We propose a method to adjust room acoustics by controlling specific acoustic impedance in a reflective environment. This approach enables control of frequency response, sound pressure distribution, and sound intensity direction in the low-frequency range. In our previous work, we also demonstrated that impedance control is equivalent to suppressing the response of a cardioid directed opposite to the incident direction. This suggests that the method separates direct and reflected sounds through directivity and reduces the response to reflections, thereby compensating for the room sound field. However, such impedance control merely separates based on directivity and thus cannot be considered a strict control method. Therefore, it is unsuitable for scenarios requiring free field conditions, such as acoustic measurements. In this study, we address this limitation by employing cylindrical harmonic expansion to explicitly separate direct and reflected waves. This allows us to construct a field equivalent to a free field even in a reflective room. Although this approach requires more control sources and microphones, we achieved highly accurate suppression of reflections. The proposed method is expected to enable robust control regardless of the primary source position within the control region. Work supported by JSPS KAKENHI Grant No. JP24K03222.
Kushiro et al. (Wed,) studied this question.