# Prediction Method of Underwater Acoustic Transmission Loss Based on Deep Belief Net Neural Network
## 4 Experimental Results and Discussion  
### Sources of Data  
To verify the accuracy of the new forecasting method, we conducted a sea trial experiment in May 2020 to analyze and process the obtained sea trial data to obtain the actual underwater acoustic transmission loss data in this sea area. Then, we compared and analyzed the measured transmission loss data with the data predicted by this method. The sea trial site is shown in Figure 6.  
The experimental area is shown in Figure 7. The experimental ship traveled from Q1 (Figure 7) to Q2 (Figure 7) for the experiments. The ship carried the towed array sonar and continuously emitted a set of signals, and the 20-element vertical receiving array at the ZS (Figure 7) station received the signals emitted by the towed array sonar. We placed a CTD at the receiving array for sound velocity profile measurement. The measured sound velocity profile is shown in Figure 7. The planned total voyage of the experimental ship was 160 km, the starting QI was 274 km away from the receiving array, and the ending Q2 point was 420 km away from the receiving array. Among them, we selected one of the sea trial data with a voyage to establish transmission loss data for verification. Figure 5: DBN training accuracy and loss curve. The signals continuously transmitted by the experimental ship during the experiment are shown in Table 4, where Continuous Wave (CW) represents a single-frequency signal, and Linear Frequency Modulated (LFM) represents a frequency-modulation signal. In each group of signals, single-frequency or FM signals are transmitted according to the law shown in Table 4, and a group of signals is transmitted with a period of 160 S. As shown in Figure 8, it is a time-frequency domain diagram of the transmitted signal received at a certain moment for the 311 m depth array element in the receiving array. As shown in Figure 8a, although it is impossible to analyze useful information on the time domain diagram, it can be seen in the frequency domain diagram that there are significantly higher peaks at the frequency of 360 Hz than the other frequency points. This phenomenon indicates that the 360 Hz signal transmitted by the experimental ship is received on the receiving array element, and the energy intensity of the received signal can be obtained in the frequency domain. Similarly, as shown in Figure 8b, the 320-400 Hz LFM signal can be found in the frequency domain, and its center frequency can be selected to obtain the energy intensity of the received signal. The remaining CW and LFM signals use the same method to obtain the received signal energy intensity. Next, according to the known transmitted signal energy intensity and received signal energy intensity, we can calculate the actual underwater acoustic transmission loss for this distance and generate the underwater acoustic transmission loss dataset. Figure 6: Sea trial site. Figure 7: Experimental area. Figure 8: Time–frequency domain diagram. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Signal Type** & **Signal Frequency** & **Duration (S)** & **Interval (S)** \\ \hline CW & 245 & 5 & 15 \\ CW & 360 & 5 & 15 \\ LFM & 220–270 & 10 & 30 \\ LFM & 320–400 & 10 & 30 \\ LFM & 600–800 & 10 & 30 \\ \hline \hline \end{tabular} (Continuous Wave (CW) Linear Frequency Modulated (LFM)).  
\end{table} 
Table 4: Periodic signal. Based on the transmitted signals in Table 4, this paper constructed a sea triainl transmission loss dataset. The sound source level (_SL_) of the CW signal was 192 dB, the _SL_ of the LFM signal was 174 dB, and the emission depth was 140 m. As shown in Figure 9, it can be clearly seen in the LOFAR spectrum that the high-brightness area represents the transmitted signal received at the current moment. The LOFAR spectrum of the 360 Hz CW signal is shown in Figure 9a. It can be seen that the duration was 10 S, which was the same as the description in Table 4, indicating that the transmitted CW signal was received at this time. The LOFAR spectrum of the 320-400 Hz LFM signal is shown in Figure 9b.