Evaluation of a mixed-order planar and periphonic Ambisonics playback implementation

Planar (2D) and periphonic (3D) higher-order Ambisonics (HOA) playback systems are widely used in multi-channel audio applications. For a given Ambisonics order, 2D systems require far less loudspeakers and provide a larger spatial resolution but cannot naturally reproduce elevated sound sources. In order to combine the benefits of 2D and 3D systems, a higher order 2D playback system can be mixed with a lower order 3D system. In the present study, a mixed-order Ambisonics playback system was realised by extending the spherical harmonics decomposition of a 3D sound field with additional horizontal components. The performance of the system was analysed by considering a small and a large loudspeaker setup, allowing for different combinations of 2D and 3D Ambisonics orders. An objective evaluation showed that the systems provided a high spatial resolution for horizontal sources while producing a smooth decrease in spatial resolution with increasing source elevation until 3D performance is reached. This observation was confirmed by a listening test (simulated concert scenario), which showed that in comparison to a conventional 3D system the perceived spatial resolution for sources in the horizontal plane can be significantly increased by adding 2D components and thereby approaching 2D system's performance. Simultaneously, frequency spectrum properties of horizontal sound sources were restored and did not show a low pass filtering effect as it is present in 3D HOA systems.