Acoustruments can also create end-user applications with rich, tangible interactive functionalities. Experiments show that the system can achieve 99% accuracy with minimal training, is robust to noise, and can be rapidly prototyped.