F. Basan et al. 
At the pontoon site in the Stockholm archipelago, projector and recorder were 
deployed at 5 m depth with a horizontal separation of 5.6—-8.3 m. Here, end-wall 
reflections are irrelevant, but the surface reflection curve becomes the dominant con- 
straint. With the larger depth and separation, usable frequencies extended down to 
800 Hz (Fig. 1). 
Signal Design and Analysi: 
Projector signals were emitted as short narrowband pulses and repeated three times 
per frequency step. Sensitivities were obtained by matched-filter detection of each 
pulse and comparison of the resulting amplitudes with the absolute pressure derived 
from the projector’s TVR and drive voltage. To reduce variance, the three repeats 
were averaged per frequency. 
Results were averaged to decidecade bands based on definitions in ISO 
18405 (2017). 
The calibration exercises covered representative autonomous recorder systems used 
in Baltic Sea monitoring programs. Table 1 lists the devices tested in the FOI indoor 
vasın (2-10 kHz), including their input voltage ranges. Table 2 provides the 
corresponding overview for the open-sea pontoon calibration (0.8-20 kHz). The spec- 
ification of voltage ranges is particularly relevant, as it determines the scaling of the 
digital counts to voltage and thus allows calibration results to be expressed relative to 
full scale as clip level, 1e., the maximum level the system can record before the audio 
files clip. 
During the second day at the pontoon, several units did not record as intended (e.g., 
logging/power supply issues), so fewer systems were successfully calibrated. As a 
result, one-to-one comparisons with the tank results are only possible for the overlapping 
devices. 
Table 1 Overview of recorders that were calibrated in the FOI tank during the first day of the 
HELCOM calibration workshop 
Country/institution 
Sweden/FOI 
Sweden/FOL 
Sweden/FOL 
Germany/BSH 
Germany/BSH 
Denmark/AU 
Estonia/TUT 
Recorder 
Soundtrap ST640 
Soundtrap ST300HF 
Sylence LP. 
Sylence LP. 
LIE. 
Soundtrap ST600HF 
Sylence LP 
Hydrophone 
HTI-96-UHF 
Insitu 
HTL92-WB 
HTI-96-min 
In situ 
In situ 
HTI-92-WB 
; Voltage range 
+1 V 
+1 V 
+2.5 V 
+2.5 V 
+3 V 
+1 V 
"45V