Toward quantitative soundscape monitoring with low-cost recorders: application to the Venice Lagoon

Underwater noise in shallow coastal lagoons arises from both natural and anthropogenic sources and is shaped by complex environmental factors. High-quality hydrophones provide precise measurements but are often limited in spatial coverage and deployment duration due to cost and logistical constraints, whereas low-cost acoustic sensors allow flexible observational networks but frequently exhibit sensor-to-sensor variability and are typically used qualitatively. Here, we present a practical framework to demonstrate that low-cost recorders can provide quantitative insights in shallow lagoon environments. A spatially distributed array of three HydroMoth sensors (Open Acoustic Devices, UK) was deployed across multiple sites in the Venice Lagoon, recording continuous acoustic data over full diel cycles. Using a co-located reference-calibrated hydrophone and a straightforward calibration workflow, recorded signals were converted into absolute sound pressure levels (SPL, dB re μPa), enabling comparisons across sensors and sites. Analyses revealed marked day-night differences and spatial heterogeneity across monitoring sites. We also found that simultaneous recordings made by the three HydroMoth units at the same location slightly differ from one another, and this discrepancy could not be entirely suppressed by calibration. Our results show that, despite some important limitations, low-cost sensors can provide frequency-limited, semi-quantitative acoustic measurements, supporting large-scale, quantitative soundscape monitoring. By enabling spatially distributed and cost-effective measurements, this approach supports scalable monitoring of underwater noise pollution in complex transitional coastal systems, contributing to ongoing efforts in marine environmental assessment and management.