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Why do many microwave ovens hum in an interval of a minor 7th? (lemmy.wtf)

submitted 11 months ago* (last edited 11 months ago) by [email protected] to c/askscience

42 comments fedilink hide all child comments

Something that I've noticed across most of the microwave ovens that I've used is that when they hum while cooking food, I can pick out 2 distinct tones. One of them is pretty clearly ~~60~~ 120 hz, the 2nd harmonic of the AC power frequency. The other is consistently a minor 7th above that (which would be somewhere around ~~106-108~~ 212-214 hz depending on the exact ratio). What causes this 2nd frequency to be produced?

Edit: after checking against a tone generator, the low frequency is actually 120 hz, double the grid frequency. The question is still the same, just an octave higher.

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[–] [email protected] 12 points 11 months ago* (last edited 11 months ago) (2 children)

Could it be the motor?

The second harmonic from 60hz is 120, which is close to where your hearing it? For what its worth, i get a peak at 100hz, which for the australian grid would make sense?

I used this app: https://play.google.com/store/apps/details?id=org.intoorbit.spectrum

I dont see a 50hz peak, but 100hz was clearly represented.

[–] [email protected] 9 points 11 months ago* (last edited 11 months ago)

I checked against a tone generator, and realized that the lower frequency was actually 120, not 60, so I'd guess you're probably hearing that. That puts the higher frequency at 212-214, which I also checked and that range matches what I'm hearing (for 100hz, it should be at around 177-180).

[–] [email protected] 1 points 11 months ago (1 children)

Interesting. I saw mostly 38Hz, 117, and 240. 100W LG microwave on US 60Hz with an audible fan.

[–] [email protected] 2 points 11 months ago

Mine is also LG, not sure what its power output is, but i think its either 1000W or 1200W, yours must take forever to heat anything 😉.