When an incident particle collides with an electron or nucleus in an insulating crystal, the recoil kinetic energy is converted rapidly into a burst of low-energy phonons. If the crystal is very pure and free of defects, and if it is very cold (T approximately 0.1 K), the phonons will propagate ballistically for distances of several centimeters. The authors report on experiments with two types of superconducting phonon sensors being considered for use on a new kind of particle detector, which is based on this effect. The device, which is called a Silicon Crystal Acoustic Detector (SiCAD), reads out phonons generated by particle scattering events.