This device uses AM radio frequency to pick up random audio signals and generate white noise. The theory is that these random signals and white noise will allow spirits to create their own vocals through the receiver (talk through the box). Newer versions of this box tunes (scans) back and forth through various AM bands.
There are many obvious problems with this piece of equipment. First, and most important, it is irritating. It is constantly picking up small clips of actual AM radio broadcasts. The investigator is hearing white noise with broken, random words. More than a few minutes of this will lead anyone running for the quiet of some dark, creepy basement.
Second, it is too easy for someone to hear and interpret a random voice as being from a spirit when it is, in fact, just a piece of AM broadcast.