Surface microphones: a calibration method proposal based on IEC 61094-5

Surface microphones are especially used on aeronautical and automotive applications. One of the main struggles concerning reliability in its usage is to ensure the maintenance of specifications through periodic calibrations. Models that are designed with a metal membrane, in principle, could be calibrated by a special type of electrostatic actuator, provided that the laboratory possess the accessories required for such a fixation and testing. However, the design and type of encapsulation that is most common does not always enable this calibration modality. In general, surface microphones are calibrated solely during the manufacturing process, so much so that just a mere sensitivity check is left to assure stability, on one or two frequencies, naturally, considering that appropriate adapters for coupling are available to the user. Still, this sensitivity check is also challenging because the physical structure of this type of microphone is such that does not allow it to be inserted directly into the coupler of a sound level calibrator. Admittedly, any adapter provided by the manufacturer will necessarily go along with problems such as volume, leakage, and acoustic insulation. In this context, the periodic calibration of surface microphones, performed by accredited laboratories, would represent a huge metrological advance, and would support confidence in the results of measurements based on these transducers, such as with any other measurement microphone. This work aims to evidence the feasibility of determining sensitivity and frequency response using one of the mounting configurations described by IEC 61094 5. In this configuration the standard microphone and the surface microphone are arranged face-to-face orthogonally to the sound field. The source is excited with a sine-sweep and the reference value is given by a standard laboratory microphone, calibrated by reciprocity in pressure field. Furthermore, the work envisions the possibility of a free-field test configuration based on IEC 61094-8.