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A reactance tube circuit acting as a sinusoidally varying capacity in the frequency range from 20 to 200,000 cps is used to compensate the capacity variations of a condenser microphone in a high-frequency carrier circuit. The microphone is driven as in the electrostatic actuator method by a voltage of the same frequency, that modulates the reactance tube capacity. By adjusting amplitude and phase of the reactance tube modulation, the total capacity change of the circuit becomes zero. The sensitivity and phase lag of the microphone response can be determined from the actuator and modulating voltages when the circuit is properly calibrated. The circuit is advantageous in obtaining an equivalent pressure chamber calibration of condenser microphones up to 200 kc. The null method makes it useful for relatively unsensitive microphones.