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?
What exactly is this sound?
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.
Under normal conditions, the voltage and current in the line change constantly, but smoothly. The change in current induces a magnetic field. Because the current changes direction 100 or 120 times a second, so does the magnetic field. If things are loose or have the right properties, this changing field will cause resonance and parts can "hum" at that frequency. Magnetostriction is often part of why transformers hum.
When you use a normal household, solid-state dimmer, the voltage and current no longer change smoothly. Instead the line is opened at the same point every cycle. Near full brightness, the line opens very near the zero point. But as the dimmer is lowered, the line opens closer to the peak. This sudden change in current means a stronger magnetic field.
The wikipedia page on dimmers has some mention of it.
When the dimmer is at 50% power the switches are switching their highest voltage (>325 V in Europe) and the sudden surge of power causes the coils on the inductor to move, creating buzzing sound associated with some types of dimmer; this same effect can be heard in the filaments of the incandescent lamps as "singing".
