> do human ear canals vary that much to need a mic?
Surprisingly, they do! It doesn't need a lot of change to have a different ear canal resonance, changing the frequency curve somewhere between 5000-7000Hz. Oluv is currently tuning Earfun IEMs to different earl canal lengths which you can then try out, and he has written some articles [0] on exactly this topic, but unfortunately its paywalled: https://www.patreon.com/oluvsgadgets/
Surprisingly, they do! It doesn't need a lot of change to have a different ear canal resonance, changing the frequency curve somewhere between 5000-7000Hz. Oluv is currently tuning Earfun IEMs to different earl canal lengths which you can then try out, and he has written some articles [0] on exactly this topic, but unfortunately its paywalled: https://www.patreon.com/oluvsgadgets/
[0] https://www.patreon.com/posts/trust-your-ear-58601998