# Automatic Spatial Audio Scene Classification in Binaural Recordings of Music +
## 2 Materials and Methods
### Corpus of Binaural Audio Recordings
#### 2.2.3 Synthesis of Binaural Recordings  
In order to synthesize the binaural recordings, monaural signals, representing individual foreground objects in raw audio material, were convolved with binaural impulse responses estimated for selected angles of incidence. Since the spatial resolution of the incorporated sets of BRIRs was low, with the exception of the last two sets of BRIRs, the binaural room responses for selected angles of incidence were calculated using the two techniques. For the BRIR sets Nos. 1-11, a standard vector-base amplitude-panning algorithm [43] was used to \"interpolate\" impulse responses. To this end, a MATLAB procedure written by [PERSON] was utilized [44]. For the remaining two sets of BRIRs (Nos 12 and 13), the wave-field synthesis algorithm was exploited, using the MATLAB sound field synthesis toolbox developed by [PERSON] and [PERSON] [45]. In the latter case, the virtual audio objects were synthesized as point sources at an intended distance of 3 m from a listener. According to the description of the _FB_ scene, presented earlier in Table 1, foreground audio objects could be, in theory, placed anywhere on the frontal half-plane. However, for simplicity, it was decided to limit the domain of possible placements (positions) to the frontal semicircle. Moreover, the panning angle was reduced from \(\pm 90^{\circ}\) to \(\pm 30^{\circ}\), with \(0^{\circ}\) being the direction indicated by a listener's nose. Some of the BRIRs employed in this study did not allow the use of panning angles beyond the limits of \(\pm 30^{\circ}\). Therefore, such restriction seemed to be a reasonable compromise between the need to span the azimuth range to a full semicircle and the necessity to validate the inter-BRIR generalizability of the method. The latter requirement implied that a relatively large number of BRIRs had to be included in the study. Note that after introducing the above simplifications, the resultant scenes still met the constraints of the basic spatial audio scene definitions, presented in Table 1.  
Interestingly, informal listening tests undertaken by these authors indicated that despite the restriction of the azimuth range to \(\pm 30^{\circ}\), the perceived panning angle of the synthesized complex binaural recordings was wider than the intended one, for some stimuli appearing to span almost all frontal semicircle (\(\pm 90^{\circ}\)). The investigation of the discrepancy between the intended and the perceived ensemble width in binaurally synthesized stimuli is beyond the scope of this study. As mentioned above, for the _FB_ scenes, the intended directions of sound incidence for foreground objects were limited to \(\pm 30^{\circ}\) (inclusive). This condition was illustrated in Figure 2a for a recording consisting of 20 foreground audio objects. For the BRIR sets Nos. 5 and 6, the panning angle had to be further reduced to \(\pm 28\) and \(\pm 18^{\circ}\), respectively, due to the reduced spacing of the physical loudspeakers. No spacing restrictions, understood as a minimum angular difference between adjacent objects, were imposed on the randomly generated distribution patterns. For the _BF_ scene, the panning angle was adjusted to the range between \(150^{\circ}\) and \(210^{\circ}\) in order to symmetrically \"mirror\" the _FB_ scene. Examples of the intended azimuth angle distributions for the _BF_ and _FF_ scenes were illustrated on panels b and c in Figure 2, respectively. For the _FF_ scene, audio objects were equally split between the front and back sectors in order to balance the number of objects in both sectors. For recordings containing an odd number of tracks, one of the objects was randomly allocated either to the front or to the back sector. In the case of the algorithm developed for the purpose of this study, intended azimuth angles were selected randomly and separately for each raw recording, so that there were no repeating spatial patterns across the synthesized excerpts. The duration of the synthesized excerpts was limited to seven seconds.