I have my theory but I am wondering what your first guess would be as to why a dimmer switch makes a humming sound (obviously when the light is on). How do you explain this to a homeowner?

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I think it is the standard answer: It doesn't know the words! The more the delay towards 90 degrees, the greater the harmonics / noise caused by the triac switching. This is in the audible spectrum. Snubber circuits are used in the output to reduce noise. I would expect that most of the noise you hear at the dimmer comes from the iron cored inductor of the snubber circuit. Think of the noise you hear coming from power transformers with some harmonics mixed in.
Joe

Explain to them that there are coils of wire in the dimmer just like the coils in a speaker, and the fluctuating magnetic field makes it vibrate and hum a small bit.

It's not completely accurate, but should make them feel better.

We also get hum from the lamp filaments, in our theatrical fixtures. Most newer lamps are designed to prevent this, but replacing all of our fixtures to work with newer lamp designs isn't cheap. Not sure if this is a problem with smaller bulbs (assuming 100 watt or less).

All those things that Joe said...and cheap light bulbs. I used to have the same problem. The dimmer has a filter to keep the bulbs from humming. So now the dimmer hums. Upgrade your light bulbs.

I was going to add what "noderaser" mentioned, but not needed now!

Looks like Lighting is more oriented towards the Entertainment Industry than previously expected, in that we have...

Singing Lamps and Humming Dimmers

Just couldn't resist that one!

Scott35

edited to fix UBB coding blunder!

[This message has been edited by Scott35 (edited 03-08-2007).]