The inclusion of piezoceramic rods in a passive polymer can greatly enhance the piezoelectric effect. We analyze this phenomenon from the point of view of the theory of composite materials, focusing on the evaluation of dh, the hydrostatic piezo-electric coefficient, dhgh, the hydrophone figure of merit, and kh, the hydrostatic coupling factor measuring the efficiency of energy conversion. We show how these quantities can be expressed as algebraic functions of a single microstructural parameter, p. In the limit of large elastic contrast (soft matrix, hard ceramic), this theory gives a first-principles explanation of the decoupling effect of the composite on the hydrostatic piezo-electric coefficient, as well as the role played by the porosity and Poisson's ratio of the matrix phase. Using a differential effective medium type scheme, we compute the aforementioned properties for two commercial polymer-piezoceramic composites. It is thus shown that this effective medium approach provides a simple yet self-consistent framework for the design of effective 1-3 piezocomposites.