Thanks Pedro. I have created feedback FB24904876 with a sample project. This is on iOS 27 and macOS 27. The source is at 0, 0, -1, listener at 0, 0, 1, wall at 0, 0, 0. with extent in the x/y dimensions and 4cm thickness on the z dimension. I noticed that when the wall is small, a tiny bit of sound is audible, but when the wall is large nothing can be heard, even though the thickness of the wall is the same, and in all cases the source is fully occluded from the listener. I would expect some sound to be transmitted through the wall no matter the size given the same thickness and positioning. Maybe I don't understand the PHASE occlusion model. Does PHASE transmit sound through occluders at all? Or does the material choice only affect reflectivity of occluders?