There are 2 ways to find it. Taking an impedance sweep and an acoustical measurement
Look between 12 and 14 kilothertz. On the impedance graph there will be a small bump.
This is a diaphragm resonance. He notch circuit is only a passive means of Equalization. It never goes away. I listen to a pair of those everyday for 5 years.
When you increase the level being played by the driver, the notch keeps it real to position.
There is no electronic means of making a mechanical problems go away.