This paper presents the theory, design, and validation of a microphone calibrator used to calibrate multiple microphones simultaneously. This work was done in conjunction with the development of an acoustic energy density probe, which was used to validate the calibrator. The probe uses multiple microphones to acquire the data needed to compute the acoustic energy density. The probe microphones are 0.006 m diameter electret microphones which typically do not have an ideal “flat" response over a wide range of frequencies as compared to precision condenser microphones. The probe microphone characteristics prompted the need for simultaneous, multi-microphone magnitude calibration. The idea behind the calibration process was to simultaneously subject each microphone on the probe to the same known acoustic pressure over the frequency range of the probe (0-2 kHz). This is done using equal-length small-diameter tubes connected to a single source at each microphone. The calibrator was modeled using and equivalent circuit model. The model results are presented and compared to measured results. The calibrator was validated to result in the same pressure along individual paths to each microphone simultaneously. Test results show that the calibrator can calibrate each probe microphone within ± 0.5 dB up to 2000 Hz, and within ± 1 dB up to 4900 Hz with a confidence level of 95%. Directivity tests were performed to further characterize the probe. Using a correction factor the probe is shown to be directionally independent.