The reflection and transmission coefficients of a horizontal double plate system were derived in terms of the reflection and transmission coefficients of a single plate case. A spectral decomposition technique was used to evaluate the spectral parameters for the incident, reflected and transmitted wave spectra. Experiments were conducted for the case of a seaward surface plate and a submerged leeward plate and it was established that the optimum degree of submergence for the leeward plate would be about 0.10 to 0.20 for minimum wave transmission.