18.10.2023
For the measurement of infrasound, a reliable infrastructure for the traceability of measuring instruments and sensors has been lacking up to now. As an important development step, a new type of primary measuring calibration device for infrasound has now been set up in Department 1.6 Sound, which uses the decrease in ambient air pressure with altitude as an excitation signal.
The reliable measurement and assessment of infrasound is becoming increasingly important, especially in the context of managing the transition to renewable energy sources. The foundation for a reliable assessment of potential sound emissions are sound field measurements on the basis of the realisation of the unit Pascal for the alternating pressure through a primary calibration method. In the infrasound frequency range, however, the established primary calibration methods, such as the pressure reciprocity method, are only applicable to measurement microphones to a limited extent.
Therefore, an infrasound calibration setup was developed at PTB’s Department of Sound within the EMPIR project Infra-AUV [1], which uses the gradient of the ambient air pressure with altitude to generate a reference signal [2]. For this purpose, a device under test is mounted on a vertically rotating disc and, thus, subjected to a sinusoidal change in altitude due to the rotation. The amplitude of the resulting alternating pressure can be determined analytically from easily measurable variables such as ambient temperature, humidity and air pressure, while the frequency of the alternating pressure results from the speed of rotation.
Device for the primary calibration of measurement microphones in the infrasound region.
The new setup covers a frequency range from 0.1 Hz to 5 Hz with a planned extension up to 10 Hz. This provides a connection to the established pressure reciprocity method, which can be used with limitations from a frequency of 2 Hz upwards for the primary calibration of microphones. The frequency range of the new measurement setup is limited at the lower end by high background noise levels. At the upper end, the signal-to-noise ratio decreases due to the onset of flow noise as a result of the rapid movement of the microphone. The measurement uncertainty for determining the sensitivity of suitable microphones is less than 0.1 dB over the entire frequency range covered.
The calibration results from this new measuring station were compared in a bilateral comparison with results from the laser pistonphone of the laboratoire national de métrologie et d'essais (LNE) and good agreement was found. Further validation is being carried out in an international intercomparison as part of the Infra-AUV project.
Literature:
[1] Webpage of the EMPIR-project InfraAUV: https://www.ptb.de/empir2020/infra-auv/home/
[2] „Primary calibration for airborne infrasound utilizing the vertical gradient of the ambient pressure“, Marvin Rust et al 2023 Metrologia 60 045001 
https://doi.org/10.1088/1681-7575/acd941
Contact:
Marvin Rust, 1.6, E-Mail: 
marvin.rust(at)ptb.de