## 4 Proposed DoM Assessment Method
### Communication Performance Assessment
Figure 7: _Cont._  
For the 1 st step DoM assessment, the sequences with \(Ls\) of 20, 60, 100, and 120 were multiplied to the whistles for the modulation. A set of assessment whistles consisted of one of the original dolphin whistles in Figure 7 and four biomimetic whistles that were modulated from the real dolphin whistles with four \(Ls\). Noises were added to the real- and the modulated-whistles with 10 and 20 dB of SNRs to analyze the noise effects to the DoM assessments. The number of listeners was 30. The ages of the listeners were from 10 to 60 s, and the age of listeners are uniformly distributed. The 1 st step DoM assessment was conducted in Table 1. The listeners graded the similarity to the real dolphin whistles by Table 4.  
For the assessment test, Terratec D/A convertor and AKG-K52 headphone were utilized. The assessment results are shown in Table 5. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**1** & **2** & **3** & **4** & **5** \\ \hline Different & Slightly different & Similar & Very similar & same \\ \hline \hline \end{tabular}
\end{table} 
Table 4: MOS grades for the 1 st step DoM assessment. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Species**} & \multirow{2}{*}{**Noise**} & \multirow{2}{*}{**Real Dolphin**} & \multicolumn{4}{c}{**Sequence Length (\(L\))**} \\ \cline{3-6}  & & & **20** & **60** & **100** & **120** \\ \hline \multirow{2}{*}{Killer} & 10 dB & 4.7 & 3.8 & 3.2 & 3.1 & 3.2 \\  & 20 dB & 4.8 & 3.9 & 3.0 & 2.9 & 3.1 \\ \hline \multirow{2}{*}{Delphinus delphis} & 10 dB & 4.1 & 4.3 & 4.3 & 4.2 & 4.3 \\  & 20 dB & 4.2 & 4.3 & 4.0 & 4.1 & 4.1 \\ \hline \multirow{2}{*}{Whistle sided} & 10 dB & 4.2 & 4.1 & 4.1 & 4.2 & 4.1 \\  & 20 dB & 4.4 & 4.2 & 4.2 & 4.2 & 4.2 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Results of the 1 st step DoM assessment. Figure 7: Mean Opinion Score (MOS) test whistle of white sided dolphin of (**a**) flat, (**b**) moderate variation, (**c**) dramatic variation; Delphinus delphis of (**d**) flat, (**e**) moderate variation, (**f**) dramatic variation; Killer whale of (**g**) flat, (**h**) moderate variation, and (**i**) dramatic variation. In Table 5, as the SNR decreased, the average MOS of the modulated whistles increased. This is because the background noise hindered distinguishing the difference between the modulated- and the original-whistles. The MOSs of real Delphinus delphis and Whitesided dolphin for all sequence lengths were approximately 4.2. As a result of the 1 st step assessment, the human does not recognize the difference between real whistle and whistle multiplied with the sequence length of 120. Thus, the maximum sequence length for Delphinus delphis and White-sided is 120, and the spreading bandwidth (\(B_{c}\)) was calculated as 300 Hz using Equation (9). The MOS score of a real killer whale was higher than whistles multiplied with the sequence. If we used a smaller sequence length, the MOS score of sequence multiplied whistle increased. In this paper, Delphinus delphis was selected as a mimetic model because the MOSs of Delphinus delphis's real whistle and sequence multiplied whistle were the same. Based on this assessment, the largest length of the sequence by the 1 st step DoM assessment was selected as 120 and \(B_{c}\) was chosen as 300 Hz. The 2 nd step DoM assessment was conducted using the result of the 1 st step. For the 2 nd step DoM assessment, eight Delphinus delphis dolphin whistles were modulated by TFSK and each whistle was multiplied by the sequence that has 300 Hz of \(B_{c}\). The spectrograms of the modulated whistles are shown in Figure 8.  
Two consecutive long-term whistle signals with a 10-s duration were made: the first signal consisted of eight whistles in Figure 8. The second one was composed of two whistles in Figure 8c,g. The first and the second signals were utilized for the MOS tests of the proposed method and the conventional method, respectively. For the fair MOS comparison tests, the BERs of the two methods needed to be the same.