Auditory Illusion Through Headphones: History, Challenges and New Solutions

The Technology of Binaural Listening & Understanding: Paper ICA2016-363

Auditory illusion through headphones: History, challenges and new solutions

Karlheinz Brandenburg(a),(b), Stephan Werner(b), Florian Klein(b), Christoph Sladeczek(a)

(a)Fraunhofer IDMT, Germany, [email protected], [email protected] (b)TU Ilmenau, Germany, [email protected], [email protected], ﬂ[email protected]

Abstract The dream of perfect recreation of sound has always consisted of two parts: Reproduction of monaural sounds such that they seem to be exact copies of an original signal and the plausible recreation of complex sound environments, the possibility to be immersed in sound. The latter goal seems to be much more difficult, especially if we consider reproduction over headphones. From standard two-channel sounds reproduced over headphones through artificial head recordings, the inclusion of HRTF and binaural room impulse responses, always something was missing to create a perfect auditory illusion. Depending on refinements like individually adapted HRTF etc. these methods work for many people, but not for everybody. As we know now, in addition to the static, source and listener dependent modifications to headphone sound we need to pay attention to cognitive effects: The perceived presence of an acoustical room rendering changes depending on our expectations. Prominent context effects are for example acoustic divergence between the listening room and the synthesized scene, visibility of the listening room, and prior knowledge triggered by where we have been before. Furthermore, cognitive effects are mostly time variant which includes anticipation and assimilation processes caused by training and adaptation. We present experiments proving some of these well-known contextual effects by investigating features like distance perception, externalization, and localization. These features are shifted by adaptation and training. Furthermore, we present some proposals how to get to a next level of fidelity in headphone listening. This includes the use of room simulation software and the adaptation of its auralization to different listening rooms by changing acoustical parameters. Keywords: immersive sound via headphones, room simulation Auditory illusion through headphones: History, challenges and new solutions

1 Introduction As long as there has been recording of sound, people have been dreaming about the perfect sound reproduction enabling the real illusion of artists being in the room. There are reports of even Edison, when marketing early phonograph systems, emphasized audio quality, even over the artistic quality of the recording [9]. He organized demos around the world where people where asked whether they were actually listening to the live artist or a recording. With the continued improvement of recording, amplifier technology and loudspeakers, today we can get a quite faithful reproduction of monaural signals. For reproducing sound from multiple sources in a room, we still cannot say that the task of a plausible recreation of the sound in a different room has been completely solved. Difficult as this task is for reproduction using loudspeakers, there is even more of a problem when using headphones. The usual result if using recordings which have been mixed to enable playing the sound via two loudspeakers is sound which seems to come from within the head. What we do desire is to hear the sound coming from a stage before us (or around us). If we succeed with this, we call the result an externalized sound. Externalization describes the perception of the position of an auditory event outside or inside the head of the listener [29, 15]. Externalization is a crucial feature to reach a plausible spatial auditory illusion with binaural headphone systems. In the following chapters, we will first look at the main reason for non-externalized sound and present older efforts to get around these problems. We then present more current work ex- plaining the extent of the difficulties and look into some newer proposals to enable perfect audio illusion via headphones.

2 Earlier work In the field of electrical sound reproduction headphones always played a major role. In the early days due technical constrains, the development of a headphone speaker was much simpler than the development of a loudspeaker. Therefore the first electrical devices allowing listening to recorded audio were based on headphone-like technology. One of the first systems that was available to a broader audience was the "Théâtrophone" developed by Clément Ader in 1881 [8]. This device allowed a transmission of a two-channel audio signal to different receiver stations were the user needed to put two earcups to its ears to listen to concerts or plays. As the recording was realized using two microphones placed in a distance to each other, the users reported a spatial impression because of the inequalities of the sound at the two ears [22]. However, as these signals do not really represent the sound pressure at the ear drums as it would occur in natural listening, researchers have been working since more than 40 years to re- semble the "real world" signals. Major cues for directional hearing are time (phase) differences

