A continuous process for the manufacture of a polycarbonate comprises: combining an aromatic dihydroxy compound stream and a diaryl carbonate stream to form a mixture; controlling a weight or molar ratio of the diaryl carbonate to the aromatic dihydroxy compound by: measuring a sound velocity, Vs, in the mixture at an operating temperature of the controlling step; and determining the weight or molar ratio of the diaryl carbonate to the aromatic dihydroxy compound in the mixture; and adjusting a flow rate of at least one of the aromatic dihydroxy compound stream and the diaryl carbonate stream, if needed, to achieve a desired weight or molar ratio of the diaryl carbonate to the aromatic dihydroxy compound; and polymerizing the aromatic dihydroxy compound and the diaryl carbonate in the presence of a catalyst to produce the polycarbonate.