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).