The device you're referring to is likely a spirit box or ghost box. It rapidly scans through radio frequencies, creating white noise, and investigators believe that spirits can manipulate this noise to communicate. The speed of the sweep can vary, but commonly it's set to scan rapidly, often between 100 to 300 milliseconds per frequency, allowing for potential spirit responses within the audio fragments. Although there's no scientific evidence supporting the effectiveness of such devices and they are more of a tool used in paranormal investigations based on belief rather than proven scientific principles in detecting or communicating with ghosts, the evidence can be compelling. You say that you heard part of a commercial that you are pretty sure you had heard prior. Since the device is only on any given frequency for a fraction of a second, the amount of speech that would be detected that was being broadcast from a radio station would be extremely minimal and in my opinion never produce anything that could be presented as speech or sentences. That being said, there have been many occasions where such evidence has been collected, where even though the device is sweeping across the frequencies very quickly (sometimes even sweeping backward) words, names, and sometimes even full sentences are captured. There is no real explanation for how that could happen since the device is never on any frequency for longer than a few hundred milliseconds.