I think that its because tweeters handle what we thought was the majority of the range. First off, I thought I could hear to 20k because I’m still young. Well I can barely hear to 15k; maybe I’m not so young anymore too thats a different conversation.
So yeah, if we think we can hear to 20k, and tweeters are crossed over at 3.5k then that’s 16.5k of a 20k range...