https://www.rtings.com/ https://www.rtings.com/ provide frequency response graphs for reviewed headphones + in-ear phones. For headphones they measure against

https://www.rtings.com/ https://www.rtings.com/ provide frequency response graphs for reviewed headphones + in-ear phones. For headphones they measure against a dummy head and against 5 individuals. They then show variation between these readings, and show the average freq responses.