Introduction

In virtual reality (VR), spatial hearing plays an important role for achieving an immersive VR experience. For being able to fully immerse in a VR scene not only visual aspects of the virtual environment (VE) are crucial but also the representation of audio and being able to localize sound events spatially at the appropriate position in the displayed scene is relevant. The term localization can be described as the location of an auditory event in terms of direction and distance (Blauert, 1997, p. 37). Sound localization is the ability to predict the position of a sound source and is based on the processing of auditory localization cues such as binaural or monaural cues (e.g. Akeroyd, 2014; Middlebrooks, 2015). Compared to visual localization, auditory localization offers certain advantages, for example it operates all around the listener beyond the field of view and other visual barriers, timelessly non-stop, even while asleep. In vision, spatial location is topographically represented by points on the retina, while in audition, spatial location has to be retrieved from the signals perceived by the two receiving points. For spatial hearing, using only two ears technically is not enough to be able to hear in all three dimensions, so our brains have to unfurl a lot of information based on the nature of sounds and picking up on other cues such as the spectral content of the sound. Some of these spectral cues are caused by the acoustic reflections by the listener’s body, especially by the pinna, head, and torso geometry. This natural occurring filtering of a sound by the listener's body is eliminated when putting on headphones. Therefore, the adding of this filter while listening through headphones in VR is most important for a proper spatially acoustic representation of the simulated VE. This filtering process is described by head-related transfer functions (HRTFs; Xie, 2013; Iida, 2019).

HRTFs are highly frequency-dependent and vary from one individual to another (Morimoto and Ando, 1980). When HRTFs are measured at the listener's own ears, this is described as "individual", "own", or "listener-specific", whereas "non-individual", "other", or "generic" HRTF refer to measurements from another listener, a dummy head, or a calculation from a model (Begault, 1994; Carlile, 1996). The difference between stereo and binaural for headphone recordings is that binaural recordings are basically stereo recordings with the addition of the disparities of time of arrival and intensity of a sound between the two ears which is the case for instance while recording a sound file with a dummy head. Whether or not it is necessary to use personalized HRTFs for an authentic, realistic reproduction of VE scenes is a much debated topic in literature. More particularly, the increasing access to advanced virtual and augmented reality technologies gives this topic a particular drive of attention. The ability to adapt to non-individualized HRTFs through training (for a review, see Mendonça, 2014) or the tolerance given by adding distance perception (for a review, see Kolarik et al., 2016), auralization (Seeber and Clapp, 2020), auditory movements (Carlile and Leung, 2016; Cohen-Lhyver et al., 2020) and cross-/multimodal perception (Stein, 2012) as well as different recorded auditory stimuli (noise, speech, music) repeatedly raise the question, about the actual influence of individualized HRTFs.

Several studies exist that discuss sound localization in VR with different sound delivery methods (mono, stereo, binaural) including different interpolation and amplitude panning-based spatial sound techniques (e.g. Lam et al., 2015; Turchet et al., 2015), though in very few of them the individuality of headphone spatialization techniques were considered. This review paper provides not only an overview of the state of the art in binaural virtual acoustics to be able to fully understand the genesis of the individualization HRTF-based spatial audio signal processing for the purpose of advanced VR audio scenes, but also a methodology with descriptive attributes which should be considered in future sound localization experiments in VR.

Evaluation of spatial auditory experience in VR applications

There are several VR studies where the focus lies on immersive VR which is helpful in health care (the more immersive the better) (Snoswell and Snoswell, 2019) or in spatial navigation memory assessment (Ijaz et al., 2019), though considerations about immersive audio are missing in these studies. To fully understand the contribution of HRTFs to the spatial auditory experience in VR applications, we would like to give a detailed overview of its histography, measurement methods and thresholds, reproduction via headphones, and evaluation methods.

