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F. Basan et al. and harmonize calibration practices was therefore needed—especially because a variety of different recorders is being used in the regional monitoring. To address this, the HELCOM Expert Group on Underwater Noise (EG Noise) organized an intercalibration workshop at the Swedish Defence Research Agency (FOI) in September 2024. The workshop brought together participants from five Baltic Sea countries, combining practical calibration exercises with discussions on rig design and data quality control. Hydrophones were calibrated using a calibrated projector in FOTI’s indoor basin, covering 2-10 kHz, and at an open-sea pontoon facilıty in the Stockholm archipelago, covering approximately 800 Hz-20 kHz. In voth cases, narrowband pulses of defined drive voltage were transmitted, and the recorder responses were evaluated relative to the acoustic pressure calculated from the projector’s TVR. The workshop also served to share experiences with measurement logistics. Participants reviewed different rig designs, highlighting key lessons. Background and Methec«“ The general principle of hydrophone system calibration is simply relating the electrical output of the system to the incident acoustic pressure in water. The practical implementation depends on the chosen method and facility. Two comple- mentary approaches dominate current practice in monitoring programs and have been tested during the workshop at FOI: pistonphone checks and broadband free- field calibrations. Pistonphone Checks Pistonphones have been used for decades as the most accessible and robust calibra- tion tool. They generate a known, stable sound pressure pe, at a single frequency, typically 250 Hz, when the hydrophone is inserted into a small cavity driven by a piston. The sensitivity of the unit under test is then derived as: S =2010g 10 (Zu) [dB re 1 V/pPa] Pref where Yu 18 the measured RMS output voltage of the hydrophone and recording chain. This direct ratio provides an absolute spot check of sensitivity. In practice, the recorded audio files do not provide direct voltage measurements, but contain digital counts. Comparing these to absolute sound pressure levels in the pistonphone’s cavity gives end-to-end sensitivity values that relate the full-scale output of the recorded audio files to sound pressure, equivalent to the highest sound pressure that the measurement system can record before clipping. This procedure is straightforward, portable, and requires little additional infra- structure, making it ideal for field checks. For national monitoring programs that