I‑Simpa is a mature, open‑source desktop application for 3D numerical sound propagation modelling, designed for scientific acoustic simulations. The system provides a full GUI (built with wxWidgets and OpenGL) for defining acoustic scenes (sources, receivers, materials, meshes), running multiple calculation cores (particle tracing SPPS, classical theory TCR, and experimental TLM), and post‑processing results via point/surface receivers, impulse responses, and sound maps. It features a tree‑based data model with XML persistence, a Python scripting layer for automation and custom tools, and supports industry‑standard file formats (STL, PLY, GABE binary arrays). The codebase is predominantly C++ with a Boost.Python bridge, and is actively maintained by the Environmental Acoustic Research unit at Université Gustave‑Eiffel. The architecture follows a classic monolith pattern with clear separation: a core calculation library (`src/spps/`, `src/ctr/`), a 3D rendering engine (`src/isimpa/3dengine/`), a GUI layer (`src/isimpa/IHM/`), and a data manager with element/property tree. Python scripts in `SystemScript/` extend the UI through a plugin mechanism. While the project is functionally complete and validated against benchmarks, it exhibits moderate technical drift: bundled MathJax 2.7.5 (2018) for documentation, some French comments and variable names, and a few Python 2 remnants. Despite this, the component design—especially the octree spatial index, GABE binary I/O, and XML element tree—are cleanly abstracted and potentially reusable in other scientific or simulation contexts. The repository also includes extensive documentation and tutorials, though the experimental directivity feature is flagged as undocumented.