In this forum might be also a title ”Singing..."
I could ask this direct from Rollin or Wolfgang but maybe somebody else is also interrested.
If we filter overtones in mouth by tonque, we get low overtones from large space and high overtones from small mouth. Makes sense, longer waves fit in large space. But when I filter overtones by opening mouth, I get high overtones by opening my mouth wide open and low by closing it. Understand? What is the physical explanation?