2 (Interaural Level Differences - ILD , Interaural Time Differences - ITD) and the direction and frequency dependent transfer function from sound source to ear drum (Head Related Trans- fer Functions - HRTF) [27]. To simulate the HRTFs in an audio recording situation so-called dummy heads are used [26]. One of the first demonstrations made to a public audience was the presentation of a mechanical man called "Oscar" with microphone ears by AT&T at the Chicago World Fair in 1933 [13]. This was the starting point for the development of different dummy heads, which are still used today [21, 14]. With the availability of dummy-heads it was possible to record sound as placing a head on a specific position. However, a true spatial impression of the recorded scenes was not perceiv- able for every listener, since there are large differences between the HRTFs of different test subjects and we usually are not very good with listening with somebody else’s ears. Therefore a next big step towards more realistic auralization was the introduction of individualized HRTFs. Over the years, different methods have been used among others: In-ear measurement using small probes in the blocked or unblocked ear canal [26]; On-ear measurements including some correction factors; Selection of HRTFs (without actual measurement) of "what works best" [33] and optical measurement of ear and ear canal geometry and calculation of an HRTF using numerical simulations [18, 19]. Another question in the usage of individualized HRTFs is the needed accuracy. There is more research than would fit in this paper on such questions. There are publicly available databases of HRTF measurements. When we look at the amount of research going into the different methods, HRTF individualization has clearly got a lion’s share of the research. Since actual HRTFs depend not only on the individual, but on the position of the sound, an important cue for auditory illusion is the change of the sound when somebody moves the head or the listeners moves within space. In literature this is typically called dynamic binaural synthesis [32]. It has been implemented using head trackers and the selection including interpolation of actual HRTFs [10]. Such systems are known for a much better auditory illusion and externalization. Another cue to help with externalization is the actual reflection pattern in a room [3]. This founded in the fact, that reflections have a major influence on distance perception [24]. To include room acoustics into the sound of a headphone Binaural Room Impulse Responses (BRIR) are used. These transfer functions can either be determined by room acoustic measurement or room acoustic simulation. Regarding the lack of externalization, we find different theories in the literature. If we include the newer results on room divergence and adaptation (see the following chapters), the authors favor the following explanation: Sounds in a room are localized via a complex interaction of simple auditory cues, the expectation in higher layers of the brain and the recognition of sound patterns (including reflections) of known signals. Whenever there is too much divergence between the expected sound pattern and the actual sound delivered to the inner ear, there is an decreased probability of externalization. Context-dependent quality parameters like room divergence, presence of visual cues, and personalization of the system influence the perception of externalization [38]. From this it is clear that externalization is not just the result of using

3 correct HRTFs etc., but also the result of complex cognitive interactions in the brain. It is hypothesized that the build-up of the experienced quality is a cognitive process which includes expectations and preknowledge of the listener. Jekosch [16, 17], Blauert [5, 4], Raake [30, 31], and others [25] propose a quality formation process with two essential processes. The quality perception path is driven by the physical nature of the event which reaches the sensory organs. The perceived auditory event is created or constructed with respect to the an internal reference path. This process includes a comparison and judgment with the internal reference (or expectation) of each individual person. The reference path describes the time dependent, context dependent, multi-sensory, and cognitive inﬂuencing factors on the quality formation process. To transfer this knowledge in new applications an extension of this process is proposed. The extension includes on the one hand the technical system to build-up the perceived quality and on the other hand feedback mechanisms from the perceived quality to the technical elements of the system [35]. The quality of the system can be described by the technical quality elements and the context of use of the system (context-dependent quality parameters like room divergence or personalization of the system).

3 Drawbacks of current solutions As described in the preceding chapter, there have been many proposals for better auditory illusion via headphones. Over the last 40 years, many groups have contributed, but still the following applies: Depending on the actual system, auditory illusion including externalization of auditory events happens only for a subgroup of all listeners, for a selection of audio signals, and for certain playback conﬁgurations (depending on the room for example). Very often the new proposals have been tested over and over again, but mainly by experienced listeners. Usually, two cognitive effects with their impact on auditory illusion are underestimated: Room divergence and auditory adaptation. Some test results regarding these effects are described in the following chapter. What will happen if these are neglected? Room divergence: The evaluation of headphone reproduction systems is usually limited to one playback situation: developers listening lab. However, in real application the context of use is a subject to change. In real use this leads to a decrease of plausibility because by the room divergence effect described later. Auditory adaptation: Auditory adaptation has to be considered when developing headphone reproduction systems. Developers optimize their systems by tuning technical parameter as mention in chapter 2. and conduct their evaluations tests inside the laboratories with the same people over and over again. When doing so, auditory adaption can shift perception and the evaluation process of the developed system is not universally valid anymore.

