Why Do Fluorescent Lights Hum?
- If you have eliminated vibration noise from loose fittings or metal parts of the fluorescent light fixture, the cause of humming may be ballast harmonics. Two types of ballasts exist with three sound ratings, but all ballasts hum to a degree. Commercial ballasts, used in industrial or commercial buildings, and ballasts for outdoor-use lights such as streetlights, carry the lowest sound ratings of B and C, respectively. These will hum or buzz louder than home-grade fluorescent lights in a library or kitchen, which carry an A rating. Light fixtures created with higher-quality parts are also less likely to hum as loudly as those made from cheap parts.
- Newer lights work on electronic ballasts, which hum less than the older magnetic ballasts. The ballast works as an inductor, reducing current into the light fixture. In magnetic ballasts, this functions the same as an electromagnet made by looping wire over a metal rod. As current flows through the coiled wire, the magnetic field squeezes the central rod, a process called magnetostriction. Over time, this alters the shape of the core material. When using alternating current, this constriction is periodic, squeezing and releasing at a rate of 120 cycles per second on U.S. power, or 120 Hz, resulting in an audible hum.
- In the United States, fluorescent lights require a transformer to raise the voltage to what the light needs. Aging transformers can become noisy. Those used in electrical ballast lights will be less noisy than the older, larger transformers used in magnetic ballast lights found in old commercial buildings such as schools. Smaller transformers also hum at a higher frequency and are more difficult for us to hear over ambient noise.
