# Surround by Sound: A Review of Spatial Audio Recording and Reproduction
## 4 Multi-Zone Sound Reproduction  
Multi-zone reproduction aims to extend spatial sound reproduction over multiple regions so that different listeners can enjoy their audio material simultaneously and independently of each other but without physical isolation or using headphones (Figure 5). The concept of multi-zone soundfield control has recently drawn attention due to a whole range of audio applications, such as controlling sound radiation from a personal audio device, creating independent sound zones in different kinds of enclosures (such as shared offices, private transportation vehicles, exhibition centres, etc. ), and generating quiet zones in a noisy environment. A single array of loudspeakers is used, where sound zones can be placed at any desired location and the listener can freely move between zones; thus, the whole system provides significant freedom and flexibility. The multi-zone reproduction was firstly formulated as creating two kinds of sound zones, the bright zone within which certain sounds with high acoustic energy are reproduced and the dark  
Figure 4: The listening room compensation using WDAF (wave-domain adaptive filtering). The free-field transformed loudspeaker signals \(\widetilde{g}\) are used in a reverberant room with the filter matrix \(\widetilde{C}\) to compensate the RTFs in matrix \(\widetilde{H}\), \(\mathcal{T}_{1}\), \(\mathcal{T}_{3}\) and \(\mathcal{T}_{2}\) represent the forward and backward WDAF, respectively. Figure 5: **(a)** an illustration of multi-zone sound reproduction in an office environment; **(b)** a plane wave of 500 Hz from 45\({}^{\circ}\) is reproduced in the bright zone (red circle) with a dark or quiet zone (blue circle) generated using a circular array of 30 loudspeakers [134]. zone (or the quiet zone) within which the acoustic energy is kept at a low level [135]. The proposed method is to maximise the ratio of the average acoustic energy density in the bright zone to that in the dark zone, which is known as the acoustic contrast control (ACC) method. Since then, different forms of contrast control based on the same principle have been proposed, including an acoustic energy difference formulation [136], direct and indirect acoustic contrast formulations using the Lagrangian [137]. The technique has been implemented in different personal audio systems in an anechoic chamber [138; 139] or in a car cabin [140]; over 19 dB contrast was achieved under the ideal condition, while, for real-time systems in the car cabin, the acoustic contrast was limited to a maximum value of 15 dB. This contrast control method, however, does not impose a constraint on the phase of the soundfield and thus cannot control the spatial aspects of the reproduced soundfield in the bright zone. A recent work by [PERSON] et al. proposed refining the cost function of the ACC with the aim of optimizing the extent to which the reproduced soundfield resembles a plane wave, thus optimising the spatial aspects of the soundfield [141]. Another issue in ACC is the self-cancellation problem, which results in a standing wave produced within the bright zone [142]. The pressure matching (PM) approach aims to reproduce a desired soundfield in the bright zone while producing silence in the dark zone [143]. The approach uses a sufficiently dense distribution of microphones within all the zones as the matching points and adopts the least-squares method to control the pressure at each point. A constraint on the loudspeaker weight energy (or the array effort) is added to control the sound leakage outside the sound zones and to ensure the implementation is robust against speaker positioning errors and changes in the acoustic environment [144]. When the desired soundfeld in the bright zone is due to a few virtual source directions, the multi-zone sound control problem can be solved using a compressive sensing idea dolwhere the loudspeaker weights are regularised with the L\({}_{1}\) norm.