Cost-effective hydrophone sensors to support marine noise monitoring

Underwater noise is an increasingly relevant anthropogenic pressure in coastal marine environments, particularly near ports, shipping routes, and areas characterized by intense human activities. Although underwater noise is formally recognized as an environmental stressor within the Marine Strategy Framework Directive (Descriptor 11), its operational assessment remains challenging. This is especially true in coastal settings, where long-term and continuous acoustic monitoring is often constrained by resource requirements of length and spatial extent.

In this contribution, we present a cost-effective marine hydrophone developed with the specific aim of monitoring anthropogenic underwater noise in coastal environments, prioritizing temporal continuity and operational robustness. The system is designed to describe noise presence, persistence, and temporal variability, allowing the distinction between episodic acoustic events and conditions of chronic acoustic pressure.

Experience gained in real coastal applications indicates that low-cost hydrophones can provide meaningful information on underwater noise exposure, particularly through indicators such as temporal persistence and recurrence of anthropogenic sources. At the same time, the intrinsic limitations of cost-effective acoustic instruments need to be interpreted in relation to their intended scope of application, as misinterpretation often arises from unrealistic expectations or inappropriate metric selection. In this context, we outline a set of minimal, operational criteria for the use of low-cost hydrophones in coastal monitoring programs, aimed at improving the interpretability and reliability of the information produced.

This work highlights how properly designed and purpose-oriented low-cost acoustic systems can provide concrete and actionable information on underwater noise pressure, supporting sustainable coastal monitoring strategies and contributing to the objectives of the Decade of Ocean Science for Sustainable Development.