## 2 Materials and Methods
### Experiment Stimuli
[37] measured the sound insulation index of 31 window elements in 1/3-octave bands ranging from 50 Hz to 5000 Hz. The sound insulation levels from three apartment building facades were also reported in Reference [38]. The frequency characteristics reported in the two studies were quite similar. Therefore, the detailed data from Reference [37] were used to design the filter in this study, as illustrated in Figure 4. For frequencies below 50 Hz and above 5000 Hz, the attenuation was designed based on the 50 Hz and 5000 Hz bands, respectively. The sound filtering was applied using the graphic equalizer tool in the Audition software (2020) [38]. The spectrogram of the filtered exterior noise clips is shown in Figure 5.  
Figure 4: Sound reduction index of the filter applied to exterior traffic noises, simulating the sound insulation afforded by the building facade (data from [37]). Figure 3: Spectrogram of noise clips extracted from the field recordings. (3) Sound level calibration  
Based on the field measurements and data reported in previous studies, the noise level of common office spaces was found to vary from 40 dB to 55 dB [26,39]. Two sound levels (L\({}_{\text{Aeq, 1 min}}\)) were considered for the traffic noises: 40 dB and 50 dB. Because six different traffic noise sources were considered in this study, there were 12 traffic noise clips in total. The sound level of the interior noises was set to 40 dB. The sound level calibration was conducted using a head and torso simulator (Bruel & Kjaer 4128-C, Denmark). The volume of each stimulus was adjusted until the sound level reached the intended level. In addition, we also examined the frequency response of the reproduced signal. At the frequency range of most sound energy (50 Hz to 4000 Hz), the differences caused by the sound reproduction system were smaller than 3 dB. The sound levels of frequency bands above 5000 Hz were lower than 25 dB, which was close to the background level. Therefore, it could hardly affect the participants' evaluation, even though there were relatively larger differences (5 dB to 10 dB) between the designed and the reproduced signal. In this study, the stimuli were calibrated at the same equivalent sound level (L\({}_{\text{Aeq,1 min}}\)). However, the peak sound levels in the noise clips varied due to temporal differences. As shown in Table 1, the peak sound level (L\({}_{\text{Afmax}}\)) varied from 45.7 dB to 56.2 dB at the same equivalent sound level (40 dB). To quantify the temporal fluctuation characteristics, L\({}_{\text{10}}\)-L\({}_{\text{90}}\) values were calculated. L\({}_{\text{10}}\) and L\({}_{\text{90}}\) are commonly used to measure the sound levels of noise events and background sounds, respectively. Therefore, L\({}_{\text{10}}\)-L\({}_{\text{90}}\) describes the sound level difference between the noise event and the background sound. As shown in Table 1, the L\({}_{\text{10}}\)-L\({}_{\text{90}}\) of road traffic noise was relatively low (2.63) due to its consistent temporal pattern without significant noise events. On the contrary, there was a distinct noise event in each rail traffic noise clip, leading to more temporal fluctuations and significantly higher L\({}_{\text{10}}\)-L\({}_{\text{90}}\) values (17.5-23.2). As vehicle speed increased and vehicle length decreased, the duration of sound events in rail traffic clips decreased, resulting in a greater temporal fluctuation. Figure 5: Spectrogram of exterior traffic noise clips after filtering and sound level calibration (L\({}_{\text{Aeq,1 min}}\) = 40 dB). Experiment stimuli production: combination of traffic sounds and interior noises  
The experimental stimuli with combined noises were created by blending traffic sound clips with interior noise clips. There were three interior noise conditions: SL (silence), AC (air conditioner), and IS (irrelevant speech). For each interior noise condition, twelve traffic noises were considered as exterior noise conditions, including six traffic noises at two sound levels (40 dB and 50 dB).