While studies of urban acoustics are typically restricted to the audio range, anthropogenic activity also generates infrasound (<20 Hz, roughly at the lower end of the range of human hearing). Shutdowns related to the COVID-19 pandemic unintentionally created ideal conditions for the study of urban infrasound and low frequency audio (20–500 Hz), as closures reduced human-generated ambient noise, while natural signals remained relatively unaffected. An array of infrasound sensors deployed in Las Vegas, NV, provides data for a case study in monitoring human activity during the pandemic through urban acoustics. The array records a sharp decline in acoustic power following the temporary shutdown of businesses deemed nonessential by the state of Nevada. This decline varies spatially across the array, with stations close to McCarran International Airport generally recording the greatest declines in acoustic power. Further, declines in acoustic power fluctuate with the time of day. As only signals associated with anthropogenic activity are expected to decline, this gives a rough indication of periodicities in urban acoustics throughout Las Vegas. The results of this study reflect the city's response to the pandemic and suggest spatiotemporal trends in acoustics outside of shutdowns.

A wide range of natural and anthropogenic events produce infrasound, acoustic waves with a frequency below 20 Hz (the approximate lower limit of human hearing). Infrasound can be used to monitor a variety of sources, including earthquakes (e.g., Arrowsmith et al., 2012), volcanic eruptions (e.g., Johnson & Ripepe, 2011, ocean processes (e.g., Fricke et al., 2014), urban activity (e.g., Bird et al., 2021), and nuclear or chemical explosions (e.g., Che et al., 2009;Pasyanos & Kim, 2019). The bulk of infrasound research focuses solely on data from ground-based sensors, but a growing area of study considers airborne stations (Bowman, 2021). Recordings from infrasound microphones attached to balloons, aerostats, or other similar crafts have been used to interrogate numerous sources, including microbaroms (Bowman & Lees, 2015), volcanism (Jolly et al., 2017, chemical explosions (Bowman et al., 2014), sonic booms (Veggeberg, 2012), and ground shaking (Krishnamoorthy et al., 2018). Relative to ground stations, balloon-borne sensors appear to mitigate background noise from wind, recording subtle, lower-amplitude details (Young et al., 2018) and at times record signals not observed on groundbased arrays (Bowman & Lees, 2017).The majority of the studies cited above included only one sensor per floating station. However, we designed our balloons to include microbarometers separated by 100 m, which enabled the elevation angle of incoming acoustic signals to be calculated (e.g., Wescott, 1964). The distance the plane wave travels between arrivals at each payload is given by the product of the lag time between arrivals at the lower and upper sensors and the speed of sound. Considering the right triangle formed with this distance acting as the opposite leg a and the tether length

The Las Vegas Infrasound Array (LVIA) is a network of eleven infrasound sensors deployed from November 2019 through September 2022. While ambient infrasound noise in high and low-noise rural environments has been well characterized, little attention has focused on similar characterization in urban areas with presumed higher background noise levels. The LVIA long-term deployment provides an unprecedented opportunity to study urban infrasound and low frequency audio (20–500 Hz). In addition, large scale shutdowns due to the COVID-19 pandemic provide the ability to discriminate between background noise sources as closures reduced human-generated noise while natural signals remained stable. Within this presentation we will provide an overview of the LVIA installation, focusing on data quality. In addition, we will discuss comprehensive background noise models in urban regions, focusing on presenting probability density functions (PDFs) and median, 5th percentile, and 95th percentile amplitude values to evaluate variations in frequency and amplitude. We will summarize observed trends in background noise over time, highlighting sharp declines in acoustic power following COVID-19 shutdowns. Both sets of analyses will be combined to evaluate periodicities in urban acoustics throughout the city of Las Vegas. [ SNL is managed and operated by NTESS under DOE NNSA contract DE-NA0003525.]

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