Underwater Noise Measurement Intercalibration Practices Experience 
GG 
90 
Pontoon calibration 
-BSH Sylence = — FOI ST300HF 
= = BSH Sylence — FOI Sylence 
—— AU ST600HF - - FOI Sylence 
- = AU ST600HF — TUT Sylence 
— FOI ST300HF = = TUT Sylence 
185 
2 180 
N ” 
© 
5 
3 175 
2 
5 
Go 
L“ 
170 
N 
165 - 
160 
MM 
—_ 
ann 
3000 5000 
Frequency [Hz] 
Sm 
+ 
u“ 
{O0 
4 
x 
20000 
Fig. 3 Clip levels from the calibration exercise at FOI pontoon facility (800 Hz-20 kHz), indicat- 
‚ng maximum level that can be recorded by the different systems before clipping occurs in the data 
reflections (Robinson et al. 2014). A 3D-printed holder used to mount the decoupled 
hydrophone along the mooring line caused strong frequency-dependent deviations, 
demonstrating that deployment design affects the response. 
Responses were generally non-flat, with device-specific features (peaks and mild 
andulations) of a few dB over frequency. The TUT Sylence LP recorder appeared to 
be the most sensitive system (i.e., lowest clip level), while the remaining devices 
clustered within a comparable range. 
Pontoon Calibration Results 
At the FOI pontoon in the Stockholm archipelago, clip-level curves were flatter 
across frequency bands, and several deviations that appeared in the tank were no 
longer evident (Fig. 3). The systematic increase with frequency seen in the tank was 
no longer observed. Most recorders showed stable responses, with some even 
decreasing at higher frequencies (e.g., TUT’s Sylence LP). 
Tank-specific peaks (e.g., the ST600HF M-shape around 3-6 kHz) were absent, 
while smaller, inconsistent peaks appeared at other frequencies. Variability between 
units was somewhat larger, likely reflecting ambient noise and open-water variabil- 
ity. Recorder curves still clustered within a + 10 dB band, consistent with open-sea- 
like conditions and more representative mounting (Robinson et al. 2014).