## 2 Materials and Methods
### Experimental Stimuli  
A listening test was designed to measure the perceptual thresholds of four BRIR parameters, Reverse Early Reflection (RER) removal, Forward Early Reflection (FER) removal, Late Reverberation (LR) removal and Initial Time Delay Gap (ITDG) extension of three different rooms. Since in the study of artificial reverberation the RIR is usually divided into two parts, one including direct sound and early reflections and the other including late reverberation [24], DS was not used as a parameter for the measured threshold in this study. Forward Early Reflection (FER) removal has been implemented through the removal of the initial reflections with subsequent tests removing further reflections forward from the initial reflections of the BRIRs. The opposite scenario, known as Reverse Early Reflection (RER) removal is achieved when the latest early reflections (those just before the LR) are removed first, and at each subsequent test render, we traverse further backwards towards the direct sound, removing the earlier reflections. LR cut off is called LR removal and ITDG extended is labelled ITDG extension. A half of a 64-point Hanning window, equivalent to a 0.726 ms transition at 44.1 kHz sample rate, is applied to the transitions between silence and the impulse response to smooth the truncations in RER removal, FER removal and LR removal. The schematic figure to explain these parameters is indicated in Figure 2, where a BRIR with 0.31 s reverberation time is given as an example. Figure 2a presents the BRIR with an extended ITDG of 50 ms. Figure 2b is the same BRIR with ERs reversely cut off by 50 ms. Figure 2c presents the BRIR with ERs forward cut off by 50 ms. Figure 2d shows the BRIR with LR cut off by 465 ms.  
Three BRIRs (with 0.31 s, 0.91 s and 1.51 s reverberation time) generated in ODEON [25] were used as the reference impulse signals as shown in Figure 3a-c, and their time frequency spectra are shown in Figure 4a-c, respectively. ODEON is regarded as a useful tool for research in objective and subjective room acoustics [25]. Further, 10 s, 44.1 kHz exponential sine-tone sweeps [26] were used to generate the BRIRs and the HRTF implemented in ODEON was Subject 21 from the CIPIC database [27]. The speaker was placed 4 m away from the front wall of the reverberant room, with the listening position at 0 degrees, 13.5 m from the source. The absorption coefficient was varied by changing the material of the room surface, thus the BRIRs for different reverberation times were measured in the same room model. The materials used in the room surface for different reverberation times are described in Table A1 in Appendix A. The room model used was a cuboid with a length, width and height of approximately 22 m, 16 m and 10 m, respectively. A brief segment of anechoic male speech audio was used as the test signal as shown in Figure 3d. The segment is two-channel anechoic audio of 2.6 s length. Its sample rate is 44.1 kHz and bit depth is 24 bit. The listening test reference audio samples were generated by convolving these three BRIRs with the anechoic male speech audio.  
Figure 2: The explanation of changing different BRIR parameters, including RER removal, FER removal, LR removal and ITDG extension. (**a**) Cut off ERs reversely by 50 ms on the original BRIR. (**b**) Cut off ERs forward by 50 ms on the original BRIR. (**c**) Cut off LR by 465 ms on the original BRIR. (**d**) Extend ITDG by 50 ms on the original BRIR.  
Contrast BRIRs were generated by changing one of the identified acoustic parameters of the reference BRIRs from above four variable acoustic parameters (RER removal, FER removal, LR removal and ITDG extension). These altered BRIRs were convolved with the same anechoic male speech audio to generate the contrast stimuli.  
Figure 3: The BRIRs and the test signal used in the listening test. (**a**) The BRIR with 0.31 s reverberation time. (**b**) The BRIR with 0.91 s reverberation time. (**c**) The BRIR with 1.51 s reverberation time. (**d**) The dry male speech audio signal.