We are developing an entirely new type of particle detector, called a silicon crystal acoustic detector (SiCAD), which senses ballistic phonons generated when an incident particle collides with a nucleus or electron in a cube of crystalline silicon. For events which deposit energy greater than about 1 keV, a 1 kg SiCAD would have spatial resolution better than 1 mm 3 and energy resolution better than 100 eV. We describe our laboratory research utilizing carbon thermistors, superconducting transition edge devices, and superconducting tunnel junctions as phonon sensors on the crystal faces.