However it may have been audible stage call cue.
I heard of a place that had a flashing red light and a siren ( in the green room ) to get the band on stage.
Needless to say it wasn't popular with musicians
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Thanks for the reply. It was WAY too loud for a stage call cue. I was in the back of the concert hall and it was sortof loud there. And there were two, different, alternating tones about an octave (or more) apart. Was not before the 1st band but as the main (3rd) band was taking the stage. I have never heard anything like that preceeding a band.
So just before the performers take the stage I hear about a half dozen Loud "Pings" similar to a WWII Submarine movie. Is this some sort of EQing going on?
All of the EQing is done well before the performance, prior to the sound check for that matter. I've never heard anything like that, communications between the stage and the FOH are usually by hand held radio.