## 4 Discussion  
One of the main findings from this study is that there were significant differences in sound environment evaluations due to traffic noise sources which were determined by the temporal characteristics of the noises in question. Most existing studies concur that rail traffic noise has a greater impact than road traffic noise. Through a field survey in residential areas, [PERSON] et al. [44] found that railway noise elicited higher annoyance than road traffic noise at the same L\({}_{\text{den}}\) level. As for the comparison of different rail traffic noises, [PERSON] et al. reported similar results. They found that the noise annoyance caused by conventional train noise was significantly lower than that associated with high-speed train noises [45]. In our experiment, we examined up to six different traffic noises. A strong correlation was found between temporal fluctuation characteristics (L\({}_{10}\)-L\({}_{90}\)) and the evaluations of the sound environment.  
Considering the interior noise, the impact of speech noise was found to be negative, while the air-conditioner noise was neutral when combined with traffic noises. Meanwhile, the difference between these two interior sounds was more significant when combined with more fluctuating traffic sounds. A reasonable explanation for this phenomenon is temporal differences. According to the model proposed by [PERSON] et al. [46], the perception of environmental noise is primarily determined by noticeable events. The speech noise exhibited more temporal fluctuations and more sound events relative to other noise stimuli. As a result, speech noise had a greater negative impact on people. When speech noise was combined with continuous road traffic noise, the background sound level (L\({}_{90}\)) increased, making noise events less noticeable. Therefore, the distinctions between speech noise and air-conditioner noise were more pronounced in the rail traffic groups than in the road traffic group. In a laboratory experiment, continuous industrial noise was found to have interaction effects with traffic noises due to the change in the temporal evolution of their combined noise [31]. On the other hand, the air-conditioner noise had more sound energy in low-frequency bands compared to the speech noise. Numerous studies have revealed that low-frequency sounds have a stronger masking effect than high-frequency sounds [47]. Meanwhile, a field study found that masking sounds were more efficient at masking fluctuating noises [48].  
This study has some limitations. Firstly, there were no control groups in this study. As a result, hearing fatigue might have played a role. In addition, there was only one sound clip for each sound source in this study due to the limitation of the experiment duration. As a result, the reasons for the phenomenon in the experiment could not be verified. For example, significant differences were observed between the speech noise and the air-conditioner noise in the experiment. However, these two noises differ not only in temporal characteristics, but also in spectral characteristics. Further research is needed to confirm the factor responsible for this phenomenon by using multiple stimuli for the same noise source.