Unlike a lot of evidence of the paranormal, it's common practice to grade voices captured as EVPs. There is one commonly agreed-upon classification scale with defined requirements to help investigators determine which category of EVP they've captured.

According to the Association TransCommunication (ATransC), any form of ITC, including EVP, can be split into two types:

Type 1 - Transformative: Manipulation of dissimilar soundsType 2 - Opportunistic: Selective use of existing voices

These two types of EVP actually have more to do with the environment the EVP was captured in than the voice recording itself.

Type 1 EVPs are transient sounds of a pitch matching a human voice that spontaneously appear amongst background noise or sounds of a differing pitch. This could be the sound of a voice coming through amongst the hiss or hum that is the ambient sound of the room, or a voice with a similar tone to a human voice heard...