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.
The following is an audio forensics piece created from tapes originating from the very haunted 13 Stairs Cemetary in Palo,Iowa.
Sounds have been processed and samples using samplers,sequencers,and other devices.If one listens closely voices can be heard.