# Estimation of the Noise Source Level of a Commercial Ship Using On-Board Pressure Sensors
## 3 Source-Level Estimation at Full Scale
### Underwater Radiated Noise (URN) Measurement  
A full-scale URN measurement of the vessel of interest (see Figure 15) was carried out by the Korea Institute of Ocean Science and Technology (KIOST) in March 2014. The test site was in the South Sea of Korea (see Figure 16), in which the depth was around 105 m. Figure 16b shows the test course. \"St.10\" (or \"St.9\") is the start/end point of the measurement, and \"St.5\" (or \"St.4\") is the start/end point of the data acquisition. \"St.8\" is the closest point of approach (CPA), which is 300 m from \"St.3\". The distance from \"St.8\" to \"St.5\" (or \"St.4\") is 173 m, and that from \"St.8\" to \"St.10\" (or \"St.9\") is 692 m. These distances are nominal distances, as in the measurement, they kept changing due to wind, tide, current, and ship operation. Therefore, relative distances were recorded using GPS and were used for the source-level calculation.  
To measure the underwater radiated noise, a buoy-type hydrophone array was deployed. A schematic view of the hydrophone array is shown in Figure 17. The array consisted of a surface buoy with GPS, three self-recording hydrophones with a depth sensor, sub-surface buoys, an acoustic release, and a weight (train wheel) at the bottom. A GPS sensor was installed in the target vessel to calculate the relative distance.  
Figure 14: Comparison between the measured and calculated source levels for conditions (a) C2 and (b) C3.  
Figure 15: Target vessel.  
The test procedure was based on ISO 17208-1, although the test area was a shallow water. For each condition, the measurement was conducted for both sides of the ship, and the results were processed as follows. Firstly, for measured data \((L_{p+n})\) and background noise \((L_{n})\), an incoherent background noise correction was made. The rules for the correction were as follows:  
* For \(\Delta L\leq 3\) dB: data to be discarded;
* For \(10\) dB \(\leq\Delta L\geq 3\) dB: \(L_{p}^{\prime}\)= \(10\log_{10}\left[10^{L_{p+n}/10}-10^{L_{n}/10}\right]\) (dB);
* For \(\Delta L\geq 10\) dB: no corrections.  
Here, \(\Delta L\big{(}=L_{p+n}-L_{n}\big{)}\) is the difference between the measured signal and the background noise in decibels. After the background noise correction, a distance correction was made using a monopole sound source assumption. This can be written as  
\[L(r)_{RN,i}=L_{p}^{\prime}+20\log_{10}d_{T}/d_{ref}\quad(\text{dB}), \tag{16}\]  
where \(L(r)_{RN,i}\) is the corrected sound source level at the \(i\)-th hydrophone in the \(r\)-th run, \(d_{T}\) is the distance between the hydrophone and the acoustic center of the ship at the closest point of approach (CPA), and \(d_{ref}\) is the reference distance (1 m). Here, the CPA is defined as the point at which the distance between the hydrophone array and the target vessel is the shortest.  
Figure 16: Test site for underwater radiated noise (URN) measurement: (**a**) location and (b) test course.  
Figure 17: Schematic view of theURN measurement setup.  
A spatially averaged value of each run can be calculated as  
\[L(r)_{RN}=10\log_{10}\left[\frac{\sum_{i=1}^{3}10^{L(r)_{RN,i}/10}}{3}\right]. \tag{17}\]  
The spatially averaged source level is again averaged for all runs, i.e.,  
\[L_{RN}=10\log_{10}\left[\frac{\sum_{r=1}^{k}10^{L(r)_{RN}/10}}{k}\right], \tag{18}\]  
where \(k\) represents the number of runs for each condition. The same operating conditions were considered as in the model-scale measurement. The shaft RPM values for the conditions were 95, 99, and 103, respectively. For each condition, the measurement was carried out twice (\(k=2\)). The radiated noise was measured with a sampling frequency of 60 kHz, and the result was processed to the source level in one-third octave bands.