Infrasound measurement – From primary calibration to field application

The reliable and traceable assessment of infrasound has become more and more important in recent years. One area of application for infrasound measurements is the worldwide sensor network of the International Monitoring System run by the Comprehensive Nuclear-Test-Ban Treaty Organization. On a smaller scale, the demand for the assessment of low-frequency and infrasound noise rises, one example being infrasound emitted by wind turbines. While the demand is there, the established calibration facilities for airborne sound do not sufficiently cover the infrasound frequency range. Furthermore, the established regulations for the assessment of noise have been designed for the audible frequency range and do not cover the infrasound frequency range. This thesis covers the full traceability chain from the realization of the unit Pascal in a primary calibration setup to the application of calibrated sensors in field environments. A primary calibration method and its realization in a measurement setup are presented which utilize the vertical gradient of the ambient pressure as stimulus. The calibration setup is capable of calibrating microphones in the frequency range from 0.1 Hz to 10 Hz by periodically moving them up and down. To disseminate this realization of the unit onto field devices, two secondary calibration setups were realized. Both setups are based on the comparison principle and utilize loudspeakers to create a significant alternating pressure in a closed cavity. These setups enable the calibration of a variety of infrasound sensors for field use, such as microphones, sound level meters, and microbarometers. In addition to traceably calibrated sensors, regulations regarding measurement procedures are necessary to ensure the reliable assessment of infrasound. Two field studies were conducted to provide a basis for the extension of regulations concerning noise assessment towards infrasound frequencies. First, microphones and microbarometers equipped with three different wind protection systems were compared in a field measurement setup near a wind park. These measurements allow the cross-validation of both sensor principles and enable a comparison of the different wind protection systems. The findings from this study highlight the need for extensive wind protection. Second, an investigation into the influence of small defects of a microphone diaphragm on the frequency response of the microphone was conducted. This study revealed that such defects can have a major influence of the infrasound sensitivity of a microphone, while being nearly undetectable with established calibrators. All in all, the results from this thesis are a valuable contribution to the calibration capabilities of infrasound sensors and to procedures regarding the reliable assessment of infrasound.