## 4 Discussion  
Whistle and burst pulses are within the human hearing range, and the duration of the signal is long enough for our sense of hearing to recognize them. However, it is not easy for humans to recognize clicks, as they are too short for us to hear their features clearly. It is also difficult to distinguish clicks from other impulse signals, such as the knocking sounds of crustacean activity and offshore piling. Therefore, we believe that the results of manual methods for whistle and burst pulse are reliable and that these methods can be used as a standard, but we do not have a reliable standard for clicks. It should be noticed that the frequency band of click is far beyond the upper limit of the human auditory threshold, and there is no clear standard that can be used to judge the recognition results. Although we increased the sampling time by 20 to render the click audible, the duration was still too short (approximately 440 us). It is difficult to distinguish a click signal from other broadband pulse signals. For example, knocking sounds of objects impacting each other are so similar to clicks that some aged dolphins with hearing loss may misjudge them [19]. Therefore, it is reasonable to assume that there were several misjudgments in the experiment. WT reduced the influence of the uncertainty principle and had advantages in terms of non-stationary signal detection. Although WT detected more click signals in the experiment, it is uncertain whether WT performed better or, instead, marked interference signals. Is it possible for WT to distinguish clicks from other interference knocks by analyzing the duration of the signal? To determine this, some knocking sound samples were collected for an experiment. In this experiment, the data were pure interference knocks without _Cetacean_ acoustic signals. We used the manual, STFT, and WT methods mentioned in Section 2 to process these data separately. Subsequently, all the detected signals were misdetections. These results indirectly reflect the anti-interference ability of the methods. The fewer misdetections there were, the better the anti-interference performance of the method in question was. As shown in Figure 5, a hydrophone was tied to an unmovable cantilever with soft nylon string and released into a barrel filled with water. The parameters and settings of the hydrophone are shown in Table 6. Knocking sounds were made in the air (point A), on the barrel wall (point B), and in the water (point C), respectively (50 times each, totaling 150 times). The time-frequency spectrum of the clicks and knocking sounds at points A, B, and C are shown in Figure 6. Being affected by the air-water interface, the knocking sounds at point A were greatly weakened and could easily be eliminated using STFT and WT. The knocking sounds at points B and C were extremely similar to clicks and were difficult for the manual method and STFT to distinguish. WT had a better time resolution and could distinguish the knocking sounds through their duration. Based on analysis of the data obtained by the manual, STFT, and WT methods, respectively, the results are shown in Table 7. It can be seen that WT had fewer misdetections of knocking sounds than the other methods. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Type of Signal** & **Manual** & **STFT** & **WT** \\ \hline Whistle & 1062 & 1062 & 1059 \\ Burst pulse & 1361 & 1361 & 1352 \\ Click & 2501 & 2382 & 3057 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Results of target signal detection obtained using different methods. Figure 5: Sketch map of the knocking test. \begin{table}
\begin{tabular}{c c} \hline \hline
**Parameters** & **Value** \\ \hline Type & SC2-ETH, Ocean Sonics \\ Memory & 256 GB \\ Frequency band & 10–128 kHz \\ Sampling rate & 256 kHz \\ Resolution & 16-bit \\ Minimum self-noise & 27 dB \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Parameters of the hydrophone for data collection. Figure 6: The time–frequency spectrum of clicks and knocking sounds. (**a**) A knock sound at point A.