Stuart, the standing wave pattern is based on volume for irregular shaped resonant objects and length for cylindrical objects. The acoustical diameter of a tube is larger on bends even if the bore is the same size. You may be on to something here. A D shaped slide is primarily cylindrical except for the two corners. The ovate slide is more constant. That still does not explain why I can hear myself better and that the audience does not hear any difference. At the end of the day, the difference in my daily form will be far greater than the small hardware difference. I guess practicing more will give us greater improvements.