Here's a brief explanation:
If you believe in ghosts or spirits and you know someplace that is haunted or known to be haunted, you should try this out. All you need is just a regular hand-held cassette recorder. I tried this once, and I believe I caught something on tape. Just walk around and record anything and everything you think might be a good spot to record at. After the day is over, review the tape and see if you caught anything.
E.V.P stands for Electronic Voice Phenomena. This Phenomena occurs when the voices of a Spirit interrupt an Electro Magnetic Field,"E.M.F's", so inturn using audio tape i.e. from a micro cassette recorder reveals these voices where as before they could not be heard by a human ear. It is the same way with a dog-whistle, the dog in this case being the audio recorder. We as humans cannot pick up the sound of this dog-whistle because the frequency is to high. The dog, on the other hand can, because its ear drum is designed for hearing higher frequencies. The audio recorder uses magnets to record onto tape when an Electro Magnetic field is interrupted such as when paranormal activity occurs. This inturn effects the tape i.e."E.V.P"