Abstract: A planar-type loudspeaker incorporating a substantially planar diaphragm constructed from a pre-expanded cellular plastic material, such as polystyrene, in which separate portions of the diaphragm have different densities. The higher density portion is designed for the reproduction of high frequencies, and the lower density section is used for the reproduction of low frequencies. In one embodiment, the diaphragm is formed by laminating together a pair of diaphragm members having the different densities to define a single sound producing region, to which a single voice coil assembly is coupled. In another embodiment, the diaphragm is formed as a unitary, one-piece structure having separate but contiguous sound producing regions, each with its own density material and voice coil assembly for reproducing a specified frequency range of sound.