A new, numerical method was developed to calculate density second virial coefficients B(T) from sonic velocity measurements in gases at low pressures. Unlike existing methods, this procedure requires no model assumption as to the form of the temperature variation of B(T). Rather, it differences the measured accoustic second virial coefficient according to a new mathematical identity. While two higher-ordered terms in the identity must be ignored to initiate the numerical calculations, the magnitude of these terms can later be found from the initial determination of B(T). This article describes new equations developed, numerical procedures, calculations with Redlich-Kwong and Peng-Robinson gases and, with a Lennard-Jones model gas, to show the accuracy of the method and th magnitude of the two error terms, and the method applied to existing sets of sonic velocity data.