We discuss the prompt (ballistic and quasidiffuse) phonon physics associated with elementary particle interactions within silicon crystals at temperatures below 1 K, and the differences in the ballistic and quasidiffuse phonon production from primary electron recoils versus primary nuclear recoils within these crystals. We then summarize the results from a growing body of direct experimental evidence on prompt phonon signals from particle detectors bombarded with alphas, x-rays, and neutrons.