A study on constructing arbitrary directivity of sound sources by fitting the spherical harmonics coefficients

We attempt to obtain room impulse responses by sound sources with arbitrary directional characteristics by directivity synthesis in the spherical harmonic domain. For this purpose, we use directional room impulse responses measured with a general loudspeaker pointed in multiple directions. Specifically, we attempt to synthesize the directivity from measurements in 24 directions, including the horizontal and elevation angles of 45 degrees each and the case where the loudspeaker was pointed directly above and below. This enables us to construct an omnidirectional sound source that strictly meets ISO standards or a more flexible directional that reflects musical instruments' directivity. For the omnidirectional case, we are also considering how much we can reduce the number of measurement directions and propose a practical index. In addition, because the proposed method uses a commercially available coaxial loudspeaker with enough sound quality and performs frequency spectrum correction at the same time as directional synthesis, it is possible to synthesize room impulse responses that can be used for auralization.