## 5 Results and Discussion
### Subjective Evaluations  
We perform subjective listening tests to assess the performance of our systems in terms of speech quality and speaker similarity of the generated speech from the five conversion methods (proposed, S1, S2, S3, and S4). It should be noted that all tests were performed under the same conditions. We first conduct the Mean Opinion Score (MOS) test, admittedly the most pertinent subjective test. For each conversion pair, 10 pairs (sentences) were randomly selected from the 25 paired samples of the test set. A group of fifteen listeners (five males and ten females) took part in the listening tests where each participant had to listen to 10 speech samples, which were converted according to each of the systems mentioned above. In this experiment, each converted sample was randomly presented to listeners who were asked to independently judge the quality of the speech in terms of speech quality and naturalness on a five-level scale (ranging from 1, for the lowest quality, to 5 for the best quality). Each participant has the chance to replay each stimulus before giving his/her note. We then performed an ABX similarity test to evaluate the recognition of the target speaker. The setup is similar to the MOS test. X was the converted sample in each pair, and A and B were the source and target samples, respectively. Listeners are requested to judge the proximity of the converted sample X to both source sample A and target sample B by giving a score of either 0 or 1 (0 for the choice of source speaker and 1 point for the choice of target speaker) after each listening session. The final result is given as a percentage for the converted voices X that is recognized as the target speaker. In the case of Male-to-Female and Female-to-Male conversion, the inter-gender conversions were easily recognized. \begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Methods**} & \multicolumn{3}{c}{**WER in (\%)**} \\ \cline{2-4}  & **BDL (Male)** & **CLB (Female)** & **Average** \\ \hline Source & 8.56\% & 7.46\% & 8.01\% \\ \hline Proposed & 28.89\% & 27.69\% & 28.29\% \\ \hline S1 & 37.39\% & 34.19\% & 35.79\% \\ \hline S2 & 32.67\% & 29.87\% & 31.54\% \\ \hline S3 & 41.33\% & 43.03\% & 42.18\% \\ \hline S4 & 50.6\% & 48.76\% & 49.68\% \\ \hline \hline \end{tabular}
\end{table} 
Table 2: ASR Word error rate (WER) (%) scores for source speech (Source), converted speech from proposed and baseline (S1, S2, S3, S4) methods for male and female speakersThe results of the subjective MOS and ABX tests obtained on the validation set are depicted in Figure 3 and Figure 4, respectively. From Figure 3, it can be observed that the S4 system yields lower MOS values than the other three methods, which confirms the results of the objective experiments. In the RMS-to-SLT conversion pair, we notice that the method we propose proved to be close and has slightly lower efficiency compared to the S2 system. Respective MOS values are 3.86 and 3.89. In all conversion pairs performed, we can further see that the MOS scores of our proposed method are higher than those of the S4 method on naturalness. Figure 4 presents the results of the similarity test obtained from the Male-to-Male and Female-to-Female conversion pairs. As can be seen, listeners preferred the proposed  
Figure 3: MOS results with 95% confidence intervals on naturalness obtained from all conversion pairs. Figure 2: Spectrograms of the target speech, the converted speech from the proposed system, and the S1 baseline system of the same sentence “Beyond refusing to sell us food, they left us to ourselves”. system and the baseline S3 since the stimuli provided by both methods seem to them more similar and closer to the target than those provided by the other methods. To evaluate the degree of preference for the different conversion systems, we also conduct an AB preference test to compare between proposed methods (designated A) and baseline methods (designated B). Similar to the ABX test, 10 pairs were randomly selected from the 25 paired test samples.