http://www.readresearch.co.uk/HTML/articles.htmThe Klipsch Modulation Distortion article is interesting -- they feed at the same time two different sine waves into a single driver and then measure distortion. Who knew there could be so much distortion? Of course, this is part advertising for horns, but it's interesting nonetheless.
I haven't read the others, but these are "famous" articles.
(Edit to fix "speakes".)
