## 2 Spatial Audio Recording Techniques for Soundscape Design
### Recording Techniques
#### 2.1.2 Microphone Array Recording  
To better capture the spatial sound field, more microphones or an array of microphones are required. This idea dated back to the 1930s, where [PERSON] and [PERSON] introduced an \"acoustic curtain\" system that consists of a wall of microphones, which are directly connected to a matching loudspeaker array on the playback end [32]. With a good matching (closely positioned) between the microphone array and loudspeaker array, the recreated sound field is more realistic. However, such well-matched recording and playback system is hardly practical in today's applications, where the configurations of the playback system (including headphone, number and positions of loudspeakers) vary greatly in different room settings. As a result, microphone recordings are no longer used directly without processing, but require real-time or post-processing to analyze the characteristics of the sound field. In general, a microphone array with signal processing could provide three types of the information about the sound field [33]: (i) the locations of the sound sources; (ii) the corresponding (\"clean\") sound source signal; and (iii) the characteristics of the sound environment, including the reflections and ambience. Therefore, microphone arrays are widely used in sound source localization, noise reduction, speech enhancement, echo cancellation, source separation, room impulse response estimation, and sound reproduction [34]. Microphone arrays are classified into two categories based on whether they are moving or stationary during the recording. Moving microphone arrays could be used for static or moving sound events. For static sound events, the movements of the microphone array create multiple virtual arrays or enlarges the array size so that more precise information on the sound field could be obtained. A facility enabling the regular movements of the array equipment (e.g., a motor) is required for the moving arrays system. In this case, the assumption that the sound field is spatially static, is very critical. In addition, the effect of recording system movement on the recordings (including the time difference, Doppler effect) should be accounted for. Furthermore, if the movement pattern of the array could be recorded, moving arrays could also be useful in recording non-static sound events. For example, the array could follow the main sound event to give it more focus. The other category of microphone arrays is the static array. For practical reasons, static arrays are widely adopted as it is easier to set up a static microphone array for recording than moving arrays. Since microphones are to simulate human ears, it is reasonable to establish static microphone arrays as sound reproduction normally assumes the listeners to be static. The most commonly-used static microphone array is the uniform linear array (ULA), which is regularly deployed in beamforming applications. Multiple linear arrays can be arranged in a plane to realize a planar array. Circular array is another commonly used geometric configuration of static microphone array. Compared to linear arrays, circular array provides a two-dimensional scan (\(360^{\circ}\) in the horizontal plane) with uniform performance. Extensions to the circular array include multiple circular arrays at the same plane with different radius, or multiple circular arrays at different planes to form a sphere (i.e., spherical microphone array). Spherical microphone arrays are attractive as they can be decomposed using spherical harmonics, which facilitates the sound field representation and interpretation. Spherical arrays with a large radius can be configured to surround the sound event, creating a third-person perspective, while compact spherical arrays are used to record sound events from the listener's perspective (i.e., first-person perspective).