Validation of a Practical Spatial Soundfield Reproduction System Using a Directional Microphone

Abstract

Spatia1 soundfield reconstruction using loudspeaker arrays has a well-established theoretical treatment in the existing literature. Theoretically, the accurate control region of a fourth-order soundfield is smaller than a listener's head at 4kHz. In real-world audio systems, loudspeaker imperfections, acoustic reflections, and the presence of an observer further reduce the accuracy. Such systems are useful for presenting acoustic scenes and have shown promise as a tool for testing consumer audio devices. This apparent contradiction is resolved by noting the theoretical metric is a tight bound on worst-case error-sufficient but perhaps not necessary. Loudspeaker arrays are often constructed for demonstrations, however very few are independently validated. Furthermore, psycho-acoustic experiments suggest that listening tests may be biased towards systems that are decorrelated to reduce spatial aliasing. As such, a suitable metric is required to assess the utility of soundfield replication for test and measurement. We propose a high-directivity shotgun microphone as an independent objective observer. The microphone's 150mm interference tube provides a smooth and frequency invariant baseline polar response when measured on a turntable. Using a 26-channel loudspeaker array, we simulate plane waves from matching directions. Comparing the two polar responses provides a visual representation of the theoretical and practical errors and limitations. Additional sensitivity analysis can be obtained by moving the microphone away from the array center. This work contributes a tool to more meaningfully frame and progress the research challenges for actual soundfield reproduction systems.