## 2 Materials and Methods
### Experiment Stimuli  
According to the research goal, there were three variables in the experiment: interior noise source (IN), traffic noise source (TN), and traffic noise level (SPL). To control the sound level of each noise source in the combined noise approach, four steps were used to create the experimental stimuli. 1. Field recording  
All sound materials were field recorded in Beijing, China. The experiment included nine noise sources: three interior noises and six traffic noises. Three interior noise conditions were considered in this study: silence (SL), air-conditioner noise (AC), and irrelevant speech noise (IS). The air-conditioner noise was recorded in an empty open-plan office to eliminate other sound sources, as shown Figure 2. The microphone was placed three meters away from the air outlet. The velocity was set to  
Figure 1: Experimental environment setup. medium, and a 30 min recording was collected after the air-conditioner noise stabilized. The irrelevant speech noise was recorded in a busy city office (volume = 500 m\({}^{3}\); number of people = 50-100). The sound recorder was placed 20 m away from the nearest office counter to prevent capturing clear voices from the visitors and officers. Longer recordings (4 h) were collected to extract clear speech clips without ambient noises. As for the traffic noises, there are two main methods for field recording. Some of the previous studies, e.g., Reference [37], have placed the recorder near the building facades. By measuring the sound insulation effect of building facades, it is possible to replicate the traffic noise situation inside the building. This method is highly accurate for simulating the noise conditions at a specific location. However, the recordings could be easily affected by ambient noise because the recording sites are very close to the building facades. An alternative method is to record the sound near the traffic line [38]. This method is quite useful for laboratory studies that compare the effects of different sound sources because the influence of ambient noises can be easily avoided. Moreover, it also allows for modifications to be applied to the original signals to simulate the effect of the sound propagation process. In this study, road traffic noise and five types of rail traffic noises were considered, including maglev (Ma), tram (T), metro (Me), conventional inter-city train (C), and high-speed train (H). To avoid ambient noises, the sound recording was carried out in quiet areas near the traffic line. The recorder was placed approximately 10 m from the traffic line and 1.25 m above the ground. A sound meter (6228+, Aihua, Hangzhou, China) was used to conduct sound recording and measurement under the same format (48 kHz, single-channel, 16 bits). (2) Sound clip extraction  
The field recordings were then manually rechecked to extract a clear sound clip for each sound condition. As shown in Figure 3, the noise recordings showed significant differences in temporal characteristics. The air-conditioner noise was quite steady. There were short-term fluctuations in the speech noise and the road traffic noise. Rail traffic noises peaked when the train passed by, causing significant temporal fluctuations in the recordings. The duration of the peak varied from 20 to 40 s, depending on the rail type. Therefore, a 1 min clip for each noise source was extracted from the field recordings. For air-conditioner, speech, and road traffic noises, continuous clips without other noise sources were extracted. As for the five rail traffic noises, the extraction was conducted based on  
Figure 2: The layout of the field recording site for speech noise and air-conditioner noise (S indicates the position of the recorder). two principles: (a) there were no other noises present, (b) there was only one train passing by in the middle of the 1 min clip. For the exterior traffic noise clips, a filter with frequency responses that mimicked the effect of sound insulation on building facades was applied. [PERSON] et al.