4 4 Results of room divergence on externalization As an example, the results on experiments regarding the room divergence effect and its effect on spatial auditory perception are summarized. The experiments show that perceived externalization is significantly lower if listening room and synthesized room are different in their char- acteristics. An acoustic congruence yields to an increase of externalization. It is also showed that externalization and localization of auditory events is influenced by training and adaptation of the listener on the context. The experiment asks for perceived externalization at different synthesis conditions. An individualized and artificial binaural synthesis of single loudspeakers at different directions is used. The acoustic congruence respectively divergence between the listening room and synthesized room are investigated. The rooms are a listening lab (HL) and an empty seminar room (SR). The main test conditions are the four combinations of synthesized room and listening room: “HL in HL”, “HL in SR”, “SR in HL”, “SR in SR”. Both rooms are approx. similar sized (approx. V=180 m3) and shaped, but the reverberation time is 2.0 s for SR and 0.34 s for HL. The listening lab follows the recommendation of the standard ITU-R BS.1116-1. An excerpt of the results in presented in figure 1. A full presentation of the results can be found in Werner et al. [38].

0° 90° * (2.8)*** (max)

1.0 *** 1.0 (4.0) (1.8)* 0.8 0.8

(3.0)*** 0.6 0.6 (1.6)* * 0.4 (1.9) 0.4 externalization index externalization index externalization 0.2 0.2 0.0 0.0 HL(IN) HL(IN) HL(IN) HL(IN) SR(IN) SR(IN) SR(IN) SR(IN) FF(KK) FF(KK) FF(KK) FF(KK) HL(KK) HL(KK) HL(KK) HL(KK) SR(KK) SR(KK) SR(KK) SR(KK)

180° 1.0 (5.1)***

0.8 ** (2.1) listening room

0.6 ● HL * (1.8) SR 0.4 externalization index externalization 0.2 0.0 HL(IN) HL(IN) SR(IN) SR(IN) FF(KK) FF(KK) HL(KK) HL(KK) SR(KK) SR(KK)

Figure 1: Externalization indices for combinations of listening and synthesized room; 95% binominal conf. int.; SR=seminar room, HL=listening lab, KK=artificial head (KEMAR) BRIRs, IN=individual BRIRs; ***significant difference at p<.01, **significant difference at p<.05, *significant difference at p<.1; number in brackets is the effect size as odds ratio; figure from [38]

5 Significant increases of externalization (using Fisher’s exact test) are observed for congruence between the listening room and synthesized room compared with divergent listening conditions. This effect is strong for the 0◦direction and for individualized synthesis of the seminar room; “SR(IN) in SR” compared to “SR(IN) in HL”, p<.01 with an effect size as odds ratio of 4.0. A similar increase of externalization is also visible for individualized synthesis “HL(IN) in HL” compared to “HL(IN) in SR”; p<.01 and effect size as odds ratio of 3.0. A similar tendency but at a lower significance level is also visible for synthesis using artificial BRIRs and the other directions. The perceived externalization is also dependent on the presented source direction. Less externalization is measured for virtual audio objects placed at directions with expected localization inaccuracies. These directions are 0◦ and 180◦ with generally high cone of confusion errors. It is hypothesized that the described room divergence effect is the result of an cognitive detection process. The formation of externalization depends on the comparison between known reflexion patterns of the listening room and the synthesized scene. If the patterns are sufficiently similar the synthesized scene is assumed as plausible and the system becomes viable. This assumption is also in line with experimental findings connected with the well known precedence and Clifton effect [11, 7]. Auditory perception and the quality of experience are based on prior experience and can therefore be a subject of change over time. In the domain of hearing research it is well known, that listeners can improve in certain repetitive tasks. For example the discrimination of frequencies [1] can be improved by training. In the case of hearing aid rehabilitation this ability can be utilized to improve for example speech intelligibility and spatial hearing [6] [39]. Hearing aids and binaural reproduction systems share limitations regarding the spatial cues which they can provide. The original ear signals are distorted and there is a limitation of the frequency range in both cases. When using hearing aids, certain spatial cues are completely lost due to the microphone placement behind the ear. In case of cochlea implants the signals are distorted even stronger due to the electrical stimulation. Even in those cases the auditory system is able to adapt to these changes and allows to identify speech and sound directions to some degree. Similar mechanisms can be identified for listening with binaural playback systems. Spatial cues are distorted when non-individual HRTFs are used which often leads to front-back confusions and high errors in the elevation perception. Various researchers found, that the listener can adapt to distorted spatial cues. For a review see [23] and [2]. Experiments at our laboratory show that listening training with visual positional feedback leads to a fast adaptation to artificial HRTFs. Especially in the median plane, localization errors are improved and some listeners show better elevation localization accuracy with trained artificial HRTFs than with their individual HRTFs [20]. Possibly the listeners focused on relearning spectral cues, because these are the main cues for front-back discrimination and elevation perception. Room related adaptation effects are also very important in the case of binaural playback systems. Binaural technology is mostly used to synthesize virtual soundscapes or to resynthesize real rooms like famous concert halls. In this application it is common that the room we hear over headphones does not fit the room we are currently located in. As mentioned before this leads to the room divergence effect and has a negative effect on the externalization perception. To synthesize a concert hall while sitting in a living room is probably not plausible for most listeners. Preliminary findings [38] show that this effect is related to our expectations about the listening room and

