2 
F. Basan et al. 
Some units lacked data on day two, limiting direct comparisons, but available 
results suggest that under realistic open-water conditions the tested systems con- 
verge to broadly similar clip level characteristics across most of the operational 
range. However, differences between the two frequency ranges (0.8-10 kHz 
vs. 2-20 kHz) remain visible, underlining that calibration results are themselves 
subject to deviations that should be further investigated to better quantify 
uncertainties. 
Comparison to Pistonphone Checks 
We compared pontoon-derived and pistonphone-based clip levels (converted via full- 
scale voltage; Fig. 4). The comparison was restricted to the overlapping 2-10 kHz 
band. Deviations mostly range between —1 and +4 dB, indicating good agreement. 
This consistency suggests that the free-field calibrations results are broadly coherent 
across systems, though each shows distinct frequency features. 
For most recorders, deviations cluster around small positive offsets, indicating 
slightly higher measured clip levels than those derived by pistonphone checks 
(Le., lower effective sensitivities). A broad minimum in deviation appears around 
[ 
f 
h 
/ 
Po 
£ 
1 
a 
Le] 
30 
SZ 
5 
— 
27-7 
2000 
200: 
3J00 
Frequency [Hz] 
— AZ 
-FOI ST300HF (pontoon))| 
-AU ST600HF (pontoon) 
—— BSH Sylence (pontoon} 
—— FOI Sylence (pontoon) 
= TUT Svience (nantnan) 
10000 
Fig. 4 Deviation between pontoon-derived clip levels and pistonphone-derived reference clip 
‚evels (measured-reference) for the overlapping 2-10 kHz band. Reference values are based on 
pistonphone checks listed in Table 3, measured at different frequencies (typically 125-250 Hz; FOI 
Sylence at 1 kHz)