EVP - or electronic voice phenomena - is an event in which human-sounding voices from an unknown source are heard on recording media (analog or digital), in radio station noise and other electronic media. The mysterious voices are not heard at the time of recording; it is only when the recording is played back that the voices are heard. Sometimes amplification and noise filtering is required to hear the voices. Some EVP is more easily heard and understood than others. And they vary in gender (men and women), age (women and children), tone and emotion. They usually speak in single-words, phrases and short sentences. Sometimes they are just grunts, groans, growling and other vocal noises. EVP has been recorded speaking in various languages. In rare instances they will come across backwards or time delayed.