A theoretical approach to the mechanical coupling between phases in metal/polymer/metal sandwich composites is developed assuming perfect interfaces. Comparisons between experimental dynamic mechanical spectroscopy and theoretical results are made with special attention to the geometrical characteristics. Unexpected results concerning the dependence on the width of the specimen are explained using the strain energy method. The polymer thickness influence is explained only by structural effects instead of the interface effects often found in the literature. The temperature of the sandwich loss factor peak is revealed to greatly depend on the specimen dimensions.