A theoretical study has been made on the free‐field correction of laboratory standard microphones mounted on a semiinfinite rod. In the analysis, we have discussed the diffraction phenomena of plane sound waves by the actual microphone system, taking into consideration the effect of a recess as well as the interaction between microphone diaphragm and medium. A new method of solving wave equations met with these boundary conditions, called the infinite‐matrix method, is presented in this paper. Our whole problem is divided into three parts. The first is concerned with the external free field, which can be solved rigorously by Wiener‐Hopf technique; the others refer to the solution of the sound field inside a recess and, finally, to motions of a diaphragm. After putting these solutions into the form of matrix equations with unknown variables, we obtain a system of simultaneous matrix equations by connecting solutions with each other. Numerical results are given for the free‐field correction of the Western Electric Company type 640AA microphone. Comparison with the experiment, made at the National Bureau of Standards, indicates that sufficient agreement is obtained.