A study was undertaken to obtain the free‐field and the coupler calibrations of several probe‐tube microphones with tubes of different sizes and to determine those characteristics of a probe tube which are necessary for accurate measurements in both a free field and in closed couplers. For accurate measurements with a probe‐tube microphone, precautionary measures must be taken. The probe tube must be sealed to the frame of the microphone in order to prevent an acoustic leak. Such a leak causes reduction in the output of the probe‐tube microphone at the lower frequencies. A probe tube with a high input impedance should be used for making measurements in a closed coupler. A high input impedance is necessary if the pressure in the coupler is to remain the same as before the insertion of the probe tube. If measurements are to be made with the accuracy of measurements made with a standard laboratory microphone, there is a significant difference between the pressure and the free‐field calibrations of probe‐tube microphones.