Presentation given by K.C. Budd at DEFCON 17, showing a 1964 acoustic modem.
From the original description: "Eighteen years ago I was given a curious device: An Acoustic Coupled modem in a wooden box. I've kept it over the years and recently rediscovered it. Thanks to the recent invention of the web, I am now able to discover something about its history; I may have the oldest working modem on the planet! At this talk I will not only show you the modem, but also demonstrate its operation. Come along for a journey 45 years back to the beginning of digital telecommunications!"
This is a very rare, and very well recorded demonstration of this technology. The original information is available at defcon.org/html/links/dc-archives/dc-17-archive.html#Phreakmonkey