The BBC on those who believe that radio and tape recording devices offer a window to the realm of the dead:
In 1969, a mysterious middle-aged Latvian doctor turned up in Gerrards Cross with a large collection of tape recordings…he had established contact with Hitler, Stalin, Mussolini and many other deceased 20th Century statesmen. The recordings – 72,000 of them – contained their voices. His name was Konstantin Raudive, and he called his technique Electronic Voice Projection, or EVP.
It wasn’t real-time interactive communication. You asked your questions, and then left the tape running, recording silence. But listening back, through the mush and static, you could sometimes just about make out people speaking.
Nowadays, EVP is a standard tool of ghost hunters worldwide. There are hundreds of internet EVP forums and many serious and well-educated people who see it as proof positive that the dead are trying to talk to us.