Hi. I'm studying radiation sound topology optimization, and my optimization object enclosed in far-field(air). Radiation sound topology optimization resulted in the reduction of the volume fraction (like picture). But I have a question. When the topology is optimized, a empty space (no-mesh space) is created between the constructed Far-Field and the object. But I wonder how the topology optimization works. Does radiation sound not transmit to empty space?? The picture below is the optimization result and I hide the far-field ( the object is surrounding in sphere far-field) I'm appreciate in advance.