Ok, note to myself: When building the next panel, save one measurement without a spline and one with and compare the direct sound components.
Next topic: Maybe if I have enough prototypes I try DMLs for a home theater installation. But that requires direct wall mounting for the surround speakers which kills the sound. I tested a bit with 5 mm sound insulation (XPS, the thing you put underneath laminate) as panel and noticed an improvement when bending it (looks a bit like the plywood panel someone constructed for wall mounting). Thinking about this I come to the conclusion that the non uniform distance to the wall prevents destructive interference with reflected waves. So it might be a good idea in general to (slightly) bend a panel to reduce negative effects of walls nearby. What do you think?