In indexing a relatively new web site on Western Great Lakes Lighthouses, I came across this page which goes into detail on the diaphone fog signal:

http://www.terrypepper.com/lights/closeups/fogsignal/diaphone.htm

This is an Excellent lighthouse site. Great depth and bredth of content. Well researched and contains directions to most lighthouses. If your travels include Lakes Michigan, Huron or Superior, you need to check out Terry Pepper's site:

http://www.terrypepper.com/lights/

Hi;John I agree Terry has a great website and the great thing is he follows up on any changes via Email.This is a great way to keep abreast of anything new when they make tours of the lights.One of his updates was a story "Reflections of Plum Island"by David Robb USCG ret.covering his days at Plun Island.Very interesting reading.Bill O'Brien