## 3 Interference simulation of different bandwidth signaling
#### 2.1.2 Analysis of Experimental Data  
The bottom of the Yellow Sea is relatively flat and the sea conditions are relatively stable in the autumn, which is suitable for sound field analysis. With a view to verify the spatial distribution of the sound field obtained from the simulation experiments, a sound field analysis experiment ExQD_1701 was performed in the Yellow Sea in the autumn of 2017.  
The depth of this experimental sea area was precisely 40 m. A signal-launching ship, which used a UW350 type transmitting transducer with a working frequency range of 20 Hz-20 kHz, was used for the transmission. The transmitting transducer was cylindrical with a diameter of 0.2 m and a length of exactly 1 m. The net weight was exactly 100 kg with its own hoisting device. The schematic diagram of the experiment is shown in Figure 4. The Xiangyang Hong 81 experimental ship was utilized to be the signal-receiving ship with five sub-arrays of the same specification for signal reception with a pitch of 1 m. When the signal-transmitting ship reached the preset position, it could transmit signals with different frequencies. The real-time positions of the signal-transmitting ship and the signal-receiving ship were recorded using GPS, which was used to calculate the relative distance between the transmitting and receiving ends.  
Figure 4: ExQD_1701 test.  
Figure 3: Signal interferograms at different bandwidths: (**a**) 1 Hz, (**b**) 10 Hz, and (**c**) 100 Hz.  
The former simulation experiments demonstrated that when the signal frequency was low, the fluctuation was large. First, the experiment processed and analyzed the low-frequency signal data below 1 kHz. During the experiment, single-frequency signals of 95 Hz and 400 Hz were transmitted, and the sound source level of the transmitting transducer was stable. In order to summarize the spatial distribution of signals, Figures 5 and 6 display the vertical distribution of sound pressure levels for single-frequency signals of 95 Hz and 400 Hz when the relative distances between the transmitting and receiving ends were different. The abscissa is the received sound pressure level and the ordinate is the water depth. By comparing different distances, frequencies, and depths, the spatial fluctuation characteristics of the signal could be summarized. When the signal frequency was low, the sea trial results fit well with the simulation results. As for the normal wave, the fluctuation law could be described as: the modes of the normal wave excited at different frequencies were different. The higher the frequency was, the greater the number of modes, and the more complicated the signal fluctuation law. This rule can be used to guide the equipment placement of low-frequency remote UAC  
the comparison of different distances, frequencies, and depths, it was discovered that when the signal frequency was above 1 kHz, the signal wavelength was short; meanwhile, the environment was greatly affected during the propagation, which was difficult to study qualitatively. The farther the horizontal distance was, the larger the vertical fluctuation range, while the deeper the equipment deployment depth, the larger the vertical fluctuation range and the sound intensity of the near-sea surface signal was slightly lower than the signal strength in water. With the increase in frequency, the signal fluctuation is reduced; however, when the signal frequency was too high, i.e., the signal wavelength was short, which was greatly affected by the scattering and reflection of surface fluctuations, and the signal absorption loss was greater than when the frequency of the signal was low. Therefore, a single-frequency signal appeared to have a violent spatial distribution. When the frequency was as high as 20 kHz, there was a 25 dB intensity difference in the vertical distribution at a horizontal distance of 8 km.