Measuring the Free Field Correction for a New Condenser Microphone

In the development of the new Bruel & Kjaer Measurement Condenser Microphone, it was found necessary to thoroughly investigate the diffraction characteristics and to use several calibration techniques to establish the difference between the microphone's free field and pressure response. The techniques and results of these investigations are described as the following: (A) Measuring the pressure distribution around a model of the microphone placed in a free sound field and the sensitivity distribution over the microphone diaphram. In conjunction with measurements made with a specially designed concentric electrostatic actuator, the effective sound pressure increase on the diaphram is calculated. (B) Measuring the microphone sensitivity as a function of frequency according to the free-field reciprocity method, and comparing the results with the pressure response of the same microphones measured by means of the electrostatic actuator method. The free-field correction obtained by the two different methods (A) and (B) showed excellent agreement.