F. Basan et al. 
Open-Sea-Pantoon FACH ran: sera air HraRs 000 BR EEE BR HER HR BEE BEE BEL BEE BER HR Sa 
Dimensioning and Facility Constraints ................. EA 
Signal Design and Analysis .... 
RESUltS are 
Pistonphone Checks ................... 
Tank Calibration ResuliS... 0... 0000: 00 RE KRAHE KEE KRERE Ka 
Pontoon Calibration Results ..................4000. 
Comparison to Pistonphone Checks 00.0 
Tank-Pontoon Comparison and Calibration Uncertainty ........0.000000000000 00er kenne 
Rig DesigNS ...........0.00000) 
Conclusion and Outlook ........... 
References ........ 
7 
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PP. 
Abstract 
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 monitor- 
ing. 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 wıth known transmitter voltage 
response in FOTI’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. 
Keywords 
Hydrophone calibration : Pistonphone : Intercalibration workshop : Frequency 
response : HELCOM - Baltic Sea monitoring 
introduction 
Accurate calibration of hydrophone systems is a prerequisite for any meaningful 
interpretation of underwater sound measurements. Without reliable sensitivity 
values across the operational frequency range, long-term monitoring data cannot 
oe compared across devices, institutions, or countries. For this reason, calibration is a