HRTFs measurement methods and thresholds

For achieving an HRTF dataset for VR applications, there exist different HRTFs measurement methods, such as acoustically, numerically or with other tests such as calculation models. For the most commonly acoustic approach, the acoustic transfer functions from free field to eardrum are measured at multiple source locations and incorporated as digital filters to synthesize stimuli. Measurements of HRTFs can be carried out with microphones placed in the ears of the listener (Møller et al., 1995, see Fig. 1). Acoustical recordings are not the only way to measure HRTFs. They can also be calculated numerically by means of the boundary element method (BEM) (Katz, 2001) using 3D photos (Ziegelwanger et al., 2015). Unfortunately, this method has not yet gained acceptance due to its complexity (using a special 3D photo scanner for depth of field, shadow, etc.). There further exist different HRTF calculation models, i.e. PCA (Principal Component Analysis) or a parametric test. With the calculation models approach, based on large HRTF databases, an HRTF dataset can be chosen which contains most of the relevant data (e.g. Baumgartner et al., 2014; Stitt and Katz, 2021).

Figure 1: Depicting the individuality of the pinna based on two examples (NH258 and NH677) with in-ear microphones (Sennheiser KE-4-211-2) for the HRTF measurement method by Majdak et al. (2007). Beneath, the head-related transfer function of the respective left ear.

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The directional perception of static audio signals has already been extensively researched: Thus, the localization along the horizontal axis is determined both by interaural time differences (ITDs) in the low frequency range and by interaural level differences (ILDs) in the high frequency range. While the ITD / ILD individuality depends mainly on the size of the head and on the distance between the eardrums, monaural spectral features are mainly due to the unique shape of the pinna (Wightman and Kistler, 1997). The reflections of the pinna produce the peaks and notches. Shoulder reflections are observed by the wave-like pattern. The reflections of the trunk and shoulders produce spatial frequency modulations up to 3 kHz (see Fig. 2). Above 1 kHz the head reflections and above 6 kHz the reflections of the pinna are decisive (Majdak et al., 2020).

Figure 2: Simplified depiction of spectral monaural cues and their corresponded frequency ranges to the human pinna, head, and torso. KEMAR head and torso simulator of GRAS used for description.

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Evaluation of moving sound sources in VR applications

In a permanently moving world (and its virtualization) one constantly has to deal with a combination of different parameters in always new and different weighting. In VR applications, the directions of the virtual sources must be shifted according to the head movement to the respectively desired horizontal and vertical axis. The detection and computational operationalization of the head movement simultaneously minimizes the risk of in-head sound localization and front-back confusion (externalization, Brimijoin et al., 2013). For the perception of moving sound sources in VE, not only temporal aspects of the sound source and its speed must be taken into account, but also other perceptual aspects. These are e.g. the influence of different or changing room acoustic conditions (reverberation, early reflections, distance to the sound source, etc.) or the influence of visual perception on the location of a sound source (cross-/multi-modal perception).

Further Perceptual Qualities

"Does it sound realistic?" This question addresses important perceptual state related to the auditory space. How do we decide whether the mental representation of some aspects of our auditory environment is realistic or not? Coming from the field of creating VE (visual and/or auditory), terms like fidelity, immersion, presence, authenticity, plausibility, and realism have been coined in order to address these and similar questions. However, it seems difficult to distinguish this terminology not only for the layman, but also for specialized researchers. Do not all of these terms mean the same thing? In the following, these perceptual quality characteristics and their meaning depending on the different discipline are discussed.

Realism

Regarding the attribute realism, "How much realism is necessary?" (Vorländer and Shinn-Cunningham, 2014) or "How real does it need to be?" (Simpson et al., 2014) are frequently asked questions in acoustical VE. In headphone-based systems, realism is enhanced with the use of listener-specific HRTFs (Shinn-Cunningham et al., 1997; Vorländer and Shinn-Cunningham, 2014). For maintaining a sense of realism among virtual acoustic objects, Begault (1994) explains that the inclusion of distance and environmental context is important and describes realism of virtual audio as one of the factors of audio quality. Externalisation is important for the realism of a virtual sound source simulation (Begault, 1992). Wightman and Kistler (1997) talk about a cue realism and as a qualitative attribute, perceived realism has been measured in auditory HRTFs studies (such as Begault et al., 2001; Iwaya et al., 2011). Nevertheless, more studies can be found to the opposite, as in the effects of sound on visual realism (Davis et al., 1999). Some researchers have found no effect of audio on perceived realism or quality of the visual display (Hendrix and Barfield, 1996) whereas others have found an effect (Storms, 1998 summarized in Kohlrausch and van de Par, 2005, p. 125). Similar to Hendrix and Barfield (1996), results of Larsson et al. (2008) suggest that the addition of non-individualized room acoustic cues to a spatialization increases presence (as well as Kobayashi et al., 2015) but not realism. They believe that sound realism is depending on well-designed source content (for example, that a bus really sounds like a bus) rather than accurate spatialization (that the bus is properly externalized and localized). In multimodal VE, realism factors are used for measuring the amount of received presence, as in for instance scene realism which refers to the size of the field of view, light sources, dimensionality, etc. (Witmer and Singer, 1998). As a perceptual attribute for comparison of HRTFs, realism is defined as sounds seem to come from real sources located around you (Simon et al., 2016). The overall result is given on the basis of a graduation scale, the so-called degree of realism.

