Experimental determination of acoustic attenuation under simulated Martian atmosphere

Abstract:

As an acoustic wave propagates though a planetary atmosphere, its amplitude is attenuated along its transmission path due to the geometric spreading of the wave front and intrinsic attenuation. Geometric attenuation is independent of the frequency. In contrast, the intrinsic attenuation depends on the wave frequency and is due to a fraction of the acoustic energy being dissipated by the gas due to viscous losses and molecular relaxation processes. On Mars, the carbon dioxide that mainly composes its atmosphere, is known to damp sounds more significantly than on Earth; this stems from the viscosity and vibrational relaxations of CO₂. The accurate knowledge of CO₂ attenuation is a major concern for ongoing infrasonic studies of NASA/InSight data, and for the upcoming NASA/Perseverance rover that includes two microphones. However, in addition to the lack of experimental data, only few theoretical studies have investigated the attenuation of Mars atmosphere [Williams 2001; Bass and Chambers 2001; Petculescu 2007] and their results are shown to significantly disagree in the infrasound and audible domains, mostly because of uncertainties in the CO₂ vibrational attenuation.

This study aims to validate existing models with experimental data acquired under a simulated Mars atmosphere (6-20 mbar, 100% CO₂) in the Aarhus Wind Tunnel Simulator II. Acoustic sensors were placed at distances ranging from 30 cm to 3 m from a speaker generating monochromatic pulses from 100 Hz to 7 kHz at five different pressures between 600 Pa to 2000 Pa. Acoustic data in a confined environment can be complex (e.g., because of echoes, near-field effects). Their processing is supported by full-wave numerical simulations under SPECFEM2D-DG, which models both the attenuation due to viscosity and the vibrational attenuation that is approached by a Zener model. Results on CO₂-induced acoustic attenuation coefficients will be presented, and compared to the literature. We will also consider additional measurements taken under terrestrial conditions, for a comparative study and the benchmarking of geometric losses in such a confined space.