# Immersive Two-Channel Recordings Based on Personalized BRIRs and Their Applications in Industry
## 2 Immersive Sound  
Spatial sound refers to a set of audio signals that, when properly presented, can give the listener the impression of being surrounded by multiple sound sources [15]. This impression is primarily possible due to the specific perception of acoustic waves by the human sense of hearing. Based solely on the signals reaching both ears, the brain can determine the location and characteristics of the perceived sound sources [16; 17]. With a thorough understanding of the phenomena of the auditory system that enable the experience of sound sensation, including spatiality, it is possible to produce any type of sound artificially, such as by positioning sound sources in a virtual soundscape. Spatial and immersive sound are terms often used in the context of advanced audio systems. While the terms spatialization and 3D sound are used interchangeably, they denote different aspects of the listening experience [18]. Understanding these differences is key to properly applying the appropriate technologies to create desired sound effects in recordings. Spatial sound refers to a sound reproduction technique designed to create the illusion of acoustic space around the listener, usually involving only one horizontal plane located at the height of the listener's ears [19; 20]. Immersive sound is a more advanced concept that goes beyond this traditional approach by registering sounds more naturally. Unlike standard surround sound, immersive sound is not limited to the horizontal plane but also considers planes at different heights, allowing sounds to be precisely located both above and below the listener. In the context of BRIRs and immersion, the research began in the early 1970s, with the first binaural recording using a dummy head as shown in Figure 1. The used technique aimed to replicate natural hearing experiences by placing microphones in the ear canals of a mannequin. [PERSON], [PERSON] et al. details the methodology of the first recordings using miniature microphones and probes placed in the ears of human subjects [21]. Nearly 20 years later, the introduction of 3D audio formats, e.g., Dolby Atmos, began: one of the most groundbreaking industrial applications of immersive audio. Object-based audio systems were the answer to enhancing spatial perception and auditory experience not only in specific loudspeaker setups but later also for two-channel playback. This could not have been possible if not for creating codecs such as MPEG-H 3D, which enabled the support of coding audio as HOA as well as both audio objects and channels. The MPEG-H 3D audio decoder also supports binaural rendering [22]. When it comes to immersion, introducing head tracking for binaural playback systems can heighten the plausibility of localization. The most important element concerning head tracking is the accuracy of sound-source localization in systems using this technology. Although some studies have not concluded that individualized HRTFs are favorable in a standard listening experience [23], some have found that there are slight differences in this aspect, but not as concerns localization in dynamic environments [24]. The most recent studies, however, point out that there is a more than random chance that a person presented with a set of their own individualized HRTFs, someone else's, and generic ones, are likely to choose their own [25]. Even though there is not statistically universal favorability in the aspect of choosing between sets of individualized HRTFs and generic ones, the problem may still lie in the method of obtaining such data and not in the aspect of personalization itself. The creation of a realistic sound image in 3D space needs to take advantage of the natural ability of humans to perceive spatial sound, allowing precise localization and tracking of the possible movement paths of sources relative to a reference point. Traditional surround sound systems, such as those used in movie heaters, were designed for a single listening plane at the height of the listeners' ears.