Class A - These are, by far, the best audio evidence and the most publicly presentable examples of Electronic Voice Phenomenon. They are loud, clear and of very high quality and the voice is easy to understand and may even be mistaken for that of a fellow investigator on the scene. For this reason, all serious investigators now make it a point to identify any sounds that they hear during EVP Sessions as well as to include the names of those present at the beginning of each recording. “Class A” EVP’s require no additional enhancement or amplification. These are generally but, not always, in response to investigators questions. Due to the lack of processing, they leave the least doubt in the mind of the listener as to their authenticity. Unfortunately, this class of EVP is comparatively rare.
Class B - The most common type of EVP. This type of EVP is less easily understood but, still very audible. Generally speaking, these often need some amount of enhancement or amplification to be heard distinctly. The voice heard is usually not clear enough to be easily understood. Some may disagree over what it said to the degree that they may question what language it is in.
Class C - The lowest quality EVP still considered as viable. Will require extensive enhancement and will still not be heard as any recognizable word/phrase. It may often be questionable as to rather there is even a legitimate EVP on the recording in the first place as it could easily be confused with other, natural noises such as shuffling feet or various mechanical sounds. In many cases, these may be indiscernible or unnoticed with out drastically speeding up or slowing down the recording. When captured on analog devices such as tape recorders, the EVP is often obscured by the recorders “background hiss“. To remedy this, it was at one time common to use out-board microphones to avoid picking up this “hiss” as well as the mechanical sounds of the recorder itself. Due to the wide spread use of digital recorders in recent years, this class of EVP is becoming less common.
Class A, B and C are the only classes that are considered clear enough to be publicly presentable. Some paranormal investigators continue to use the now widely dismissed Class D and Class G classifications as well although, due to the poor quality of the EVP's and the amount of processing required, they're too subjective to be of any real scientific value. They are more likely to be proof of the researchers wishful thinking than anything else.
Class D - These are sometimes called "Class R" EVP's as well. They're extremely poor quality and highly questionable as EVP's. These usually turn out to not even be EVP's at all. This class is characterized as "EVP chatter", "whispers", "breathing noises" and "airy sounds". Like Class C EVP‘s, they are sometimes only noticeable when the audio is sped up or slowed down, sometimes drastically. Some investigators use the term "Class D" as a temporary, or "working" classification until additional "cleaning up" is done. Class D EVPs are not usually candidates for consideration as evidence for public display but, are occasionally shared with other researchers as "Possible EVP's" although this is discouraged.
Class G - For most investigators, "G" stands for "garbage". These are of such low quality that they require excessive enhancement, filtering, etc. are almost certainly not really EVPs. Although these may sometimes be maintained in ones personal files with the hope of validation at some later date, the extreme enhancement needed to bring them out is enough to cast them in doubt. In short, if it requires that much work to clean up a sound, it's probably not an EVP.