Laser resonates with ceiling speaker to create a horrible ?ground loop?

We have no idea how the system is wired or where the hum is coming from.

You should go to the site and inspect the problem before posting.

Is the hum line frequency? Someone could use a cell phone and send you the sound.

On a sunny day (Sun, 30 Oct 2022 14:47:22 -0000 (UTC)) it happened "Don" snipped-for-privacy@crcomp.net wrote in snipped-for-privacy@crcomp.net:

I once had that problem in a building, differential audio (2 opposite phase and ground), but somebody had cut one of the audio wires... As it is far away looks like a possible connection problem.

You got me back on-track Fred, thank you. It helps to show up onsite with a battle plan. It's been years (not long enough in other words) since the last time Valcom speakers were debugged by me. And, of course they use the cheapest twisted pair available - not the luxurious audio line imagined by my mind. So, yes, somewhere in the ceiling the twisted pair for the obnoxious speaker must parallel the electrical outlet used by the medical laser. The Valcom speaker system is no longer used, so the simplest solution is probably to disconnect the twisted pair from the Valcom LM386 amp.

For the benefit of others in this thread, a "ground loop" implies a frequency of 60 Hz. Danke,

Not necessarily. A 50 Kilowatt switching power supply can generate all manner of frequencies. I've also encountered 180 Hz, caused by swapping a power neutral (white in the US) with safety ground (green) on a 19: relay-rack cabinet.

Joe Gwinn

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Yes, and if the laser has a simple line cord, a common-mode clamp on bead would be the first thing to try.