## 4 Discussion  
Through the analysis of the collected data, according to Tables 15 and 16 in Appendix A, the maximum thresholds of LR removal at a long reverberation time appear most frequently. Further, some maximum thresholds of LR removal appear at a medium reverberation time. Although the maximum LR removal threshold does not occur at short reverberation times, the average LR removal threshold for the short reverberation time is close to its LR length. The results suggest that people's perception for LR removal is therefore insensitive, and that only when LR is completely or almost entirely removed are people able to perceive a difference to the reference reverberation. Through analysis, RER removal and ITDG extension have the most influence on perceptual reverberation of the parameters assessed. ERs improve speech intelligibility by increasing the loudness of the direct sound [32]. Table 17 in Appendix A shows a comparison between the energy corresponding to the reference BRIR versus the BRIR with early reflections removed at different reverberation times. The energy values were computed through integrated Loudness according to EBU R 128 standards [33] (in Loudness Units relative to Full Scale (LUFS)) and the ITU-R BS.1770-4 [34] (still refers to Loudness, K-weighted, relative to full scale 'LKFS', but LKFS and LUFS are equivalent). ITU-R BS.1770-4 [34] also defines that the LKFS unit is equivalent to a decibel in that an increase in the level of a signal by 1 dB will cause the loudness reading to increase by 1 LKFS. It can be seen that the energy of the BRIR is reduced by approximately 0.6 to 3 LUFS when the FER or RER is removed at the corresponding threshold (the one exception is that the energy of the RER removed increases by approximately 1 LUFS at 0.31 s reverberation time). It is shown in [35] that listeners can distinguish a change in sound level of about 1 dB in their most sensitive sound level range (about 35 to 80 dB SPL). So the difference one hears when the early reflections are removed may be due to a difference caused by the level. For a speech signal, the ERs are limited to around 50 ms. For RER removal under different reverberation times, their average resolution thresholds are from 15 ms to 18 ms. This means that when ERs are reversely removed 30% to 36%, people can perceive the change of the reverberation. For FER removals under different reverberation time, their resolution thresholds are from 27 ms to 34 ms. When ERs are forward removed over 50% to about 68%, people can perceive the change of the reverberation. Compared with FER removal, RER removal should be a greater consideration when designing reverberation algorithms. When ITDG is extended by about 18-30 ms, the difference of reverberation can be perceived clearly. To contextualise, research shows that ITDG is typically in the range of 20 ms to 60 ms in a large concert hall and from about 8 ms to 27 ms in chamber music halls [36]. ITDG extension thresholds of 18-30 ms is enough to affect the perceptual reverberation, so ITDG should be another major consideration in designing artificial reverberation algorithms. For LR, the average thresholds measured at different reverberation times are 435 ms, 771 ms and 1276 ms, respectively. Before a difference was perceived, the LR was more than halved at all three reverberation times, and at 90% removal for the short reverberation time, suggesting that people are less sensitive to the reverberation difference of male speech signals caused by LR changes. Therefore, to a certain extent, in the design of reverberation algorithms, it is not necessary to give priority to some small changes in the later reverberation, if these changes do not have major impact on the perception of reverberation overall. Theoretically, expert listeners and those experienced with acoustic experiments can more accurately distinguish the impact of BRIR parameters on perceptual reverberation.