Perhaps this is a pressure node rather than a displacement node.
The screen is placed at L/4 in a tube of length L. In the octave
mode (first overtone) there are pressure nodes at the ends and
center of the tube. The L/4 point is halfway between these nodes.
The screen is at a displacement node. We toyed with the idea that
the thermal time constant of the screen might be involved in the
excitation. I think that more experimentation is called for.