The following classifications of EVPs are generally accepted and used by paranormal researchers and investigators:
Class A - This type of EVP is loud, clear and of very high quality. The voice is easily understandable and does not need enhancement or amplification. Class A EVPs are also often (but not always) in direct response to a question being asked.
Class B - This is
the most common type of EVP. This type of EVP is of somewhat lower quality and clarity
than a Class A EVP but still very audible. Class B EVPs often do need some
amount of enhancement
or amplification to be heard clearer. The voice may not be clear enough to be
totally understood or there may be disagreement as to what it is saying.
Class B - EVPs are often not in direct response to a question.
Class C - This is the lowest quality EVP. With a Class C EVP even the best enhancement and amplification may not be sufficient to make the voice audible or clear. There may even be debate whether or not an EVP is actually present.
Investigators go further but that can get complicated and the above
three classification does work and has a scientific approach.