Microphones and Their Calibration

Abstract

The condenser microphone continues to be the standard against which other microphones are calibrated. A brief discussion of the theory of the condenser microphone, including its open-circuit voltage, electrical transfer impedance, and mechanical response, is given. The most precise method of calibration, the reciprocity pressure calibration method for laboratory standard microphones is discussed in detail, beginning with the principles of the reciprocity method. Corrections for heat conduction, equivalent volume, capillary tube, wave motion, barometric pressure and temperature are necessary to achieve the most accurate open-circuit sensitivity of condenser microphones. Free-field calibration is discussed briefly, and in view of the difficulties in obtaining more accurate results than those provided by the reciprocity method, references are given for more detailed consideration. Secondary microphone calibration methods by comparison are described. These methods include interchange microphone comparison, comparison with a calibrator, comparison pressure and free-field, and comparison with a precision attenuator. These secondary calibration methods, which are adequate for most industrial applications, are economically attractive and less time consuming. The electrostatic actuator method for frequency response measurement of working standard microphones is discussed with some pros and cons presented. An example to demonstrate the stability of laboratory standard microphones and the stability of a laboratory calibration system is described. Appendix A discusses acoustic transfer impedance evaluation, while appendix B contains physical properties of air, which are necessary for microphone calibration.