# Automatic Detection and Unsupervised Clustering-Based Classification of Cetacean Vocal Signals +
## 2 Experiments and Results  
### Detection  
A total of 194 audio files have been analyzed by the proposed automatic detection method. An initialized signal waveform of the common dolphin is shown in Figure 2b, and the corresponding time-frequency spectrogram is displayed in Figure 2a. The settings of this spectrogram are 81.6 KHz sampling rate, 50% overlap, and a total of 256 frequency points used to calculate the discrete Fourier transforms. Then, the dual parameters-dualthresholds endpoint detection method is employed for the acoustic signals after initialization. There are three transformations in endpoint detection. Firstly, EMD is used to reconstitute the initialized audio signals into IMF components to reveal the intrinsic mode of the vocal signals. Figure 2: The result of dual parameters dual thresholds endpoint detection based on EMD. **(a)** The signal spectrogram of common dolphin after initialization. **(b)** The signal waveform of common dolphin after EMD decomposition. **(d)** The signal waveform of common dolphin after EMD decomposition. **(e)** The mean value of Teager-Kaiser energy operator. **(f)** Zero crossing rate of the signal. (The vertical solid lines represent the start points and the dashed lines represent the endpoints. The horizontal solid lines and the dashed lines are two-grade thresholds. Color indicates normalized log power (dB)). Reproduced with permission from [PERSON] et al., published by IEEE International Conference on Electrical, Automation and Computer Engineering (ICEACE), 2023 [33]. The audio signal spectrogram and waveform reconstructed by EMD are shown in Figure 2c. Secondly, the average TKEO energy for each IMF component is calculated according to the frame size, and the frame length is set at 25 ms. The IMF components are transformed into the TKEO shown in Figure 2d. The horizontal solid and the dashed line are two grade thresholds computed by the mean value of TKEO or ZCR for each frame but in different sizes. The first grade threshold solid line is used for rough estimation and set higher than the second threshold. The second grade threshold dashed line is slightly lower than the threshold for more accurate estimation. For example, the value of the first grade threshold in Figure 2d is set as \(3\times TE\), and the value of the second grade threshold is set as \(1.5\times TE\). The numbers 3 and 1.5 are multiples of the thresholds, and they are also the empirical value set according to the experiment. The lines in Figure 2e are plotted in the same way. The value of the first grade threshold in Figure 2e is set as \(0.9\times ZCR\) and the value of the second grade threshold is set as \(0.8\times ZCR\). The variables \(ZCR\) and \(TE\) have been defined in Appendix A. Finally, another parameter threshold, short-time average ZCR, is measured frame by frame from the initialized audio signals shown in Figure 2e. After the energy transformation, the energy amplitude at each moment is compared with the average TKEO and short-time average ZCR of the signals. When the value exceeds two grade thresholds of two parameters, it is considered as the beginning of the vocal call. Two-level decisions are judged for whether they are in a vocal segment. The start points are recorded in vertical solid lines, and the endpoints are recorded in vertical dashed lines. The segments between the start and endpoints are the potential vocal signals. We compare the different forms of the acoustic signals of CD and show the picked vocal signals in Figure 2.  
In the experiment discussed above, we assume that the spectrograms are free from distortion. However, in oceanic environments, the PAM signal may experience frequency drift due to the Doppler effect, which results from the relative movement between cetaceans and the monitoring sensors. Detecting and compensating for this frequency drift is a challenging task. Fortunately, researchers have developed effective solutions. A recent advanced method by [PERSON] et al.