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Calibration Of Infrasound Measurement Devices

Infrasound measurement devices like low frequency microphones and dynamic pressure sensors are widely used in many branches of trade like aerospace industries (airflow mechanics) or acoustical engineering (infrasound measurements of geophysical events, emissions of wind energy plants, etc.). This paper presents a calibration system that allows the calibration of such devices in a frequency range as low as 0.1 Hz to 31.5 Hz. It is based on a special low frequency pressure chamber that uses a similar working principle as a pistonphone but is adapted to the requirements of this low frequency range. The chamber has a size that allows to place the whole device under test (DUT) inside the chamber. So no mechanical adaption between DUT and pressure chamber is required and thus the DUT can have a nearly arbitrary shape. Other than a pistonphone which is based on the assumption that the sound level can be calculated from the physical dimensions of the chamber by means of the adiabatic gas law, this calibration system requires the use of a reference standard sensor. This is due to the fact that in the low frequency range the gas compression becomes more and more an isothermal process the lower the applied frequency gets. Thus a pressure sensor is used as reference standard allowing high precision measurements (uncertainty < 0.05 dB). The paper discusses the question how the measurement uncertainty over the whole frequency can be determined although the reference pressure sensor is only calibrated by static pressure calibration. It also shows how other parameters like the homogeneity of the pressure field can be determined that contribute to the measurement uncertainty. Finally some calibration results will be presented.