I have been looking at these. I saw one once and it
seemed to respond well to noise in a room, such as from
lighting. I have been making a noise detector for a while, which
has selectable types of noise, electrostatic, magnetic, light, and
vibration. I really need to just get one of these and experiment, but
the Progressive (now Tempo or Textron or Greenlee ??)
probe say inductive while I really thinks its a basic electrostatic
probe. I'm thinking of modifying it since its so inexpensive.
I don't know if it has any filtering of frequencies in the circuit.
Anyone know about these ?

greg