Topic-123: Explaining Perturbation Theory According to perturbation theory, resonance occurs in a uniform tube where one end is closed and the other end is open (just remember the source filter idea. This theory tells us whether each resonance frequency increases or decreases when a small modification occurs in the diameter at a local region of the tube (tract. As a result the resonance frequency of the particular resonance mode decreases when a constriction is located at an anti-node of that resonance mode and the resonance frequency of the particular resonance mode increases when a constriction is located at anode of that resonance mode. Keeping in mind this idea of perturbation theory, we can derive that the resonance frequencies will change (decrease or increase) as per the position (modification in the size and nature) of the vocal tract (tube.
