Abstract: A wave simulator including an enclosed receptacle filled with non-mixing liquids so that tipping of the receptacle simulates wave action and incorporating at least one flexible diaphragm that flexes to prevent excessive liquid pressure buildup within the receptacle while also maintaining the interior of the receptacle completely filled by the liquids. The receptacle has a two-piece construction injection molded from clear polycarbonate plastic and includes a unitary bottom wall and an upper wall member having unitary top, side, and end walls. Edges of the bottom wall are secured to the side and end walls of the upper wall member and an intermediate portion thereof includes a pair of openings which respectively receive a pair ofthe diaphragms. One preferred construction of the diaphragm includes annular steps that extend upwardly so as to be flexible downwardly to accommodate for liquid expansion.