Konstantin Raudive was a Latvian psychologist and student of Carl Jung and university lecturer in Sweden who devoted the last 15 or so years of his life to communicating with the dead through electronic media.
Late on in his research Raudive thought a parakeet might also be channelling messages from the other side, and said he sometimes heard telepathic messages when he wasn't recording, but that's another story.
Capturing what he defined as fragmentary telegram-like messages in many languages using a running mic, radio white noise and an untuned crystal set through more than 100,000 laboratory recordings, Raudive popularised the idea of electronic voice phenomena, or EVP.
Electronic voice recorders are now standard issue in any ghost hunter's kit, and just the latest link in a chain of technology (modern magic) used to contact the spirit world that goes back to the Victorian spiritualists and their notorious yet mesmerising ghost photography.
Raudive published two books before his death in 1974, the bestseller Breakthrough: An Amazing Experiment In Electronic Communication With The Dead (1968) and Do We Live After Death (1971, pictured), though Raymond Bayless, another occult-investigating sound experimenter, trumped him on title alone with his 1979 book Phone Calls From The Dead. Kudos name-wise should also be given to William O'Neil's 1980 contraption, The Spiricom, built to a psychic spec and apparently capable of two-way spirit conversations.
Read Jared Keane Feldman's essay Specters of The Spectrum for more on Raudive