This paper describes a precision ac null‐detecting arrangement that has a resolution of better than 0.001 dB for reciprocity calibration of condenser microphones. Systematic errors are analyzed in depth. The uncertainty of the absolute open‐circuit sensitivity is predicted to be less than 0.005 dB, which is approximately an order of magnitude improvement over conventional calibration arrangements. With international standardization, a comparison accuracy of a few thousandths of a decibel can be achieved.