## Human-Performed Classification
### Classification Method  
The participants were requested to classify each stimulus as representing one of the three spatial scenes. Initially, a single-stimulus paradigm was followed whereby the listeners were exposed to only one audio excerpt at a time. However, according to the pilot results, omitted in the paper due to space limitations, the test proved to be too challenging, yielding the results close to statistical \"noise.\" Therefore, in the improved version of the test, reported below, a triple-stimulus paradigm was adopted. The reason for such modification was motivated by the well-known phenomenon that humans have a weak discrimination ability when they are asked to characterize a single auditory stimulus at a time, but they are very acute at hearing differences between stimuli if they have an opportunity to make side-by-side auditory comparisons [41]. Therefore, in each trial of the test, the listeners had access to three stimuli, representing the three spatial scenes of a given multitrack recording. To facilitate the test, a custom-designed web-application was developed. Figure 1 illustrates its graphical user interface.  
Prior to the commencement of the listening test, the participants were informed about its purpose, terms, and privacy policy. All subjects gave their informed consent for inclusion before they participated in the study. Then, they were requested to provide information about their previous listening test experience (if any), hearing difficulties, and the model of the headphones used. The data were collated anonymously.  
In the next phase, the listeners were provided with a tutorial, graphically explaining the differences between the three basic spatial audio scenes (see Figure 1a). Subsequently, they were requested to undertake a guided equipment check, in order to verify the playback system and adjust its volume. This procedure also included the left and right channel checks.  
During the listening test, the subjects were sequentially presented with triplets of stimuli, which could be auditioned by pressing one of the three buttons, labeled as \"audio 1\", \"audio 2\", and \"audio 3\", respectively, as illustrated in Figure 1b. These stimuli represented the three spatial scenes (_FB_, _BF_, and _FF_) of a given multitrack recording (e.g., a jazz recording). The items could be auditioned in any order and switched at any time, at the discretion of a listener. The switching was undertaken synchronously, with a 10 ms cross-fade (with a raised cosine function) to avoid sound distortions. The task given to the listeners was to \"_match audio buttons with acoustic scenes by dragging them from the grey container and dropping on the acoustic scenes images_\", as shown in Figure 1b.  
The test consisted of 10 trials. Consequently, each listener was asked to classify 30 items in total (10 trails \(\times\) 3 items), randomly drawn from the repository of 1560 excerpts. Since 212 subjects took part in the test, each excerpt was classified at least four times (1440 items were classified four times, whereas the remaining 120 excerpts were classified five times). There were no time limits imposed on the participants. The mean duration of the listening test was equal to approximately 8 min.  
Figure 1: Graphical user interface exploited in the developed web application: (**a**) excerpt from the instructions for the listeners; (**b**) interface used for spatial audio scene classification.  
The experimental protocol employed in the listening test was approved by the Ethics Committee of the Bialystok University of Technology (Project Code 2/2020).