# Auditory Localization in Low-Bitrate Compressed Ambisonic Scenes
## 1 Introduction  
Immersive audio technology is an inevitable element of modern digital media. It is present in cinematic, music and installation arts, broadcast, computer games, virtual reality, and augmented reality applications. With the rise of 5G mobile networks, it is also expected to become a key element of communication services. Typical use case scenarios of using immersive audio in mobile technologies require binaural playback to spatialize the sound. For example, in _mobile VR_, where a mobile device is attached to the VR headset (e.g. Samsung Gear VR, [[https://www.samsung.com/global/galaxy/gear-vr](https://www.samsung.com/global/galaxy/gear-vr)]([https://www.samsung.com/global/galaxy/gear-vr](https://www.samsung.com/global/galaxy/gear-vr))), or both are integrated (e.g. Oculus Quest, [[https://www.oculus.com/quest](https://www.oculus.com/quest)]([https://www.oculus.com/quest](https://www.oculus.com/quest))), spatial audio is delivered through headphones or miniature speakers built into the headset. Some recently introduced headphone products enable the use of motion sensors paving the way for interactive audio rendering without the visual presentation (e.g. Bose Frames, QC35, [[https://www.bose.com/en_us/better_with_bose/augmented_reality.html](https://www.bose.com/en_us/better_with_bose/augmented_reality.html)]([https://www.bose.com/en_us/better_with_bose/augmented_reality.html](https://www.bose.com/en_us/better_with_bose/augmented_reality.html))). With the current state of technology these wearable products require an external mobile device working in tandem, acting as a real-time audio processing unit. Such products might soon enhance general use navigation apps or help improve the accessibility for blind and visually impaired users. The recent introduction of immersive audio streaming for films and television programs for home cinema setups also indicates the need for mobile-based spatial audio solutions. In addition, mobile spatial audio might become a large-scale delivery medium for music, superseding the existing formats originally intended to use multiple loudspeakers for audio playback. State-of-the-art immersive audio rendering systems are expected to give users the sensation of being in another acoustical space, as well as realistically render virtual sound sources as they would exist in the real world. To create a convincing auditory experience a set of sonic attributes needs to be provided, e.g. natural (or plausible) timbre, sound externalization, convincing acoustics, and precise sound source localization. For high quality immersive and interactive experiences, the accurate presentation of virtual sound source location is critical as it is required for directing user's attention and providing coherent visual and auditory cues. Key external factors affecting the auditory localization in immersive audio reproduction include: Head-Related Transfer Function (HRTF) mismatch, the frequency response of headphones or loudspeakers, and audio rendering method. For the latter consideration, one of the main techniques which is employed in mobile spatial audio rendering is Ambisonics [1]. It is used by content delivery services (e.g. YouTube, Facebook) to stream spatial audio for 360/180-degree videos. Current perceptual audio coding standards officially supporting Ambisonics are MPEG-H 3D Audio [2] and Opus [3]. However, such perceptual audio coding can also introduce spatial distortions leading to degradation of localization cues. This is the primary focus of this paper--the evaluation of auditory localization within Ambisonic scenes with respect to perceptual low-bitrate coding and different reproduction methods. Specifically, the subjective differences between Ambisonic scenes encoded with Opus at different bitrates and Ambisonic orders are investigated in terms of localization precision of virtual sound sources in loudspeaker and headphone-based presentations. This paper is organized as follows.