Transmitted phonon drag signals through n-p-n silicon structures differ at 77°K by as much as a factor of 3 depending on whether the silicon surfaces are polished or mechanically lapped. This suggests that polished surfaces act like mirrors for the relevant phonons. Direct evidence for the mirror effect is obtained from a structure in which the two n-type layers, which act as transmitter and receiver for phonons, are located on the same sample surface while the opposite one is the phonon reflector. Based on these results, it should be possible to fabricate phonon lenses in the form of spherically polished surfaces, and to obtain phonon diffraction with reflecting gratings. The possibility to construct a Rowland circle arrangement and a solid state Michelson interferometer is discussed. © 1967.