# The Influence of Binaural Room Impulse Responses on Externalization in Virtual Reality Scenarios
## 1 Introduction  
Headphone-based three-dimensional (3D) audio is becoming increasingly important due to the growing market for mobile devices, virtual/augmented/mixed reality (VR/AR/MR) applications, etc. [1]. A general way to create headphone-based virtual sounds is by convolving an input signal with a pair of binaural room impulse responses (BRIRs) [2]. Binaural room transfer functions (BRTFs) are the frequency domain representation of BRIRs. A BRIR describes an acoustic impulse response between a sound source and listener's eardrums in a room, and can be divided temporally into direct- and reverberant sound parts (early reflections and late reverberation). The direct sound part can be characterized by a head-related impulse response (HRIR, the time-domain representation of head-related transfer function (HRTF)), which contains relevant acoustic cues for sound localization [3].  
Perceived externalization, i.e., out of the head [4], is one of the most important acoustic attributes related to the overall quality of headphone-based virtual sounds [1; 5; 6]. A vital factor for the perception of externalized sound images is the acoustic congruence between the binaurally synthesized room and the real listening environment. The degree of externalization decreases as the acoustic divergence between them increases, which is known as the room divergence effect (RDE) [7; 8; 9].  
A typical way to demonstrate the RDE is to record test stimuli (or synthesize virtual sounds with BRIRs) in one room and play them back in a different room through headphones [7; 8; 9]. [PERSON] et al. [10] investigated the effect of incongruent room-relatedauditory and visual information in externalization of headphone-based virtual sounds. In their study, listeners were divided into two groups to assess externalization of stimuli. One group of subjects was blindfolded, but could hear sounds from a speaker in the playback room. In contrast, subjects in the second group could see the room but could not hear sounds in the room (except for test stimuli played back over headphones). In this way, the first and second groups received only auditory and visual information about the playback room, respectively. Their experimental results reveal that the congruent auditory information is more important to perceived externalization than the visual impression of the listening room. According to the localization model proposed by [PERSON] [7], the room-related auditory information is stored in human short-term memory and adapts quickly when the listening environment changes. The adaptation of this information is actually the familiarization phase to the listening environment.  
The results of the above-mentioned studies [7; 8; 9; 10] are important for binaural listening in AR/MR applications because listeners are aware of the acoustics of their listening environments. In order to perceive well-externalized sound images, the auditory information provided by the binaurally synthesized room should match that of the local listening room. However, for VR applications, listeners are introduced to a virtual environment that may differ (both acoustically and visually) from their local environment. VR users get a visual impression of the room, but may not be aware of its auditory information. It is unknown whether the acoustic congruence between the binaurally synthesized room and the visual-only virtual listening environment is important for externalization.  
The present study aimed to investigate the influence of BRIRs with congruent and divergent auditory information on externalization in VR scenarios. The remainder of this paper is organized as follows. Section 2 introduces the experimental setups. The subjective evaluation results are presented and discussed in Sections 3 and 4, respectively. Finally, Section 5 concludes this study.