Underwater Noise Measurement Intercalibration Practices Experience 
operate multiple autonomous recorders, pistonphone checks provide a rapid way to 
verify system sensitivity before and/or after deployments. 
However, the method provides information only at one frequency, yet assumes 
that the hydrophone and the entire recording chain exhibit a flat frequency response 
across the full operational range. This assumption may not hold true. Commercial 
autonomous recorders often incorporate preamplifiers with their own gain and 
frequency characteristics that can produce measurable deviations. In addition, 
some systems implement built-in filtering (e.g., low-frequency roll-off or anti-alias 
filters, and manufacturer-defined equalization) that a single-frequency pistonphone 
check will not reveal. This is consistent with IEC 60565-2 (2019) and ANSI/ASA 
S1-20-2012 (2012) which formalize pressure field versus free-field procedures. 
Consequently, reliance on pistonphones alone may obscure systematic biases in 
long-term data sets, especially when monitoring frequency-dependent metrics such 
as decidecade band levels, which is required by the MSFD. 
Free-Field Calibrations 
In general, free-field calibration determines the system’s sensitivity by relating the 
recorded output voltage to the absolute acoustic pressure at the hydrophone position. 
This pressure can either be measured using a calibrated reference hydrophone or 
calculated from the known output of a projector with calibrated transmitter voltage 
response (TVR). Both approaches are described in IEC 60565-1 (2020) and provide 
frequency-dependent sensitivity values under controlled propagation conditions. 
Free-field calibrations can be carried out in different environments. Laboratory 
tanks provide a controlled setting, free from weather dependence, but are inherently 
limited in usable frequency range by their dimensions. Open basins and sheltered sea 
sites allow lower frequencies and more realistic propagation, but introduce logistical 
challenges and environmental variability. 
Facilities and Setup 
The HELCOM calibration workshop took place in September 2024 at the facilities 
of the Swedish Defence Research Agency (FOI) in Stockholm. Complementary to 
pistonphone checks, two distinct facilities were employed: FOT’s indoor acoustic 
basin and an open-sea pontoon site in the Stockholm archipelago, where free-field 
calibrations could be conducted. 
Projeciu- 
The projector used in this calibration is an ITC 1007 from Gavial. It has a resonance 
at 11.5 kHz with a peak transmit voltage response of roughly 147 dB re 1 uPa/V.