Jim,
The sign sounds like a great idea!
Even then, some folks will still come up and ask. I have an inepensive Radio Shack mic that I keep ready for event providers that wish to make announcements. The volume is kept down to a minimum, it's always turned off and never out where anyone can see it. Kinda' like the Out of Sight--Out Of Mind mentality.
Cheers,
Gary
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PSR-S950, TC Helicon Harmony-M, Digitech VR, Samson Q7, Sennheiser E855, Custom Console, and lots of other silly stuff!
K+E=W (Knowledge Plus Experience = Wisdom.)