We introduce Acoustruments: low-cost, passive, and powerless mechanisms, made from plastic, that can bring rich, tangible functionality to handheld devices. Through a structured exploration, we identified an expansive vocabulary of design primitives, providing building blocks for the construction of tangible interfaces utilizing smartphones’ existing audio functionality. By combining design primitives, familiar physical mechanisms can all be constructed from passive elements. On top of these, we can create end-user applications with rich, tangible interactive functionalities. Our experiments show that Acoustruments can achieve 99% accuracy with minimal training, is robust to noise, and can be rapidly prototyped. Acoustruments adds a new method to the toolbox HCI practitioners and researchers can draw upon, while introducing a cheap and passive method for adding interactive controls to consumer products.