I have an ordinary residential
ceiling-light that emits an audible electrical hum that increases in amplitude as the dimmer switch on the wall is turned down.
In other words; as the room grows brighter, the noise fades away.
What exactly is this sound?
Best Answer
It is called the 'mains hum'. It is usually emitted by transformers or arcs. The cause of this sound is that the electricity coming to your bulb is constantly changing direction/polarity 60 times (or 50 if you're outside the US) a second. The electricity is moving either the transformer if you're on a CFL or LED, or somewhere around an arc or loose connection if you're on an incandescent bulb.
tl;dr 1. It's the mains hum. 2. If you're on an incandescent bulb, you have to start worrying about it.