Abstract
The demand for reliable and traceable measurements of airborne infrasound has risen, one major application being the International Monitoring System run by the Comprehensive Nuclear-Test-Ban Treaty Organization. However, the current calibration methods do not sufficiently cover the infrasound frequency range. In this paper, we present a calibration method for microphones in this frequency range and its implementation as a measurement setup. The method is based on the vertical gradient of the ambient pressure as a stimulus. A DUT is subjected to an alternating pressure by periodically changing its altitude. The measurement setup realizes such a periodic altitude change by means of a vertically rotating disk and is capable of calibrating measurement microphones in a frequency range from 0.1 Hz to 5 Hz with a planned extension to 10 Hz. A measurement uncertainty of 0.07 dB at maximum could be realized. Particular attention was paid to the mechanics of the measurement setup to ensure that the DUT moves in a precisely determined orbit. We present example calibrations and an uncertainty budget for a Brüel & Kjær 4193 measurement microphone in the frequency range from 0.1 Hz to 5 Hz. Finally, we demonstrate the performance of the calibration method by comparing the acquired results to other calibration techniques showing an agreement better than 0.1 dB.
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