EVP

Electronic voice phenomenon or EVP as it is known, is when the voices of spirits are caught using a recording device. Some of the voices are not heard at the time, but can be heard later on the recording, as the voices are too low to hear with the human ear.
EVP is often used on ghost hunting shows. Usually they are able to pick up a few words or phrases from recordings. 
It has become very popular in recent years. You can perform your own tests at home, using the recorder on your phone. Simply switch the recorder on and ask some questions to any spirits who may be in the room. Leave a gap for any answers, You can even leave it in the room alone and see if it picks up anything.