Underwater Noise Measurement Intercalibration Practices Experience 
Table 2 Overview of recorders that were calibrated at the FOI pontoon during the second day of 
the HELCOM calibration workshop 
Country/institution [Recorder 
Sweden/FOI " Soundtrap ST300HF 
Sweden/FOL 
Germany/BSH 
Denmark/AU 
Estonia/TUT 
Sylence LP 
: Hydrophone 
In situ 
HTI-92-WB a 
1+2.5V 
HTI-96-min 
| Voltage range 
[+1 vV 
In situ 
HTI-92-WB 
+2.5 V 
‚A1LV 
+25 V 
Results 
The calibration exercises produced clip-level curves for autonomous recorders used 
by several HELCOM countries. These results, while not exhaustive or statistically 
representative, provide valuable insight into the performance of different systems 
and illustrate the added value of free-field calibration compared with pistonphone 
checks. 
Pistonphone Checks 
Several pistonphone checks were conducted prior to free-field calibration. 
Pistonphones of different types were used by the participating institutions, operating 
typically at 250 Hz and generating a known reference pressure level. 
These spot checks provide an end-to-end sensitivity value at a single frequency, 
from which the corresponding clip level can be derived using the system’s voltage 
range. Not all measurements could be performed during the workshop itself; for 
some recorders, the pistonphone checks were carried out afterwards at the respective 
institutions using the same procedure. 
Table 3 summarizes the pistonphone-based sensitivities and the corresponding 
clip levels for all recorders included in the calibration exercises. 
Tank Calibration Results 
None of the recorders showed a flat response; most clip levels increased with 
frequency (Fig. 2). Several units showed a pronounced clip-level peak around 
3.5 kHz (e.g., AU’s ST600HF, FOI’s ST640HF, and TUT’s Sylence LP) and a 
secondary peak near 6 kHz. As these patterns were absent in the pontoon calibration, 
they likely reflect tank artifacts (see Fig. 3).