Figure 3: Summary of the chapter perceptual qualities: fidelity, immersion, presence, authenticity, plausibility, and realism. The diagram is intended to illustrate their meaning, their context and their demarcation from the point of view of the auditory field of research in conjunction with virtual reality.

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Concluding Remarks

As you can see from the discussion above, there are many different interpretations to each qualitative attribute fidelity, realism, authenticity, plausibility, immersion, or presence. Every field of research has its own view on the terminology and within every field of research there are different definitions to each attribute. Even experts familiar with their field of research are often confused by, or interchangeably use for instance immersion and presence (Bowman and McMahan, 2007) and also use some attributes such as fidelity and immersion synonymously (Poeschl and Doering, 2013). Although all of the attributes are subjective, some of them also have an objective side. For instance, realism can be viewed as system-focused objective realism (in terms of realism of audio reproduction and simulated models) and on the other hand as user-focused perceived realism (in terms of the subjective view on how realistic audio is perceived). However, the perceptual attribute presence definitely is only subjective and has no objective part in it. The attribute immersion is a measure of the psychological sensation of being surrounded as it is to Begault et al. (1998), although immersion has also an objective part in it (such as the objective measurable apparent source width or the listener envelopment). Precisely this makes it different to the attribute presence, which is defined as the measure of psychological sensation being elsewhere as proposed by Sheridan (1992) and others. As to Brinkmann et al. (2017) and Gilbert (2016), authenticity is a measure of expectation matched based on the listener. The same applies to fidelity, which is a measure of psychophysical reproduction based on the system, e.g. the hardware implementation.

In the field of localization of static and moving sound sources with individual and non-individual HRTFs, a broad state of research is shown. Nevertheless, the parameters for directional perception of dynamic sound sources in combination with other perceptual aspects are far from being finally clarified. Especially, the influence of individualization on the listening experience leaves many questions unanswered. In particular, due to the development in the field of VR, more detailed investigations of the multimodal perception with individual HRTFs are rendered possible.

For the direct description of sound signals and the assessment of quality in binaural virtual audio reproduction, various perceptual quality features are used. These descriptive terms mentioned above are most important attributes for measuring descriptive qualitative differences between HRTFs. These attributes go beyond the simple question of localization, which is often used to evaluate HRTFs, but does not cover the variety of perceptual aspects involved in VR applications. There is more than localization, namely realism, immersion, presence etc. and if researchers want to include them in future measurements, then they can operationalize these parameters as follows: immersion is measured on a continuous scale with the end points immersive to non-immersive as a feeling of being located in the middle of the audio scene, also realism is measured on a continuous scale with the end points realistic to non-realistic as sounds seem to come from real sources located around you and so for presence these variables are crucial. The approaches of the following authors are promising (e.g. Katz and Parseihian, 2012; Lindau et al., 2014; Simon et al., 2016). HRTFs and the associated characteristics for static and moving sources in VR are without a doubt elementary factors in VR audio perception. But when it comes to listener-adequate estimation of a VR audio environment, the subjectively perceived quality of the audio experience ultimately depends on perceptual qualities, that can be gathered in listening tests based on psychological scales, like immersion, presence, authenticity, and plausibility.

Acknowledgments

Many thanks to Vice President of the Institute for Acoustic Research of the Austrian Academy of Sciences Piotr Majdak and the research group "Psychoacoustics and Experimental Audiology" for the valuable information especially for the measurement of binaural HRTFs as well as many references.

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