## 4 Experimental Results
### Evaluation  
The perceptual evaluation of speech quality (PESQ) [14], signal to distortion ratios (SDR in dB), short-time objective intelligibility (STOI) [15], and the extended STOI (eSTOI) [16] are used to evaluate the quality of the enhanced speech signal. For the simulated speech data, the clean speech is available, while for the real speech data, the ground-truth clean speech is not available. Thus, we use the close-talking microphone recordings (i.e., channel index 0) as the underlying clean speech. We use the pretrained SEGAN and the OMLSA for comparison. In addition, the noisy speech is also considered for comparison (denoted by None). The SDR, STOI, and eSTOI scores for speech quality test on the development set and evaluation set are shown in Figure 4, in which the dev and eval are short for development and evaluation set, respectively. The results demonstrate that our approach outperforms the SEGAN and OMLSA for the simulated data, and yields comparable performance as the traditional state-of-art OMLSA algorithm for the real data. For example, in the simulated data of the development set, compared to SEGAN and OMLSA, the proposed method achieves about 26.01% and 21.10% relative gain in terms of SDR. Another example is in the real data of the evaluation set, the eSTOI score of the proposed method is 0.34 while the eSTOI score of the SEGAN and OMLSA are 0.28 and 0.31, respectively. The results in Figure 4 are the average scores of all environment condition. It is necessary to investigate the details of the compared methods in the different types of environments, i.e., BUS, CAF, PED, and STR. In this experiment, we use PESQ as the criteria to evaluate the performance. The PESQ results in Table 1 show that, for the real data, all the compared methods get the highest scores in PED, and get the lowest scores in the BUS. We have listened to and analyzed the recorded audio. This is due to the bus environment being much more noisy than the pedestrian area. From the results, we can conclude that the proposed method is not sensitive to noise types, and is more robust than the SEGAN. Finally, we have conducted an informal subjective preference evaluation comparison between the SEGAN enhanced, the OMLSA enhanced and the proposed enhanced speech for 100 noisy speech utterances that random picked from the evaluation set. Twelve listeners (9 male and 3 female) are \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Method** & **Environment** & **Dev-Simu** & **Dev-Real** & **Eval-Simu** & **Eval-Real** \\ \hline \multirow{4}{*}{None} & BUS & 2.15 & 2.03 & 2.16 & 2.17 \\  & CAF & 1.87 & 2.18 & 1.88 & 2.37 \\  & PED & 2.12 & 2.33 & 1.93 & 2.35 \\  & STR & 1.95 & 2.11 & 1.97 & 2.38 \\ \hline \multirow{4}{*}{SEGAN} & BUS & 2.08 & 1.96 & 2.10 & 1.98 \\  & CAF & 1.96 & 2.01 & 1.89 & 2.26 \\  & PED & 2.03 & 2.20 & 2.03 & 2.21 \\  & STR & 1.88 & 2.03 & 1.91 & 2.32 \\ \hline \multirow{4}{*}{OMLSA} & BUS & 2.18 & 2.06 & 2.21 & 2.19 \\  & CAF & 1.91 & 2.21 & 1.92 & 2.32 \\ \cline{1-1}  & PED & 2.32 & 2.40 & 2.32 & 2.42 \\ \cline{1-1}  & STR & 2.02 & 2.27 & 2.02 & 2.46 \\ \hline \multirow{4}{*}{Proposed} & BUS & 2.20 & 2.03 & 2.32 & 2.28 \\ \cline{1-1}  & CAF & 2.08 & 2.30 & 2.08 & 2.51 \\ \cline{1-1}  & PED & 2.36 & 2.45 & 2.48 & 2.43 \\ \cline{1-1}  & STR & 2.08 & 2.26 & 2.13 & 2.50 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: PESQ scores for speech quality test on the development set and evaluation set for each environment. Figure 4: SDR (dB), eSTOI and STOI scores for speech quality test on the development set and evaluation set. (**a**) SDR (dB), (**b**) STOI, (**c**) eSTOI. recruited for the evaluation. The tester is asked to select the one he/she prefers for each enhanced speech. Thus, there are totally \(100\times 3\times 12=3600\) selections, whose statistical results are shown in Table 2. The results show that the preference ratio for SEGAN, OMLSA, and the proposed is 14.0%, 14.8%, and 71.3%, respectively.