https://arxiv.org/ftp/arxiv/papers/1811/1811.03389.pdf that one comes with a big decision to try and position the rig at a fixed distance(to get a specific resonance from all subjects). so added to all that cool work, we have to consider the extra variable that is insertion depth and total ear canal length. so basically the situation is worst than what they show. I had another one that was quite complementary to that study, but the link is dead(that's what I get for bookmarking instead of downloading everything). from not so reliable memory, it was saying something very intuitive. like how beyond the length and total volume, the variations in shape didn't impact the low freqs much but started to have more impact above 1 or 2kHz(I think) and rapidly becoming quite significant as we increase the frequency.