Underwater Noise Measurement Intercalibration Practices Experience

In September 2024, the HELCOM Expert Group on Underwater Noise (EG Noise) convened an intercalibration workshop at the Swedish Defence Research Agency (FOI) to demonstrate and practice calibration procedures for autonomous hydrophone systems used in Baltic Sea continuous noise monitoring. While many national programs still rely on single-frequency pistonphone checks that implicitly assume a flat frequency response, the workshop explored broadband, projector-based free-field calibration as a complementary approach. Calibrations were performed using a projector with known transmitter voltage response in FOI’s indoor basin (2–10 kHz) and at an open-sea pontoon in the Stockholm archipelago (800 Hz–20 kHz). The resulting frequency-response curves for multiple recorder systems showed deviations from flat sensitivity and between-system variability, underscoring the need for regular broadband checks to interpret long-term monitoring data. Beyond calibrations, participants reviewed rig designs, deployment geometries, and noise-mitigation practices, and distilled practical rules for recorder deployments. This chapter synthesizes these practices and presents representative curves from the workshop. It clarifies the limitations of pistonphone calibrations and thereby highlights the needs for regular broadband free-field calibrations.