Posted the EMT plate as a counter-example of a truly useful, well-engineered, understood, intelligently applied "resonant audio enhancement device". I probably should have noted that more clearly though. I was going to include a photo of a tuned diaphragm absorber or helmholtz acoustic bass trap in the same post, as installed in a studio control room, as another counter-example of these principles, intelligently and measurably applied), but they don't look like much more than a flat panel or a slot in the wall.