I worked on a product that used an in ear microphone to adjust the model used in noise cancellation on the fly. In a control theory sense, the system varies a

I worked on a product that used an in ear microphone to adjust the model used in noise cancellation on the fly. In a control theory sense, the system varies a lot so your poles can move around and need a different controller, even on the same head depending on fit. The product produced white noise to tune the model which greatly improved the noise cancellation accuracy. It’s also not just the ear canals but the rest of your skull that effects how you hear. There are apps that use machine learning to fit a model/eq to the shape of your head from a photo. I would think that actually measuring in the ear would give a more accurate representation of this.