Laser-based comparison calibration of laboratory standard microphones

Laboratory standard microphones, which have characteristics that are specified in international and U.S. national standards, are condenser microphones used for highly accurate sound pressure measurements and calibrations. A precision laser-based comparison calibration method for these microphones is described. This method uses laboratory standard microphones calibrated by the pressure reciprocity method as transfer standards, or reference microphones, to calibrate other laboratory standard microphones, or test microphones. Electrical drive current and microphone diaphragm velocity are measured while the microphones are driven as transmitters/sources of sound; the microphone diaphragm velocity is measured using scanning laser-Doppler vibrometry. The reference and test microphones are measured consecutively. Sensitivities determined using this method display very good agreement with those determined directly by the pressure reciprocity method for seven such test microphones at 250 Hz and 1000 Hz. At these frequencies of calibration, the expanded (coverage factor, k = 2) uncertainties of this comparison calibration method for these microphones are ±0.05 dB at both frequencies. These expanded uncertainties compare favorably to those of the reciprocity-based comparison calibration service done with a large-volume acoustic coupler at the National Institute of Standards and Technology, which are ±0.08 dB at 250 Hz and 1000 Hz.