My understanding is that electromagnetic waves can have just about any frequency.

They can be basically electric such as radio waves that can be detected with the apppriate type of radio receiver.

Or they can be basically magnetic. And those are generally more difficult to detect. The system discussed at my Web site runs about $2000. And I believe that it is largely detecting magnetic waves.

I expect that it could be duplicated with a relatively simple system costing under $100 that would use a specially configured coil of wire. But that part of this research is still in its infancy. And there is no time to work on it at the present.

At the moment I am trying to get my computer programs developed to the point where governments could be using them to process Ear Tone data collected from their own local populations. They should be good indicators of approaching earthquakes. And the data and computer program would be free. You can't beat that!