# Immersive Two-Channel Recordings Based on Personalized BRIRs and Their Applications in Industry
## 4 The Importance of Externalization  
The biggest difference between headphone and loudspeaker listening is how a person determines the position of the sound source. Proper conversion of the signals, in such a way that preserves the impression of a real acoustic scene in three-dimensional space is, therefore, very important. In the case of loudspeakers, the sound source is most often easy to locate, since it is physically located at a specific distance from a person's ears and is also visible to their eyes [37]. Playback of stereo recordings using headphones, without a prior conversion method, on the other hand, causes the source, regardless of its original position during recording, to appear to be located inside the head of the listener. Such a phenomenon is called internalization and completely disrupts the sense of presence in a three-dimensional space. Figure 2: HRIR and BRIR comparison. When recording sounds in a room, the stereo base, that is, the distribution of microphones in relation to each other and the angles at which they record signals coming from the selected sources, is important. If these angles, or the distances between the source and the recording device, are too small, the picture of the room's acoustics when listening can be severely narrowed and distorted. In such a situation, during headphone listening, the recording, instead of imitating the acoustics of the room and the position of the speakers used to achieve it, seems to have its source inside the head of the listener, which, as mentioned earlier, is called internalization. The internalization effect seems to be particularly audible, for example, in diotic listening, when the same signal is fed to both channels in the headphones. As an impression, this effect may be desirable for recording designers in music mixes, but in the case of processing multichannel 3D recordings, internalization is a problem that should be eliminated. In conversions where it is particularly important to capture the characteristics of a given acoustic scene that is physically outside the head of the listener, it is essential that it be preserved in the same place for headphone listening. In the case of binaural synthesis of recordings, the effect that describes maintaining impression that the sound source is located outside the person's head during listening is externalization. From a more psychological point of view, it can be described as a natural correspondence between the signals received by the listener and their expectations of the characteristics of the acoustic scene in which they are located. This is in contrast to internalization, in which such a view is a violation of the listener's conviction about where the sound should actually come from [38]. It is natural that the effect of perception by means of the sense of hearing of actual sound sources is the impression of their presence outside the head of the listener, which is called externalization; and in this case, listening to recordings through headphones is a factor that completely disrupts this impression. The reason for this is the reduction of the amount of interaural cues, responsible for locating sound sources, which are naturally filtered and identified by the listener's ears. By covering the ear canals and depriving the ears of the ability to collect and process sound waves on their own, the internalization effect intensifies, and with it the sound source moves from the three-dimensional space straight into the head. Both the interaural cues and the components of the spatial impression can be artificially recreated. Using a processing algorithm that will mimic the physical phenomena taking place in the room in combination with the information we have about the interaction of acoustic waves with human ears, it is possible to recreate the externalization sensation with headphone listening. Researchers have found that there are some improved externalization effects for listeners when using a set of personalized BRIRs compared to generic BRIRs [35].