5 
F. Basan et al. 
The comparison calibration method has been used (IEC 60565-1 2020), and the 
calibration value is estimated according to: 
S =20- 10g 109 (7) +20- 10g ‚o(d) — 
out 
IR 
where Yan is the measured signal level by the receiver, Yu: 18 the applied voltage on 
the ITC 1007, d is the separation between the transmitter and receiver, and TRV is 
the transmit response voltage of the ITC 1007. 
Calibration Uncertainty 
The transmitter (ITC-1007) had been calibrated in free field using the reciprocity 
method, both in FOT’s tank and in Lake Hornavan, thereby providing traceability to a 
primary calibration standard. For the tank experiments, an uncertainty budget was 
established following the principles of IEC 60565-1 (2020), ISO/IEC 17025 (2017), 
and Hayman et al. (2016). 
The main contributions arise from the calibration of the transmitter response, ge0- 
metric alignment and distance uncertainty, voltage monitoring, and measurement repeat- 
ability. Minor terms (e.g., sound-speed varlation, wetting) contributed <0.5% each. 
Combining these components yields a combined standard uncertainty of approx- 
imately 2.5% (16), corresponding to an expanded uncertainty of about 5% (26) for 
Ihe tank calibration. This value characterizes the acoustic source uncertainty and 
provides the link between the secondary free-field calibration and the underlying 
primary reciprocity standard. 
In addition to this source-related uncertainty, each recorder contributes an indi- 
vidual component arising from alignment, orientation, and repeatability of measure- 
ments, which is considered separately in the graphical representation of the results. 
For the pontoon calibration, the overall uncertainty is expected to differ slightly due 
to open-water variability (temperature gradients, rig motion, and ambient noise) but 
is of similar magnitude as the tank estimate. 
indoor Basin (Tank Calibration) 
The indoor basın at FOI is a controlled laboratory environment with stable water 
conditions, well suited for repeatable tests. Its dimensions (length 8 m, depth 4 m) 
impose limits on the lowest usable frequency, but within the range of approximately 
2-10 kHz, reliable free-field conditions could be achieved. 
The measurement chain consisted of a projector with known TVR and the recorders 
under test, which were positioned at a distance of 2 m from the projector and at a depth 
of2 m.