6 that these expectations can be shifted by familiarization with the synthesized room. This basic principle was researched earlier by means of dynamic processes of the precedence effect. In [7] Clifton states “[...] expectations are most likely based on the listeners accumulated experience in highly variable acoustic environments [...]" and sudden or unexpected changes of these environments stand in conﬂict with the listeners experience. In case of the precedence effect an raise of the echo threshold is interpreted as an expectation conﬁrmation while a descend of the echo threshold is interpreted as violation of expectations.

5 New Solutions The proposed new solutions for spatial audio systems deal with an adaptation of the quality elements and/or the context parameters of the audio system to reach a plausible auditory illusion and create a viable spatial audio system. The (re-)synthesized auditory scene is shifted towards the actual listening conditions [28]. Three approaches are presented which change acoustic parameters of the used transfer functions or train the listener. The (re-)synthesized auditory scene can be shifted towards the actual listening conditions by changing acoustic parameters like Direct-to-Reverberant-energy-Ratio (DRR), adaptation of the used transfer functions, additional visual cues and a lot of others to reach a more plausible illusion in the context of use. A first approach describes the adaptation of the Direct-To-Reverberant-energy-Ratio (DRR) of the binaural synthesis towards the listening room. Recently an experiment has been conducted where the assessors adjust the DRR of a synthesized room until perceptional congruence between the simulation and the expectation is reached [36]. The adjusted DRR should be similar to the measured DRR of the synthesized room if congruence between synthesized and listening room occurs. The test persons are able to adjust the DRR of the synthesis to the DRR of the listening lab. The interquartile distance (IQDs) of the DRR adjustment is in the range of the just noticeable difference of DRR perception. We conjecture that the adjustment of the DRR is a valid method to adapt a binaural synthesis on the context listening room because of the high inter-rater-reliability visible in the small IQDs. The synthesis is shifted towards the DRR of the listening room without destroying the reflection patters of the re-synthesized room. Further investigation are planned to analyze the relation between the adaptation of acoustic parameters of the synthesized scene on the perception of externalization under divergent room conditions. As a second approach a changing or shaping of a time based parameter of recorded BRIRs to change perceived distance and externalization of auditory events is proposed. A controlled modification of the distance perception in non individualized binaural headphone reproduction is evaluated in an experiment in our laboratories [37]. The algorithm change systematically the time based distance dependent property Initial Time Delay Gap (ITDG) of BRIRs [12]. Listening tests show that measured BRIRs and the newly synthesized BRIRs cause a similar distance perception. This or similar methods are for example meaningful to change the size of the synthesized scene to match the expected distances of the listening room without destroying the spatial composition and reflection patterns of the synthesized scene.

7 As a third approach, speciﬁc training could be used as a tool to improve auditory perception. Depending on application there could be various ways to realize such a training. In computer games for example, a tutorial could serve as audio-visual training session. With increasing pop- ularity of Head Mounted Displays, these device could be used to provide audio visual training environments with visual feedback about the correct sound directions. Front-back confusions, localization errors and in-head localization could be decreased before using critical applications.

6 Conclusions While there has been research on plausible reproduction of sound in a room via headphones for several decades, we still cannot report that the problem has been completely solved. There are some commercial systems [34] which, given enough training for the listeners, produce a convincing illusion for certain application scenarios (like listening to 5-channel sound via headphone instead of five loudspeakers visible in the same room). We have shown that the actual environment (visible and audible including remembering the room) has substantial influence on the possibility for externalization of sounds and such on the possibility of a convincing audio illusion. Newer own experiments indicate that the context parameters acoustic room divergence, visual influences, and personalization are statistically independent from each other [38]. There is ongoing research on finding quantitative descriptions and models for these effects. In the moment we can just state that, among others, the existence of a plausible audio illusion is affected by technical parameters (like using correct HRTFs/BRIRs), the content, the actual listening space, the listener, and the experience (both short term and long term) of the listener. The currently best systems use head tracking, individualized HRTFs, room simulation and some local training of the listeners. Still, the authors have not yet encountered any system giving a convincing illusion for every content and every surrounding. There is more research to be done in this area.

7 Acknowledgments Thank you to all test participants for taking part in the related listening tests and your interest in this research topic. Furthermore we thank all students for their assistance during measurements and listening test conduction. This work is partially supported by a grant of the Deutsche Forschungsgemeinschaft (Grant BR 1333/14-1).

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