Honeydripper vowel sounds

[nieradka]

I recently built a honeydripper, and it works great/exactly as it should. Im trying to analyze the circuit, im a little hazy on how the vowel sounds are being made, can anyone explain the filter section, or better yet  point me to any resources? (more than, http://www.geofex.com/Article_Folders/ECFtech/ecftech.htm) -- Im working on trying to design a envelope filter to emulate the ehx talking machine open-AH setting. And the poke and hope method of changing values on bread board isnt working so well.

[nieradka]

Well I found this which helps: http://www.geofex.com/article_folders/sing-wah/sing-wah.htm but if anyone has anything else, id appreciate it.