## 3 Repository of Binaural Audio Recordings  
In this work, we used the same collection of binaural audio excerpts as in our previous study [13]. To the best of the authors' knowledge, this is currently the only audio repository intended for research into human or machine perception of spatial audio scenes in binaural recordings available in the public domain. The repository consists of the two sets of the audio excerpts intended for the development of machine learning algorithms (training) and their evaluation (testing), respectively. To create this repository, initially, 152 multi-track studio recordings were gathered, out of which 112 were allocated for training, whereas the remaining 40 items were destined for testing, as shown in Table 1. These multi-track recordings represented a broad range of music genres, including classical music orchestral excerpts, opera, pop music, jazz, country, electronica, dance, rock, and heavy metal. The number of individual foreground audio objects (musical instruments or singers) in the group of recordings intended for training ranged from 5 to 62, with a median of 9. In the case of the recordings selected for testing, the number of foreground objects varied from 5 to 42, with a median being equal to 10. The binaural excerpts were synthesized by convolving the abovementioned multi-track studio recordings with 13 sets of binaural-room-impulse-responses (BRIRs). Each multitrack recording was convolved in three variants, forming the following three spatial scenes:  
* _Foreground-Background (FB) scene_--foreground content (music ensemble) located in front of a listener and background content, such as reverberations and room reflections, arriving from the back of a listener. This is a conventional \"stage-audience\" scenario [33], often used in jazz and classical music recordings. * _Background-Foreground (BF) scene_--background content in front of a listener with _foreground_ content perceived from the back (a reversed stage-audio scenario). While it is infrequently used in music recordings, it was included in this study for completeness, as a symmetrically \"flipped\" counterpart of the previous scene. * _Foreground-Foreground (FF) scene_--foreground content both in front of and behind a _listener_, surrounding a listener in a horizontal plane. This scene is often used in binaural music recordings, e.g., in electronica, dance, and pop music (360\({}^{\circ}\) source scenario [33]). While the number of the aforementioned scenes may initially appear to be small, it has to be emphasized that these three scenes are considerably challenging to identify aurally, \begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Destination**} & **No. of Multitrack** & **No. of BRIR Sets** & **No. of Binaural** \\  & **Recordings** & & **Excerpts** \\ \hline Training & 112 & 13 & 4368 \\ Testing & 40 & 13 & 1560 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Constitution of the binaural audio repository (adapted from [34]). in particular, when head-movements and individualized head-related-transfer-functions (HRTFs) are not incorporated in a binaural playback system. Head-tracking facilities and the technique of individualization of HRTFs were deliberately excluded from this work to maintain the ecological validity of the experiments as most of the ordinary listeners of binaural audio content do not use head tracking or individual HRTFs. In total, 4368 excerpts were synthesized for training purposes (112 raw recordings \(\times\) 13 sets of BRIRs \(\times\) 3 scenes) and 1560 excerpts were synthesized for testing (40 raw recordings \(\times\) 13 sets of BRIRs \(\times\) 3 scenes), as indicated in Table 1. While the BRIR sets were the same for the training and testing repositories, the studio multi-track recordings were different for both collections. Hence, there were no common music recordings shared across the two repositories.