Acoustic transfer admittance of cylindrical cavities in infrasonic frequency range

Abstract

Demand for calibration at infrasonic frequencies has emerged in response to earth monitoring problems. The primary standard for sound pressure is defined through the reciprocity calibration method specified in the International Electrotechnical Commission (IEC) Standard 61094-2:2009. This method is based on the use of closed couplers and is routinely applied by the National Metrology Institutes for a large frequency range; however, infrasonic frequencies below 2 Hz have not been explored until recently. The acoustic transfer admittance of the coupler, including the heat conduction effects of the fluid, must be modelled precisely to obtain accurate microphone sensitivity. IEC 61094-2:2009 provides two standardised solutions for the correction of heat conduction. However, researchers have noted significant deviations between these corrections at low frequencies in plane wave couplers, indicating that one or both techniques incorrectly calculate the influence of heat conduction. In this paper, the limitations of the standardised formulations at infrasonic frequencies are identified and two alternative solutions are proposed. An experiment is also reported, which highlights the discussed limitations of the standardised formulations for acoustic transfer admittance, while also demonstrating the validity of the proposed alternative formulations at frequencies down to 0.04 Hz.

Low Frequency Acoustic Pressure Meaurements Based on a Fabry Perot Refractometer

“…𝑇 0 being the ambient temperature, 𝑃 0 the ambient pressure and 𝛾 the ratio of isobaric and isochoric heat capacities [3]. This relationship enables to calculate temperature variation due to pressure variation.…”

“…𝛼 𝑇 being the the thermal diffusivity of the enclosed gas [3], with a value of 𝛼 𝑇 ≈ 2.11 × 10 −5 𝑚 2 • 𝑠 −1 for a cylinder. The Figure 5 shows the evolution of the depth of the thermal boundary layers in terms of frequency: In the lowest frequency range (i.e., below 10 mHz), the thermal boundary layers are large than the internal cavity diameter (22 mm).…”

“…Between these two "extreme" and ideal regimes (isothermal and adiabatic), the relationship between temperature variations and acoustic pressure is more complex but can be modelled [3]. The IMEKO 24 th TC3, 14 th TC5, 6 th TC16 and 5 th TC22 International Conference 11 -13 October 2022, Cavtat-Dubrovnik, Croatia 5 of 5 implementation and validation of the associated corrections in our all-optical infrasonic measurement system is in progress.…”

“…However, for infrasound applications, the extension of the reciprocity method to the lowest frequencies of interest is not straightforward and results in implementation problems particularly because of low signal-to-noise ratios, acoustic leakages, and unsatisfactory models used in their implementation [3], [4].…”

Natural and Anthropogenic Sources of Seismic, Hydroacoustic, and Infrasonic Waves: Waveforms and Spectral Characteristics (and Their Applicability for Sensor Calibration)

“…Both microphones/microbarometers and hydrophones can be calibrated under laboratory conditions, using comparable methods. On the one hand, reciprocity calibration is often used (Vincent et al 2018 ); on the other hand, methods based on optical interferometry are applied (e.g. Hayman et al 2013 ).…”