Fluorescent Ballast

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Frequently Asked Questions...

Fluorescent light bulbs...why does one hum?

I just replaced two fluorescent bulbs in my ceiling fixtures tonight...the first one popped in and lit up nice and brightly. When I installed the second one, in a different fixture, it lit up just fine but has a loud hum.
Anyone have a clue why? Or what I can do to quiet it?

Best Answer...


All fluorescent lights require a ballast to function. All ballasts hum to some minor degree.
Ballasts for 48 inch, 72 inch and 96 inch slimline lamps are sound rated C and D and are the least quiet. Both magnetic and electronic fluorescent ballasts give off a slight humming noise; harmonics is the technical term.
Slimline ballasts should only be used in industrial/commercial applications where noise is not a problem. Energy saving ballasts, residential ballasts, and solid state (electronic) ballasts are sound rated A.

By design an electronic ballast has reduced harmonics and therefore the hum is less noticeable than when using a magnetic ballast. You may hear them in a quiet room. However, in most applications, noise from sound rated A ballasts is covered up by the usual amount of ambient noise in a typical room.

A loose magnetic ballast can be the cause of ballast hum. Check that ballast mounting screws are tight. If the hum is louder than usual the ballast may need to be replaced.