The Impact of Interstitial Air Pressure on Sand Acoustic Emissions in the Context of Mars Exploration

Abstract

Desert sand acoustic emissions are produced when a “sonic sand” is sheared locally or by a natural dune slipface avalanche, resulting in a brassy sound between 50 and 400 Hz. This type of sediment exhibits particular granulometric, shape and surface characteristics, due to the grains’ erosion and transport history, and emits sounds when the sheared grain layer vibrates in a synchronized manner, much like the membrane of a speaker. Recording such sand acoustic emissions on Mars (and perhaps other planetary environments) using rover microphones could thus become a new form of observable for scientists to estimate the surface sediment’s characteristics and history from a distance, but also the granular flow dynamics taking place. To determine whether this approach could be viable in the future, it is essential to evaluate how the Martian environment may affect sand acoustic emissions differently than on Earth. After showing that the muted Martian soundscape would likely allow rovers to detect such signals from a few tens of meters, the present thesis studies the impact of the interstitial air pressure within the sand bed on the sound emission mechanism of such sonic desert sands.

In this project, silent and sonic desert sand shear flows are induced under a range of pressure levels, from terrestrial ambient pressure to Mars-like pressure, within two separate, manually operated vacuum chamber setups: a smaller chamber shaken to create the sounds, and another longer chamber that better replicates avalanche-like sand flows. The motion applied and sound produced are measured using an accelerometer and a microphone inside the chamber. Metrics in the time and frequency domains are defined to analyse the changes in sound energy, amplitude, and frequency components produced at different pressure levels. Firstly, the silent sand tests are used to establish how the air pressure level within the experimental setup affects the regular sound of sheared sand (i.e. grains impacting one another) and more generally the sound emission of “normal” sounds, whose emission mechanisms do not depend on grain packing and synchronized motion. Then, a simplified theoretical model of how the sound pressure level (SPL) of a sound evolves with decreasing acoustic impedance, is derived and validated using the silent sand measurements performed. Finally, the sonic sand measurements are compared to the SPL model and silent sand measurement results, which are used as a baseline for nominal sound production behavior, to evaluate how the interstitial air pressure affects the amplitude and signal energy of the sheared sonic sand emissions. Furthermore, differences in the sand acoustic emissions’ frequency spectra and time duration across pressure levels provide information about the possible physical changes occurring in the granular flow dynamics of the sheared sonic sand.

In both experiments, the dominant frequency very closely follows the trend of the motion metrics used, as described in the literature, and remains very consistent across pressure levels. This suggests that the maximum sheared sonic sand layer thickness is independent of the interstitial air pressure. Then, the sonic sand emissions see an increase in the sound amplitude and signal energy related metrics from ambient pressure to 413.25 mbar, unlike the gradual decrease predicted by the SPL model and the trend of silent sand measurements with decreasing pressure. Below 413.25 mbar, the results suggest a stabilized behavior, with the acoustic metrics of the emissions following the model. Furthermore, in the avalanche-like emissions, a new frequency component slightly higher than the dominant frequency emerges as the chamber pressure decreases. These observations are evidenced in the time-domain, where the sand acoustic emissions seem to initiate earlier in the granular flow at 413.25 mbar and below, resulting in greater acoustic pressure levels being produced, compared to those at terrestrial pressure. It is hypothesized that more sheared sonic sand grains synchronize at 413.25 mbar and below (compared to terrestrial air pressure), and thus increase the amplitude of the sound wave produced. For avalanche-like flows, the new frequency component that appears with decreasing pressure level seems to suggest that the minimum sheared layer thickness threshold required to produce an emission is lowered at lower pressure, which leads to a higher frequency produced initially until the full layer forms, ultimately decreasing the frequency. Further research is required to confirm these preliminary findings and theories.