Daniel Ballou and Dashdot are developing a system that transforms the humble cinder block into a high fidelity speaker. Concrete is a low resonance material that minimizes vibrations and reduces coloration of pure sound delivered by the drivers. A drawback of course is that it’s heavy to handle and ship, and labor intensive to mold. However, cinder blocks are available throughout the world for about a buck (or free in a construction site trash pile).
Source: Ballou Projects