# Automatic Spatial Audio Scene Classification in Binaural Recordings of Music +
## 2 Materials and Methods
### Corpus of Binaural Audio Recordings
#### 2.2.2 Database of Binaural Room Impulse Responses  
Table 2 gives an overview of the thirteen databases of BRIRs used to synthesize binaural recordings. The exploited BRIRs represented a broad range of acoustical and technical conditions (venues and dummy heads), which was essential in order to reliably validate the inter-BRIR generalizability of the proposed method. It can be seen in the table that the reverberation time (RT60) of the BRIRs ranged from 0.17 to 2.1 s. The three types of dummy heads were used to capture the BRIRs, namely B&K HATS Type 4100, Neumann KU100, and KEMAR 45 BA. It was hypothesized by these authors that the head-and-torso simulators might have \"imprinted\" some spatial or spectral characteristics onto the BRIRs, and, therefore, it was decided to include in the experiments the BRIRs captured using at least three types of dummy heads. The BRIRs used in the study also differed in terms of their spatial resolution. In the case of the BRIR set No. 1 (sbs), 15 measurement positions were available [36]. However, in order to maintain front-back symmetry, only 8 BRIRs, out of 15, were incorporated in the study. They represented an octagonal configuration of loudspeakers, with the speakers positioned horizontally at the angles equal to 0, \(\pm 45^{\circ}\), \(\pm 90^{\circ}\), \(\pm 135^{\circ}\), and \(180^{\circ}\), respectively. A similar, octagonal configuration was also adopted in the case of the BRIR set No. 2 (mair) [26]. For the BRIR sets Nos. 3 and 4 (thkcr1 and thkcr7) [37], binaural impulse responses from only two physical loudspeakers were available, positioned at the angles of \(\pm 30^{\circ}\), respectively, which was adequate to synthesize the \(FB\) scene. In order to extract BRIRs for \"missing\" rear loudspeakers,indispensable to synthesize the remaining two scenes (_BF_ and _FF_), binaural impulse responses obtained for the front physical loudspeakers were also exploited, however, with the data obtained using the dummy head rotated to 180\({}^{\circ}\), and with the impulse responses between the left and right loudspeakers being swapped. In this paper, we refer to this technique as front-back \"mirroring.\" This approach could be legitimately used assuming that the dummy head was placed at the acoustical center of a room and also assuming room symmetry. Although these assumptions were moderately violated, it could be argued that while such procedure might have slightly affected the way room reflections were perceived, it did not influence the perception of the foreground audio content (due to front-back symmetry of the loudspeakers with respect to the dummy head), and hence did not affect the validity of this study. This supposition was confirmed by the informal listening tests undertaken by the authors. In the case of the BRIR set No. 5 (thksbs), the three physical loudspeakers located at the front of a dummy head were used to capture the binaural responses [37]. These loudspeakers were positioned at the azimuth angles equal to 0\({}^{\circ}\) and \(\pm\)28\({}^{\circ}\), respectively. The next set of BRIRs (No. 6, thklbs) was obtained using only two physical loudspeakers, placed at the angles of \(-\)18 and 0\({}^{\circ}\). They represented front-left and front-center loudspeakers, respectively. In line with the recommendation of the authors of that database [37], the binaural room response for the virtual front-right loudspeaker (+30\({}^{\circ}\)) was obtained by swapping front and left signals of the front-left loudspeaker (left-right \"mirroring\" technique). For both BRIR sets described above (Nos. 5 and 6), the binaural room responses for the virtual rear loudspeakers were obtained using the front-back mirroring technique. The BRIR set No. 7 (calypso) contained the measurements acquired using a standard five-channel loudspeaker layout [38], with the speakers located at 0\({}^{\circ}\), \(\pm\)30\({}^{\circ}\), and \(\pm\)110\({}^{\circ}\), respectively.