Konstantin Raudive was the first prominent EVP researcher to attempt to categorize the voices he captured into three classes of audibility. He described the scale he used in his 1968 book, 'Breakthrough: An Amazing Experiment in Electronic Communication with the Dead', which was printed in English in 1971.

He wrote, "This grading and my comments are but a rough guide in the present stage of our approach to the psycho-acoustic aspect of the investigation."

Raudive's scale was not widely adopted but is a forerunner to more popular EVP classification methods. Raudive ranked EVPs based on the ease of hearing the voice. Later classification systems would take into account the context of the words spoken by the voice.
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The Raudive Scale


Class A
Voices can be heard and...