50 Hz test tone: is your hum on the grid or an octave up?

According to Wikipedia's Utility frequency article, "As the 20th century continued, more power was produced at 60 Hz (North America) or 50 Hz (Europe and most of Asia)." If your power is 60 Hz, start with the 60 Hz tone page instead.

A 50 Hz grid makes 50 Hz the obvious first guess, but that's not where hum usually lands. According to Wikipedia's Mains hum article, hum's usual pitch is double the line frequency, which is 100 Hz on a 50 Hz grid.

Keep the level down and raise it in small steps, since a loud low tone can distort a speaker. Pause the music, switch the tone on, and listen for the buzz underneath it. If the buzz sounds like this tone, that's the less common result, and a listening match doesn't tell you the cause. If the buzz sounds an octave up, the Mains hum article ties a ground loop to that pitch. Play the 100 Hz tone next. The article also says the sound often has heavy harmonic content above the line frequency. To compare those pitches too, the exact frequency box in the full Frequency Generator can play 150 Hz and 200 Hz beside this tone.

Utility transformers can also hum on their own, from mechanical vibration caused by magnetostriction in their cores. That's sound from the transformer itself, not from your speakers, so this tone can't tell you whether one is behind a buzz you hear.

One cycle of this tone takes 20 ms. That's longer than a 60 Hz cycle, which takes 16.67 ms.