Ken & Drew. Actually his second device. 1st was a take on the old fashion ear horn, bur apparently users had a problem getting proper seal at the ear, but still effective as a 1 to 1 device.
As for the newer device, speak into a microphone that processes thru an electronic board to a speaker. Anyone within range can hear. Of course they would require their own to speak back. To be honest, a fair amount of the technological explanation ( Keith is an electrical engineer) was lost on me. Still interesting.