The boundary conditions of soft-sphere DEM are usually perfect reflectors of acoustic waves, leading to an unrealistic accumulation of energy. This situation is usually dealt with by global damping. In some situations, this solution is undesirable, so we present an alternative. If the grain-wall contact is made soft and dissipative, most acoustic energy incident on the boundary will be trapped and dissipated there. We show that these boundary conditions can efficiently damp both high and low frequency waves.