The frequency- and direction-dependence of transmission coefficients at interfaces was investigated theoretically. The interfaces are formed by having two Lennard-Jones (L-J) materials differing in mass and interatomic potential equally divided at the center of an FCC lattice system. A single frequency wave-packet is generated at one end of the system and allowed to propagate through the system until all interactions with the interface are complete. The transmission coefficient is then calculated by comparing the energy of the packet that is transmitted with the original wave-packet. Results show a difference in transmission when the wave-packet originates from opposite sides.