Gaseous speed of sound is an important thermophysical property measured with high precision. The ideal gas heat capacity and the second virial coefficient can be derived from the speed of sound. The systematic errors in measurements of sound speed using fixed path interferometers are analyzed. Mathematic modes are proposed for the influences of fixed absolute error (δFAE) and fixed fractional error (δFFE) on thermodynamic properties derived from gaseous speed of sound. The influences from the errors in measurements are also simulated. The results show that the heat capacity ratio of ideal gas varies with 2δFAE by the effect of the fixed absolute error

but the influence on isobaric heat capacity of ideas gas may be higher than -100δFAE and increases with temperature for polyatomic gas. The second acoustic virial coefficient is affected by the fixed fractional error. Substances have the same absolute deviations in the second acoustic virial coefficients at the same temperature and fixed fractional error. The influences of fixed absolute error and fixed fractional error on the second virial coefficients are lower.