I've got some hanging lights with "Fat Albert" bulbs that are plugged into a dimmer switch. When they're dimmed to certain levels, the bulbs make a soft buzzing sound. The buzz is amplified and directed straight down because of the shape of the shade. It's quite annoying if you're under them!
Here's a photo / blog post about these lights: http://lukecyca.com/2010/factory-lights.html
Why do the bulbs buzz when dimmed? Are there other bulbs that don't buzz when they're dimmed?
Why do the bulbs buzz when dimmed?
When you pass AC current through the filament of an incandescent bulb, it produces a magnetic field. This magnetic field tends to push against itself, so there's some mechanical force acting on the filament itself which causes it to vibrate slightly. When running at full power, the AC is fairly close to a sine wave (either 50Hz or 60Hz, depending on where you are) so this vibration is pretty much constant and generally barely noticeable.
A dimmer will truncate and clip part of that sine wave; this introduces a lot of higher-order harmonics, which in turn changes the characteristics of the magnetic field, changing the way the filament vibrates. Depending on how the filament is supported, this may cause resonance at certain dimmer settings, leading to the vibration becoming noticeable.
See this article for some more background information on how dimmers work, including a mention of this phenomenon.
Are there other bulbs that don't buzz when they're dimmed?
If you prefer incandescent lights, look for rough service bulbs; the filament and its supports are designed to withstand more vibration than normal bulbs. You could also try halogen bulbs or even dimmable CFLs.
I've used standard-base chandelier bulbs in the past to solve this problem, like this:
Their filament is longer and floppier than the one in a standard bulb, so the vibrations mentioned by Niall C. don't make any noticeable noise. Of course they look different than a standard bulb so they might not be applicable in your fixture. In ours they complemented the fixture nicely, so it was a win-win.
The buzzing could be from the vibrating filament, but it could also be from the leads inside the metal base. Inside the base there are leads welded to the side/tip that pass into the sealed glass bulb. I confirmed this idea by experiment on one particular offending bulb (halogen). By removing the metal base and applying dimmer output directly to the leads I was able to make the buzzing disappear. (Don't try this at home!)
Even though rough service bulbs may have a thicker filament and more supports, being less likely to vibrate, the leads at the base may actually be the culprit. When I tried using one particular brand of rough service bulbs to combat buzzing (advertised for garage door openers) I found that they still buzzed.
So in summary it appears you may need to try multiple brands to find one that works. I finally found some energy saving halogen bulbs that did the trick.
