I find it makes a difference where you put them. Dampers seem to work best near the top of the tube, not in the middle. Hey, maybe that's where the "top hat" idea came from.
But thinking of the glass as a transmission line (or guitar string), this makes sense. You minimize reflections, and hence resonances by damping the ends, where refections occur. In fact, some of the spots in middle (or harmonic fraction thereof) could be null points and have no effect. Depends on the resonance mode, if fundamental or higher. Only way to damp them all is to do it at the top or bottom.
But thinking of the glass as a transmission line (or guitar string), this makes sense. You minimize reflections, and hence resonances by damping the ends, where refections occur. In fact, some of the spots in middle (or harmonic fraction thereof) could be null points and have no effect. Depends on the resonance mode, if fundamental or higher. Only way to damp them all is to do it at the top or bottom.