Large performing arts bldg just completed. On the stage we have isolated ground show switches. We are measuring 45-88khz @ 110-180 milivolts between the two grounds and neutral. Doesn't matter which you pick. This noise is not acceptable to sound equipment in theater. Everything is connected properly. This building is very close to major airport, and sits adjacent(less than 50') to light rail tracks that is DC driven. Could the building be picking up this high freq from the metal roofing? Or do I have a bad ground somewhere that we are creating a ground loop? The High Freq is a mystery to me.