The Art of Speaker Design

http://www.nutshellhifi.com/library/speaker-design1.html
http://www.nutshellhifi.com/library/speaker-design2.html
these legacy pieces (2002) makes for an interesting read
lots of additional info on http://www.nutshellhifi.com/ 
  { Lynn Olson is still with us and hangs out on DiyAudio  eg https://www.diyaudio.com/forums/multi-way/100392-beyond-ariel-1490.html }