## 4 Spatial Audio in Virtual and Augmented Reality for Soundscape Design  
Due to the importance of audio-visual interaction in soundscape perception, the audio techniques reviewed in this paper are commonly paired with a variety of visual mediums. Increasingly, virtual reality (VR) and augmented reality (AR) systems have been employed in soundscape research. Visuals recorded using omni-directional cameras, or designed in 3D modelling software, when rendered through head-mounted displays, are perceived to be immersive and realistic. To complement the immersive visuals, the accompanying audio has to possess a minimum degree-of spatialness to achieve natural realism. This argument stems from the perceptual nature of soundscapes and thus, a disjoint in the spatialness of the audio and visual elements will degrade the ecological validity [87]. Hence, the general rule of thumb for audio with VR and AR should correlate with the receivers' movements (mostly head movements), to create an interactive and immersive perception of soundscape. Although spatial audio techniques for VR are discussed in detail in various studies [88; 89], sufficient care is needed when applying these techniques into soundscape research. With the above rule in mind, the spatial audio recording techniques that is suitable for VR (or virtual sound in AR) must be able to capture the acoustic environment from all directions. From this perspective, ambisonics recording is the most commonly used recording techniques for VR/AR, though general microphone arrays with suitable post-processing can also be applied. The spatial audio reproduction for VR/AR is usually realized through binaural rendering with head-tracking, though multichannel playback systems with an ambisonics decoder is also possible. In the binaural rendering system, when the human head moves, these HRTFs must be updated accordingly to account for the changes of the sound directions, which help to create a more interactive and natural listening experience. Note that the binaural rendering of the ambisonics recording is implemented by virtual loudspeakers in the ambisonics reproduction setup. For instance, a stationary VR scene that allows for head-track views of the visual scene (e.g., user is standing still in the virtual environment but able to turn the head to \"look\" around) needs to be accompanied by head-tracked binaural reproduction. An example of a reproduced VR scene with head-tracked binaural reproduction over headphones is shown in the right half of Figure 8. Recently, this approach has been used in soundscape research since it allows more immersive reproduction of the acoustic environment. One good example that soundscape research has benefitted from the use of VR techniques is in the SONORUS project [90]. [PERSON] et al. used VR for evaluating the influence of visual characteristics of barriers and wind turbine on noise perception [23; 91; 92]. Along with VR, the rising popularity of AR devices can be attributed to its availability in the consumer space. Devices such as the Hololens and Meta 2 have the capability to render holograms through head mounted displays [93; 94]. With the consumerization of AR systems, soundscape researchers and practitioners have access to tools that can virtually augment visual and audio elements in the real environment. The general rule for applying audio to AR should still be adhered to, for achieving the desired perceptual accuracy during auralization in the soundscape design process. There are several ongoing research works to achieve a high degree of perceptual accuracy for AR audio. [PERSON] et al. [95] described an augmented reality audio headset using binaural microphones to assist the listener with pseudo-acoustic scenes. [PERSON] et al. [96] also developed a mixer for equalizing and mixing virtual objects with real environment. [PERSON] et al.