Ambient sound directionality on a deep-water drifting platform: Measurement and model comparisons using hydrophone arrays and vector sensors

Between 2022 and 2024, multiple deployments of deep-water, drifting, opto-acoustic platforms were conducted near the New England Seamounts (∼4000 m depth) and in the Southern California Bight (∼1000 m depth). The platforms were equipped with conductivity, temperature, and depth (CTD) sensors, vertical and tetrahedral hydrophone arrays, and a three-axis acoustic vector sensor. For one deployment in Southern California, an 80-m thermistor string was suspended from the platform, providing high-resolution temperature profiles complementing the acoustic measurements. The drifters continuously recorded mid-frequency (500–20 000 Hz) ambient sound for 1–4 days at depths ranging from 100 to 800 m, entering a “hibernation” mode to minimize self-noise. The directionality and spatial coherence between hydrophones were computed and compared to analytical models and numerical simulations of surface-generated noise. Data between the hydrophone arrays and the acoustic vector sensor were also compared. Temporal and spatial variations in the acoustic environment, particularly 15–20 min oscillations in the surface noise directionality, were observed at both sites. The seamounts data displayed more dynamic changes in surface-generated noise directionality but less bio-acoustic activity. These results highlight the utility of these platforms for characterizing complex acoustic environments and cross-checking beamforming and vector sensor measurements. [Work sponsored by ONR TFO.]