## 3 Results
### Subjective Evaluation
#### 3.2.4. MOS of Intelligibility and Naturalness  
The results of subjective evaluation were obtained from the 250 test utterances. The subjective evaluation includes MOS on speech's intelligibility and naturalness. Mandarin Chinese is a typical tonal language where tone influences the listeners' perception of speech's intelligibility and naturalness. The higher the tone accuracy, the higher the MOS of intelligibility and naturalness. The intelligibility and naturalness MOS are shown in Figure 5 and Figure 6, respectively.  
In Figure 5, EL represents the intelligibility MOS of EL-FT and EL-VT; CLDNN represents the intelligibility MOS of CLDNN-FT and CLDNN-VT; VC3 represents the intelligibility MOS of VC3-FT and VC3-VT; and 2C1T2C represents the intelligibility MOS of 2C1T2C-FT and 2C1T2C-VT. The confidence interval here is 95%.  
The experiment results from Figure 5 show the perceptual intelligibility of test speech. The intelligibility of EL-VT speech is higher than that of EL-FT speech because tone variation influences the comprehension of Mandarin speech. Our approach achieves higher intelligibility of enhanced speech than other approaches. Meanwhile, the intelligibility of enhanced speech with variable tone is higher than that of enhanced speech with fixed tone.  
In Figure 6, EL represents the naturalness MOS of EL-FT and EL-VT; CLDNN represents the naturalness MOS of CLDNN-FT and CLDNN-VT; VC3 represents the naturalness MOS of VC3-FT and VC3-VT; and 2C1T2C represents the naturalness MOS of 2C1T2C-FT and 2C1T2C-VT. The confidence interval here is 95%.  
The experiment results in Figure 6 show the naturalness of enhanced speech. The naturalness of EL-FT and EL-VT are close. Both enhancement and tone variation can  
that Mel-Spectrogram parameters and the combination of features do not differ greatly from each other in enhancing continuous Mandarin EL speech. Furthermore, we use WaveNet as the neural vocoder to directly resynthesize the Mel-Spectrogram parameters into waveform signals. Our research can be applied to improve the communication ability of the laryngectomees. Additionally, this proposed method can also be used to popularize the application of the Electro-Larynx.  
The objective and subjective evaluation results show that the proposed approach performs better than traditional parallel-dependent VC in enhancing Mandarin EL speech. In particular, the proposed approach substantially improves tone accuracy and reduces WER. Moreover, the intelligibility and naturalness MOSs of 2C1T2C-FT are much higher than those of EL-FT, CLDNN-FT and VC3-FT; the intelligibility and naturalness MOSs of 2C1T2C-VT are much higher than those of EL-VT, CLDNN-VT and VC3-VT. This is because the proposed approach takes advantage of contextual semantic information based on the whole input speech. Our approach improves both the frequency and temporal information of Mandarin EL speech.  
Although the proposed approach effectively improves the intelligibility and naturalness of Mandarin EL speech, the training process still takes up more than 10 GB of graphics processing unit (GPU) memory and at least 168 h. The time cost of our proposed method, CLDNN-based VC and VC3 are close. In the future, it would be an important task to explore a quicker training process requiring smaller GPU memory. Delay of processing is one major shortcoming of our approach. In the future, we would explore ways to reduce the delay, which would be more useful for laryngectomees.