## 4. Actively Augmenting Mobile Devices
### Active Augmentation Framework
## 5 Articulating the Hybrid Mobile Instrument

Current technologies allow one to blur the boundary between the physical/acoustical and the virtual/digital world. Transforming a physical object into its virtual approximation can be done easily using various techniques (see Section 1.5). On the other hand, recent progress in digital fabrication, with 3D printing in particular (see Section 1.6), allows us to materialize 3D virtual objects. Even though 3D printed acoustic instruments don't compete yet with \"traditionally made\" ones, their quality keeps increasing and they remain perfectly usable. This section generalizes some of the concepts used by the BladeAxe[42], where sound excitations made by physical objects are used to drive physical-model-based virtual elements. It allows for instrument designers to arbitrarily choose the nature (physical or virtual) of the different parts of their creations. We introduce a series of tools completing the framework presented in this paper to approach musical instrument design in a multimodal way where physical acoustical parts can be \"virtualized\" and vice versa. First, we give an overview of our framework to design mobile hybrid instruments. We provide a set of rules to help the instrument designer to make critical decisions about the nature (acoustical or digital) of the different parts of his instrument in the context of mobile devices. Then we introduce the Faust Physical Modeling Library (FPML), \"the core\" of our framework, that can be used to implement a wide range of physical models of musical instruments to be run on a mobile device (e.g., using faust2 smartkeyb). Finally, we demonstrate how custom models can be implemented using mesh2 faust[56] and FMPL. ### Framework Overview

#### 5.1.1 From Physical to Virtual

In Section 1.5, we gave an overview of different physical modeling techniques that can be used to make virtual versions of physical objects designed to generate sound (i.e., musical instruments). The framework presented in this section is a bit more limiting and focuses on specific modeling techniques that are flexible and computationally cheap (which is a crucial feature for mobile development). Figure 10: _Bouncy-Phone_ by [PERSON], _Something Else_ by [PERSON], and _Mobile Hang_ by [PERSON] (from left to right). Various linearizable acoustical physical objects can be easily turned into modal physical models using their impulse response [28]. [PERSON] et al. implemented irzfaust [57], a command-line tool taking an impulse response in audio format and generating the corresponding Faust physical model compatible with the Faust Physical Modeling Library presented in Section 5.2. This technique is commonly used to make signal models of musical instrument parts (e.g., acoustic resonators such as violin and guitar bodies, etc.). Modal physical models can also be generated by carrying out a finite element analysis (FEM) on a 3D volumetric mesh. Meshes can be made \"from scratch\" or using a 3D scanner, allowing musical instrument designers to make virtual parts using a CAD model. mesh2 faust[56] can be used to carry out this type of task. Modal models generated by this tool are fully compatible with the Faust Physical Modeling Library presented in Section 5.2. While this technique is more flexible and allows us to model elements \"from scratch,\" generated models are usually not as accurate as the one deduced from the impulse response of a physical object that faithfully reproduce its harmonic content. Even though it is tempting to model an instrument in its whole using its complete graphical representation, better results are usually obtained using a modular approach where each part of the instrument (e.g., strings, bridge, body, etc.) are modeled as single entities. The Faust Physical Modeling Library introduced in Section 5.2 implements a wide range of ready-to-use musical instrument parts. Missing elements can then be easily created using mesh2 faust or ir2 faust. Various examples of such models are presented in Sections 5.2.2 and 5.2.3.