## 3 Spatial Audio Reproduction Techniques for Soundscape Design
### Physical Reconstruction of Sound  
The aim of physical reconstruction of sound is to create the sound field in the listening area as close as possible to the desired sound field [59]. The oldest and one of the most popular methods of sound reproduction for soundscape uses two speakers in stereo configuration, i.e., placed at an angle of \(\pm 30^{\circ}\) from the listener [60]. In soundscape studies, the stereo configuration was used by [PERSON] and his group. [PERSON] et al. [63] used stereophonic sound technique to study soundscape for French cities in 1997. [PERSON] used a 2.1 channel system for testing the Perceived Restorativeness Soundscape Scale (PRSS) [64].  
Figure 7: Evolution of spatial audio reproduction systems.  
Multi-channel reproduction methods became popular in consumer devices from 1950. The international standard ITU-R BS.1116-1 establishes the recommendation for reproducing sound and assessment methods for multi-channel sound systems [65]. These methods have been widely used in the reproduction of acoustic environment in soundscapes. [PERSON] et al. studied the subjective perception for playback through different multi-channel setups [66]. The speakers were arranged in different spatial configurations with a hexagon shaped prototype listening room created to test out different reproduction methods. The results of the experiments showed that while frontal perception was best for a 1-D configuration, the spatial definition of audio was best reproduced with 2-D configuration consisting of 6 speakers located at edges of the hexagonal room. In another article, [PERSON] et al. [16] showed that multichannel reproduction is more suitable for processing complex auditory scenes, and playback of urban soundscapes in laboratory conditions, as compared to stereophonic reproduction techniques.  
Wave field synthesis and ambisonics are the other two physical reconstruction techniques, which aim to create the same acoustical pressure field as present in the surrounding. Ambisonics, introduced by [PERSON] et al. [49] is based on the decomposition of a sound field using spherical harmonics. Ambisonic reproduction is flexible to be applied in any sound playback configurations, which makes it very attractive to be used in a wide variety of applications [67]. [PERSON] et al. [21] used the first order ambisonic microphones to record the background ambient soundscape and used a monophonic microphone to record the foreground sounds separately. The soundscape synthesis was carried out using a simulation tool. It allowed the sounds to be layered with each other and effects, like reverberation and reflections, to be added in real-time [68]. An eight-channel loudspeaker setup was used in a semi-anechoic chamber for playback. [PERSON] et al. [58] described the usage of 16-channel audio configuration for playback of soundscape recording done in different parts of New York using sound field tetramic for recording. Moreover, there is increased support for ambisonics in VR/AR through leading industry players, including Google [69], BBC [70], and Facebook [71]. The ambisonics systems mentioned thus far are in the first-order configuration, with satisfactory but limited spatial resolution as compared to higher-order configurations, as discussed in Section 2.1.4. However, higher-order ambisonics systems are still pre-mature and costly today.  
In a study for comparing the ecological validity among the different reproduction methods related to physical reconstruction of sound, [PERSON] et al. [16] compared the stereophonic and ambisonic reproduction techniques. The study analyzed verbal data collected through questionnaires, and compared it to the field survey using semantic differential analyses for different sound samples. The study concluded that a \"neutral visual environment\" along with spatial immersion of recreated soundstage is essential to ensure high ecological validity.